Monday, August 31, 2015

Sides of a Tropical Cyclone, Part 2 Meteograms.

In Part 1 of this topic, we looked at a way to study the wind and pressure on both sides of a tropical cyclone (TC) using any grib viewing program. There are many such programs available and the method outlined does not require any special features. Any of them could be used.  In this note we look at another way to study this that takes advantage of a special meteogram feature of the navigation and weather program Expedition.

In the presence of an approaching TC, we can judge where we are relative to its path by watching the wind speed, wind direction, and pressure from the perspective of being on one side of the storm's path or the other. In the Northern Hemisphere (NH) the right-hand side when facing in the direction of the storm's motion is called the dangerous side, whereas the left-hand is generally more manageable, at least if some distance away from the center, and it is called the navigable side. Part 1 included background on this and references.

The names are in large part based on the fact that the approaching winds on the dangerous side tend to push you into its path, whereas on the navigable side the approaching winds push you away from its path. The sea state on the dangerous side is also notably steeper and more chaotic.

A meteogram is just a plot of the wind, pressure, temperature, etc as a function of time. Time can be plotted forward (a forecast) or backward, a history.  The latter plots are a nice way to see, for example, that wind direction undergoes a prominent veer at the passing of any front.  Using a meteogram as a forecast we can show graphically the behavior expected on each side of a TC, as shown in Figure 1.

Hurricane Guillermo on Aug  3-5, 2105. The same example was used in Part 1. The background and meteograms are from Expedition, version 10.0.12.
Here we see in a much more direct manner what we discussed in Part 1. The blue stripe in each case marks the conditions we would observe from a stationary position over the day the system approached.  The two side points are about 70 nmi to each side, with all 3 being about 200 nmi off—which before any further discussion we have to say are all three very close to this system.  Such intense systems to not emerge suddenly, or if so, extremely rarely.  In this case we would have had at least 3 days earlier to know where this storm was going to be at this time. Thus how we ended up in one of these positions is an entirely different issue!  In short, we would have to say that we are primarily thinking about rare situations in which we did not have contact with the outside world and thus did not have the official forecasts.  But then, even with that assumption, we have to assume further that we did not have a $200 SW radio on board that would have told us the storm location and motion at least once every hour. Such a radio runs on batteries and is not dependent on the ships power.

Looking at conditions during the blue bar, we see first one important factor.  In all three locations we did not see much interesting at all for the first 6 hr, and that is because the closed isobars of the system had not yet reached us.

Once the closed system reaches us we see the expected behavior. On the right the wind builds and starts to veer. On the left we see the wind backing, but it actually falls off a bit, and in any event this far off the axis it never does build much.  This illustrates the value of getting as far as possible from the center of the storm as it passes.

In all three cases we see the pressure going down as the Low center approaches. There is more discussion in Part 1. The main goal here was to shown this new way of studying this behavior.  As it turns out, we have three very large systems in the Pacific right now, so I will try to grab some data and make a video of the use of these metorgrams.

There are other ways to generate meteograms using online resources, once you know the Lat-Lon and time you want. Most of these must be near live times, however. One example is  but I recall others that are easier to use.  I will add them here as i find them.

Wednesday, August 26, 2015

Commercial Echart Update Notice is Food for Thought

The echart industry has a long way to go in the field of intelligent marketing, which only adds to the challenges we face moving into the realm of modern navigation.

From an August 26, span email ad.

If they do indeed make 2,000 corrections per day, what is it they are correcting? 

The true charts themselves are not changing anywhere near that rate, even on a global basis. So this simply means they are mostly correcting their own errors in their own proprietary electronic charts. [*]

Thus we learn several things. Good—these charts are getting better every day. Bad—they indeed do have a lot of errors in them. (See related note.)

So the recommendation we make remains the same. When using electronic charting we are still better off with raster charts (RNC) when available. These are direct copies of the latest official charts, which these days are updated every week at no charge. Then if we want the convenience of some commercial vector chart such as this one in a packaged set of electronics of software program, we can then check it with the latest RNC or ENC.


[*] To be fair, maybe they are not just correcting actual errors but adding new information, to an otherwise correct chart. That would have to  be checked, and evaluated. If the addition is something that is already on the official charts, then is that an "error"? If it is adding something that is not crucial, then that is what it is. 

The reason vector charts (official or commercial) will eventually be our main resource is they offer so many layers of information, and are indeed easy to update, even remotely. Some of the information in deep layers is not crucial often, but could be in special circumstances. 

So the 2,000 updates a day shows they are working on this.  My comment here has more to do with the choice of marketing idea. My first impression was with that many updates, they must have a lot to fix.

Sort of like advertising ones level of experience in navigation by citing all the hurricanes they have sailed through.  I would tend to listen more to the person who has sailed all their lives and never been in a hurricane.

Monday, August 17, 2015

Mariner’s Weather Checklist Before Departure

 It is fundamental to safe efficient weather work at sea that these items be checked off and tested before departure. Having it all onboard without prior testing does not count!

(1) Barometer. This is key instrument to any ocean or coastal passage. Check it over a large pressure range using free service at,  or just interpolate the isobars to get the correct pressure for your location at the synoptic times from unified map at For electronic barometers, be sure it is on and running for a few days before departure. Ideally, record the pressure for a week or so before leaving and compare what you recorded with the official values mentioned above.

(2) Wind instruments.  To measure apparent wind speed and apparent wind direction, and they should be calibrated.

(A nice backup to both of these is the small handheld Kestrel 3500. The barometers on these that we have tested have been excellent. It is easy to check the windspeed driving along in a car holding it out of the window. Using one of these to check your onboard instruments, remember that wind at the masthead can be notably different from wind on the deck.  Might have to take it up the mast.)

(3) Calibrated knotmeter and magnetic Compass, and calibrated heading sensor.

(4) True wind from Apparent solution. We need some procedure to convert apparent wind to true wind that is well tested and practiced. This can be done within the wind instruments themselves or some other electronics box they plug into, or it can be done by inputing the data to a computer and computing it there,. Or it can be done by hand with a calculator, which is best done with one that is programmed to do this. There are numerous smart phone and computer apps for this.

(5) Laptop computer. There are certainly ways to do safe weather work without one, but  a computer makes everything related to weather very much easier, more versatile, and better. I would consider it a required item since so many of our best resources are obtained via the computer. In fact since computers are so inexpensive these days, it is not even unreasonable to set up one with all your nav and wx software and connections and then just clone it to another one which you take as a back up.  The back up could be a notebook size that might cost only some $300 or so.

(6) Communications.  This would mean either a sat phone or an HF radio with Pactor modem.  The sat phone is definitely easier, but day to day usage is more expensive.  HF radio work could be near free once set up, but the investment would be notably more than a sat phone to set up from scratch.   It would be nice to have both, that is if the boat is already set up for HF radio then add a sat phone for convenience and dependability.  If not set up already, it would be harder to justify the expense compared to sat phone only.

There are these days all sorts of other communications options, from satellite broadband to systems that connect everything on the boat wirelessly.  However, the basics are in item (6) and they should be there regardless of the other luxuries.  In a sense, the more you have the more that can go wrong, so you want to be able to pick up the sat phone and make a call, or plug it into your computer and send an email.

(7) Software for email. We need software to send and receive emails with attachments… with no restrictions on what we say or include, which rules out the basic ham radio programs. There are numerous options. Airmail from Saildocs is one. Ocens, Xgate, or UUPlus are a couple other options. Each has unique services or features; the prices vary somewhat, but not a lot.

With (6) and (7) in place you can send and receive emails and email attachments by satellite phone.  All of these offer the option in setup to switch between communicating by sat phone or communicating by Internet.  Underway you will use the sat phone, but once in a harbor with Internet, you switch to that.

On the other hand, for getting set up and tested before departure you would also primarily use the Internet connection, but it is crucial that you just spend the air time and do several transfers by sat phone. You want to be 100% certain that this system is working, i.e. send and receive emails with attachments. Turn it on and off, computer on and off, reboot, close all, start again, switch back and forth between sending by sat phone and by Internet, try multiple types of files (discussed below), etc. The main point is to be sure this is all working well before leaving and that you have used it for a couple of weeks at least getting your weather information, as outlined below.

(8) Software for GRIB viewer. Many, if not most, echart programs include the option to overlay GRIB formatted weather data right on the echart you are navigating with.

I assume you will have some echart program in a laptop for navigation that is tracking your GPS position. ( If this has not been planed, i.e. you have a console unit from your navigation electronics doing this tracking, then you may want to consider adding the laptop navigation option. You can do very much more with your own echart program in a laptop than possible with any of the packaged units which are then restricted to just the chart formats they provide as well as limited navigation tools.)

In any event, it pays to have a stand alone grib viewer in your laptop. There are numerous options. For the PC there is Viewfax from Saildocs, Ocens Grib Viewer, Ugrib, and many others. For the Mac there are fewer choices, but both very good. LuckGrib is one and Ocens GribViewer for Mac is another.

(9) Source for grib data. Establish and practice with some source of grib weather files. Saildocs is an excellent source. Many of their download options are included in a convenient Viewfax utility called “Get Data”, but they have much other data that are not listed there that must be obtained by email request to saildocs.  Ocens offers convenient download options from within their pay for use WeatherNet service.  The unique and very valuable option WeatherNet offers is the ability to download the ASCAT scatterometer wind data.  Ocens WeatherNet also now includes NDFD grib data as well. NDFD are also available from Saildocs.

And again, it is crucial to download some of the files and learn to use these forecasts before departure.

(10) Text forecasts. Don’t forget the text forecasts. Practice receiving by email the crucial text forecasts, as outlined in another note in the references below.

(11) Seawater temperature. When sailing in the Gulf Stream or any other strong current system, or when sailing in the tropics in general, it is extremely valuable if not crucial to have a through-hull seawater temperature measurement, or some other permanent way to continuously read and display seawater temperature.  For ocean currents the water temperature marks the boundaries of various features, and for the tropics the water temperature is a measure of TS development, i.e. hurricane formation usually require seawater > 80º F.


High Seas Forecasts and Tropical Cyclone Alerts by Email Request

Weather by Satellite Phone

Modern Marine Weather

Barometer Handbook

Mariner's Pressure Atlas

Sailor's Logbook



Thursday, August 13, 2015

High Seas Forecasts and Tropical Cyclone Alerts by Email Request

In several articles we point out that standard analysis and forecast maps of tropical systems do not have nearly enough information on them to make weather routing decisions.  We need more information. One approach is to download a GRIB formatted numerical prediction. The GFS is the most popular; the NDFD would be superior if that were available to your waters.

With or without these GRIB files, it remains valuable to also obtain the text forecasts from the NWS, in part because the GFS—or any one single model—can sometimes be quite a bit off on the tropical system intensity.

We have three basic text reports that might be of interest: high seas forecasts, high seas discussions, and tropical cyclone alerts. The basic ones we need to supplement the maps are the forecasts, and these are easy to get.

In principle we can get all of the text reports we need from the NWS FTPmail service, but this requires a very specific format and list of file names. An alternative is the excellent services of Saildocs, which have proven very dependable and versatile; it is relied upon by thousands of mariners worldwide, so we will limit the source to this one.

The eastern half of the Pacific Ocean is in the IMO GMDSS forecast zone called Metarea XII; the western half of the Atlantic Ocean is in Metarea IV. 

To get text report and forecast for Metarea XII (North Pacific)

send an email to with the body of the text blank except for 
send met.12  You can put anything in the subject line, which can serve for finding the mail for a repeated request. Be sure there is no signature or other text or graphics in the body.

This is all there is to it.  Try it here.

You can also subscribe to this request to have it sent to you automatically every day.  Send a blank email to for details.

For other metareas, use these Saildocs abbreviations

Code Metarea description
Met.1a      North Part of North Atlantic Ocean (High Seas)
Met.1b      Northeast Part of North Atlantic Ocean (Offshore)
Met.2       East Part of North Atlantic Ocean
Met.3e      East Mediterranean Sea
Met.3w      West Mediterranean Sea
Met.4       West Part of North Atlantic Ocean
Met.5       North Part of South Atlantic Ocean
Met.6n      South Atlantic Ocean North of 60S
Met.6s      South Atlantic Ocean South of 60S
Met.7       Southeast Atlantic Ocean + extreme SW Indian Ocean
Met.8n      North Indian Ocean
Met.8s      Southwest of Indian ocean
Met.9       Red Sea, Gulf of Aden, Arabian Sea, Persian Gulf
Met.10ne    Northeast of Australia (Pacific ocean)
Met.10n     North of Australia
Met.10w     Southeast of Indian ocean
Met.10se    Southeast of Australia (Pacific ocean)
Met.11ior   West part of the North Pacific Ocean (from China)
Met.11por   West part of the North Pacific Ocean (from Japan)
Met.11sW   West part of Pacific Ocean, 0-12S 90E-142E approx
Met.12      East part of the North Pacific Ocean
Met.13      Northwest of Pacific Nord and part of Arctic waters
Met.arctic  From SW corner 67N, 44E to NE corner 80N, 165W
Met.14s     South Pacific south of 25S
Met 14trop  South Pacific north of 25S
Met.15      Southeast Pacific
Met.16      Southeast Pacific between 18S and 3S)

We can also see these reports online two ways. Directly from the IMO see

and we have similar reports from the NWS from this folder:

The files matching metarea XII and IV are  north_pacific.txt  and  north_atlantic.txt

If you are sailing across one of these oceans, you would want to set up a schedule with Saildocs to send you the text report at least once a day, automatically.  This will supplement your maps and GRIB file weather analysis.

You can of course get this information from the HF radio. The USCG broadcasts voice versions of the above forecasts 4 times a day. See

* * *
Any of these services that might potentially be used underway 
should  be thoroughly tested before departure. 

* * *

Tropical Storm Side Determination

In principle if we have all of our weather resources working properly, then we can see where we are relative to an approaching tropical storm or hurricane, and from its forecasted motion we could determine which way to best proceed.

Unfortunately, life is never quite so simple in the real world. First, what are these “resources” referred to here?  We have more details elsewhere, but they would include the weather maps (surface analysis and forecasts), and they would include the GRIB files of numerical weather predictions such as those from the GFS.

We shall see in the videos below, however, that the maps themselves are often not detailed enough if we happen to indeed be inline with, or close to, a TC. The GRIB model predictions on the other hand, give us very detailed forecasts for wind and pressure, but we must be aware that these unvetted model predictions could be wrong!  This is actually more likely with a TC than with more climatically normal forecasts, and indeed more likely to be wrong during the first day or so of formation.

Thus it turns out that perhaps the most crucial official weather information we have underway in a case like this are the email text or radio broadcasts that give the official NWS numerical information on the location, motion, and extent of the systems. These are available at least 4 times a day, and often hourly on the storm warning broadcasts segments of the WWV and WWVH HF radio.  We can pick these up with a standard, battery-operated SW receiver.  The voice broadcasts are usually a voice synthesized rendering of the text reports. Here is a note on how to get these text forecasts by email.

From these text reports we learn the Lat-Lon of the system at a specific time, and we learn the speed of the system in kts, and the true direction it is moving—this might be stated as “northeast or 050,” in which case the numbers are the more specific value; this phrase is not a range of directions or an uncertainty statement.

For the best work here we need an accurate barometer and calibrated wind instruments. Many places elsewhere we discuss barometers and how to check that they are reading properly. This is fundamental to ocean voyaging.

Wind instruments are frankly harder to calibrate properly, but the more accurate they are at this point, the better off we are. The crucial point here is we need to know true wind speed (TWS) and true wind direction (TWD). We get this from the apparent wind speed (AWS) and apparent wind angle (AWA) along with our COG and SOG from the GPS. Some instruments will make this conversion for us, which is a great convenience once it is tested to be working properly. Otherwise, we can use a calculator to convert the AWS and AWA to TWS and TWA, then we can combine the TWA with COG to get the TWD.

Without any wind instruments, you can look into the waves to estimate the true wind direction. The TWA will always be aft of the AWA. That is if you have instruments telling your the AWA is near the beam; the TWA will be more on the quarter, etc. Then add this to the true heading of your vessel at the time, being the compass heading corrected for variation.

The barometer is dropping and the wind is building. Which side of the storm am I on?

Simple enough question, but it needs all kinds of clarification. First of all a reminder that we are talking primarily about “tropical” systems, either within the tropics, or those that have moved out of the tropics…. and we are discussing another rarer type of system which is a “tropical storm” that formed well outside the tropics.  An example of the latter is TS Claudette from July 2015 that formed off the coast of NC and moved NE.

The key characteristic we are calling “tropical” means a small and well defined system with closed isobars. A typical extra-tropical storm in the North Atlantic or North Pacific does not qualify.  Which side of these is rarely a question we face—the other side could be more than a thousand miles away. A tropical storm is typically a few hundred miles across, or less. These are the ones whose sides we care about in this discussion. We can also have the barometer dropping and wind building with an approaching frontal system, but this discussion would not apply to that situation at all.

When sailing in the tropics, however, we are not often confused by what is going on. We are used to glorious trade wind sailing, with the wind and barometer pretty stable. In fact, the barometer is actually more stable than the winds. We can see the trades vary from 10 to 20 kts with not much change in the barometer.  Throughout the tropics summertime pressures will be 1011 to 1014, depending on month and location, and more to the point, the standard deviation (SD)  in this pressure will be only 2 or 3 mb. In the absence of a tropical system, a pressure drop of 2 SD occurs only 2 or 3% of the time. Thus if you see the mean pressure drop 4 or 5 mb then you have a very good warning that a system could be approaching, even if the wind has not started to build. Note we have to say mean or average pressure because within the tropics there is a semi-diurnal variation of the pressure with an average amplitude of 1.7 mb.  Worldwide pressure statistics and how to interpret them is covered in the Mariner's Pressure Atlas.

Now a video review of the key issues of wind shifts as a closed system of isobars approaches and passes.

Storm Side Detection Part 1, Background and References (10 min)

Step by Step Procedures
(1) On a chart or universal plotting sheet, plot the latest known location of the storm.  Also then, look into your logbook to see where you were located at that time, and plot that position on the chart as well.

(2) Using the forecasted course and speed of the storm and your known position now, plot your present position as well as the DR position of the storm at the present time.

(3) Now plot the forecasted track of the storm and its forecasted locations.  We are getting this information from the text reports if we have them (see below).

(4) If we have this data, then we have a picture of what is taking place and where we are relative to the storm. More specifically, you see how far away the storm is and how close it will pass if you do not move.

(5) Now the crucial check to see if the forecasts are correct by recording our barometer and wind speed and direction at least every hour.  With a good barometer, you might learn something on an hourly basis. (Examples in the videos below.)

(6) You might also at this point do a reality check with the Buys Ballot law (refer back to Video 1). Namely, with your back to the true wind direction, i.e. true wind blowing against your back, put left arm out and rotate it forward by 25º. Now check the compass to see which way you are pointing and record that, then convert that compass direction to a true direction.  Then you can go back to your plot and draw a line from the boat to the storm and see if that direction is the same as you just measured with Buys-Ballot.  If yes then you can try this again once the wind direction changes enough to detect it.  If still pointing to the storm, then you are indeed inside the closed isobars of the storm.

If the Buys Ballot direction does not match your plot, and you checked both, then chances are you are not quite into the closed isobars.  This could also be indicated by a pressure that is not dropping or dropping very slowly.

Now we have a couple specific examples.

Storm Side Detection Part 2, Hurricane Guillermo 2015 (17 min)

Storm Side Detection Part 3, TS Claudette 2015, Value of a Good Barometer (15 min)

Now that you see the pressure dropping and wind maybe building, and you have from Buys Ballot a good check on the direction to the storm, you are prepared to consider a course of action.  They key issue is almost always getting as much separation from the storm center as possible at the time it passes you… or with confident knowledge, the possible choice of getting across the front of it before it gets to you. This is especially true with these TC that have strong winds. These strong winds are usually located fairly close around the system… that is, we see that 80 or more nmi gain is usually a huge safety factor.

The video examples we have show this, but we can use the data from a typical TC to see this for other cases.  You can also go to  and choose any of hundreds of storms to check this.

Here are a couple examples.  For tropical storms or for TS stage of a hurricane, the safety radii are smaller, so it is easier to get out of the way of them.


0900 UTC MON AUG 03 2015



64 KT....... 30NE  25SE  20SW  20NW.
50 KT....... 70NE  40SE  25SW  35NW.
34 KT.......115NE 100SE  50SW  70NW.
12 FT SEAS..300NE 200SE 270SW 310NW.

(skipping a few reports)

FORECAST VALID 04/0600Z 17.5N 148.1W
50 KT... 50NE  30SE  15SW  30NW.
34 KT... 90NE  80SE  40SW  60NW.

2100 UTC TUE JUL 14 2015



34 KT....... 60NE  70SE  60SW   0NW.
12 FT SEAS.. 60NE 120SE  60SW   0NW.


Once you see where you are and how the storm is moving, you can then figure the best plan of action.  Here is a video showing a couple choices. It uses the program Expedition, which does automatic routing, but that powerful feature is not required for a good judgement. Also since the sea state will likely be much larger than usual, chances are any polar diagram you have might be an overestimate of performance.

Storm Side Detection, Part 4 Maneuvering from a Known Side (15 min)

Before making a major decision such as trying to cross in front of a system you are nearly in line with, it is best to run for a while in the chosen direction to know true SMG and CMG possible. In some cases, like the example shown, it will be pretty clear if you can or cannot make it across to the navigable side.

That choice, however, is crucially tied to the proper identification of where you are relative to the storm path… and projected storm path.  Recall if you do not have any data at all from the NWS, we must rely to some extent on the known average trend of a storm turning poleward at some point.  The statistics of storm speeds and directions as a function of age is discussed in the text book, and hundreds of actual examples are online at the NHC to test these averages.

Please post your questions or past related experiences as  comments and we can follow up here with that discussion.

Sunday, August 2, 2015

New GRIB Viewer for Macs

For years we lamented to our Mac friends and students that there were so few good marine applications for the Macintosh that they were better off biting the bullet and buying a PC for their marine navigation and weather applications. This litany followed us right into these modern times when we ourselves started running our own PC programs on virtual machines in a Mac.

But sure enough, when the demand grew high enough, these apps have started to appear. In the weather department, our neighbors at Ocens were the first to bring high-quality weather products to the Mac with a series of valuable applications.

And the bar keeps rising, in this case with a new GRIB weather data viewer for the Mac called LuckGrib. As it turns out, this is also a product of the Pacific Northwest, though its developer Craig McPheeters happens to be underway sailing in the South Pacific at the time his product will first appear in the App Store sometime in August.

In keeping with Mac standards, LuckGrib has an elegant design and convenient interface, with several unique features added to the standard functionality we expect from a high-quality weather tool. Besides its versatile display options, it also offers a way to download the data from an Internet connection. Built in data sources include: GFS, WW3, and CMC Ensemble.

Though we tend to reply mostly on wind and pressure for routing, and the 500-mb data for evaluation, and sometimes the precipitation, this new LuckGrib app offers very much more. In a deep Advanced Option you can request any of the 320 weather parameters predicted by the GFS system, including temperatures, humidity, cloud cover, convective properties, and many more at essentially any altitude level in the atmosphere. You would have to be a professional forecaster to make use of much of this info, but it can be found here if desired.

Other outstanding features include:

• Fast operation and smooth performance on all options using even relatively large files. 

• Convenient data selection and request interface. It is always, however,  the navigator’s job to select only the data needed, only over the region needed.

• Versatile track pad pinch zoom and pan. A slider bar can also adjust the zoom level, and standard mouse click and drag also works to crisply adjust the map view.

• User added identification notes to stored data files, along with clear organization of downloaded files to access for updates.

• Many options for personal preferences in color schemes and actual data display configurations available for each of the data types.

• A dynamic feedback option that shows the values at the cursor not just of wind arrows, but also wind speed contours, pressure, or any parameter or combination of parameters. A very informative way to look at the distribution of data.

• A min and max point display within the wind contour option offers nice way to see strongest parts of a storm.

• Option for selecting GFS resolution of 1º, 0.5º, or 0.25º.

• Time to next update for each model. A very nice feature that prevents users from extraneous data downloads underway.

• A draw route leg option, with ticks at specified user-selected intervals. This can be used as an electronic range and bearing line to read the maps, or as a route leg to check wind along a path at a fixed time—or it has the very useful application of setting a day or two route leg, and then sliding the time marker to continuously watch the winds all along that path.

• Time control allows for interpolation of wind fields at times intermediate to the forecast times, which is valuable for comparing with satellite wind data, or making wind predictions at specific times.

• Low price—which has to be considered a practical outstanding feature. It is predicted to be $20, which would make this a must have app for all sailors using Macs for weather work. There is one other high-quality GRIB viewer for the Mac called Grib Explorer for Mac from Ocens, which has many outstanding features of its own. In the ideal world we would have both of these fine tools, but at $199 for the Grib Explorer it requires a larger budget plus an external source for downloading the data.

We have used the beta version of LuckGrib extensively for several months. The LuckGrib viewer is elegant, fast, and functional. Regardless of other weather tools you might have, this tool will add new functionality that will improve your weather work.  If you have not used GRIB forecasts at all, LuckGrib is an excellent way to get started—but we must remind all GRIB users that unvetted numerical weather prediction data (such as GFS GRIB files) should always be compared with the actual professional forecasts prepared by the NWS and their counterparts around the world.

Tech support and related information can be found at

We will add illustrations later and a video on standard and special operations.

Tuesday, July 28, 2015

The New Inland Navigation Rules

Last year about this time, the USCG changed the US Inland Navigation Rules and their Annexes. These are the rules that apply inside most point-to-point lines across coastal bays or inlets leading to the ocean—notable exceptions are all of Puget Sound, AK, and HI waters. There has been little notice of these changes despite a couple details of interest to sailors. Knowing these details could save you up to one hundred dollars; not knowing them could cost you a couple hundred dollars. The official discussion of the changes is an interesting insight into the rules themselves.

The goal of the changes was to bring the wording and format more in line with that of the International Maritime Organization (IMO) Convention on the International Regulations for Preventing Collisions at Sea, known as the COLREGS, or in many places simply the International Rules. Another goal was to address recommendations made by the Navigation Safety Advisory Council (NAVSAC), which we are proud to say one of our past Starpath instructors, Pamela Hom, was a member of for several years.

The changes were intended to “harmonize domestic and international law by reducing and alleviating equipment requirements on vessels, addressing technological advancements, such as wing-in-ground craft (WIG), and increasing public awareness of the US Inland Navigation Rules.”

They also changed the title of the USCG publication previously called NAVIGATION RULES, INTERNATIONAL — INLAND to now read, with a new subtitle as shown below:

The new subtitle reflects the handbook nature of the new publication in that several documents often needed by professional mariners are now in this one book. The book’s cover design is unchanged but for the new title. The inclusion of the edition date in the title might portend an easier way to tell when new editions become available, but that is just a guess. Previously, it was difficult to tell if you had the latest edition.

Following the recommendations of NAVSAC and other professionals, the proposed changes where published, and the public invited to make comments.  Apparently only ten public responses were received, addressing 49 specific points. They were described as mostly favorable, with most addressing the key issue of nomenclature, ie use of “section” versus “part” versus “subpart” versus specific CFR (Code of Federal Regulations) article reference. This aspect has definitely been improved, but anyone looking at this book for the first time would have to wonder what it looked like before improvement.

In a sense with the new handbook content and ubiquitous reference to CFR article numbers, ie 33 CFR §83.25(d)(i), the book itself looks more complex now than it was. But not to worry about that; there are only 18 half pages of content we need to know to be safe in most encounters, namely the Steering and Sailing Rules—though needless to say, proper identification of lights and sounds could also be crucial at other times. Flash cards or special pubs can help with that.

WIG Craft

First the easy part, which does not affect sailors unless they are testing for a USCG license. WIG craft rules are now all the same for Inland and International waters. WIG rules used to apply only to International Waters, so this must be looking ahead to seeing more of these “vessels” here. These strange boats that can fly are more popular in Asia and Europe than in the US.

Rule 1(g) on Carrying a Copy of the Rules

Next there was a proposed change that they did not make, but the official discussion of the decision brings up several interesting points. The USCG considered, and then rejected, a NAVSAC proposal to require vessels down to 16 ft overall length to carry a copy of the Rules on board. At present only (self-propelled) vessels of 39 ft or longer are required to carry “a copy of the Rules for ready reference” on Inland waters. The argument for smaller motorboats and sailboats to carry the rules was the obvious one that they also must obey the Rules; the argument against it that prevailed was “lack of quantifiable benefits to justify a high regulatory burden on recreational vessels.” They stated that 6.5 million vessels were within this (39 ft to 16 ft) category and they listed the GPO Nav Rules book price at $23, and then they multiplied the two to get what was described as “unnecessarily costly” boater expense of 150 million dollars. (With the actual street price of $10, the actual cost to boaters would be more like one six-pack of beer in the lifetime of each of these boats.)

They went on to point out that the skippers of only 14% of boating accidents (in this vessel category) that led to a death had ever taken any navigation training, and only 9% of those were sanctioned courses, and noted that fatal accident statistics have gone down as sanctioned training has gone up over the years. Thus they argued that more navigation training is more important than requiring Rules carriage.

This is obviously true, but I must add that the Nav Rules were not designed to prevent fatal accidents, but to prevent collisions, and I state again our premise that the Navigation Rules is the most important book in navigation.  They also failed to point out that every collision (between vessels of any size) involves the violation of at least one of the Rules by both vessels involved­—in other words, it is statistically impossible to have a collision if you obey all the Rules, regardless of what the other guy does.  But you have to know the rules on what to do when he does not obey the Rules.

I agree that it does not make practical sense to extend the carriage Rule to smaller vessels. Furthermore, it does not matter; the Rules must be obeyed by all vessels, regardless of having a copy on board, and in fact regardless of even having read them. I would hope that somewhere in the state boat registration process there is at least a statement saying you must know and obey the Rules. This would be interesting to track down.

The requirement for larger vessels (>39ft) on Inland Waters to carry the Rules at least gives mariners a chance to look up relevant rules after the fact as a means of ongoing education. There will always be encounters, lights, or sounds that we wish we knew better at the time. Why there is no similar rule in the COLREGS is beyond US law or USCG control.  The international consensus must be that International Rule 1(a) that you must obey the Rules is enough to go with. There are IMO regulations that all commercial vessels must carry a copy of the Rules, but that is independent of the Rules themselves.

With all that said, if you did not know about this Rule, do not feel too bad. It was always the law, but it was buried in Annex V, §88.05. Now with the new changes it is up front in Inland Rule 1(g).
To me the most interesting part of the discussion of the carriage requirement included the phrase “electronic copy,” but only as an afterthought, in an awkward sentence structure, without any implied price to that option, which would have befuddled their cost analysis. In short, a “copy of the rules for ready reference” could indeed be an electronic copy, and the USCG offers a free ebook copy online (pdf) that would run quite well in smart phones or tablets. Furthermore, in another part of the USCG website we find this direct statement:

“Electronic copies of the Navigation Rules are acceptable, however, only if they are currently corrected to the latest Notice to Mariners and can be made available for ready reference.  The unwritten rule of thumb: ‘readily’ means that you are able to avail yourself of a Rule(s) within 2 minutes of the need to do so.”

[Note that the "unwritten rule" seems now to be quite well written.]

"Two minutes" is a cake walk with a well designed ebook version (ie Starpath Pocket NavRules) and even trivial with the USCG pdf version that you can mail to yourself and open in your phone. Click the attachment and choose open in iBooks (a free app from Apple), which will save it there.  Practice once or twice looking up a rule, and you are always legal.

Rule 8(a) — Action to Avoid Collision

This rule is the fundamental description of how to maneuver to avoid a collision. It was always intended to be the same in both Inland and International rules, and it remains so, but there was uncertainty in the wording of the Inland Rules. Now the wording is the same in both International and Inland rules to read (italics and underline added):

Rule 8(a). Any action taken to avoid collision shall be taken in accordance with Rules 4-19 and shall if the circumstances of the case admit, be positive, made in ample time and with due regard to the observance of good seamanship.

They added the precedence statement (shown here underlined) to the inland rule.  Also interesting, though not part of the rule itself, this USCG document also effectively defined “in accordance with” by adding this sentence to their description of the rule change: “It is our intent that Rule 8(a) should be taken with full knowledge and compliance with Rules 4–19” (italics added).

Rule 33 — Equipment for Sound Signals

In the previous version for Inland Rule 33(a), it was required to carry a whistle and a bell for vessels ≥ 12m (39 ft), but the International Rules required the bell only for vessels ≥ 20m (65 ft). The new inland rule now matches the International rule, so you do not need a bell on board till 65 ft or longer—so off to the swap meets with these, or make dinner bells at home. A brass bell that meets USCG requirements can easily cost $100.

This rule change affects several of the USCG’s trickier license exam questions, so keep in mind an older data set of these questions may now have the wrong answer.  The new dataset of Rules questions online have these answers changed.
Rule 35 - Sound Signals in Restricted Visibility

Rule 35(g) and 35(h) in both International and Inland rules calls for ringing a bell when anchored or aground in restricted visibility. The rules do not specify vessel lengths. In the old inland rule you had to ring a bell for 12 to 20m, now you don’t, because you don’t have to have a bell at all. Now you are forgiven for all the times you were anchored in the fog and did not ring your bell!

Rule 25 - (Lights on) Sailing Vessels Underway and Vessels Under Oars

Here we have another seemingly very simple Inland Rule change that has notable implications to sailors of smaller boats, or dinghy rowers running back and forth to larger boats. It is also notable in that it goes contrary to the harmonizing goal of the changes in that it willfully makes a US Inland Rule different from the corresponding International Rule that was previously the same.

The new US Inland Rule reads (with underline and italics added):

Rule 25 (d) (i).  A sailing vessel of less than 7 meter in length shall, if practicable, exhibit the lights prescribed in Rule 25(a) or (b), but if she does not, she shall exhibit an all around white light or have ready at hand an electric torch or lighted lantern showing a white light which shall be exhibited in sufficient time to prevent collision.

The lights of 25(a) referred to are sidelights and a sternlight; the lights of 25(b) are a tricolor on the masthead.  They have added the all-round white light option to the Inland Rule, otherwise the two rules would be identical.

Now inland-waters boats, under sail or being rowed, that are less than 23 ft (7 m) long can run with a steady, all-round white light. If you choose that option, it definitely means only the all-round white light; no other lights allowed. It also of course means no engine or outboard running, just actual sailing or rowing.

So what are the implications of this change? There are certainly a large number of boats in this category that could end up sailing or rowing (ie a dinghy) at night. We still have the previous options of full sidelights and a sternlight, or no lights at all with just a bright flashlight at hand to warn approaching vessels. But now we have a third (Inland) option of a fixed, all-round white light.

For many sailboats (again <23ft) you can now sail at night running your anchor light from the masthead—but turn off your sidelights off if you have them. The anchor light was used by some sailboats in the past, but it was not legal then (sidelights and a masthead light looks like a power-driven vessel). Those who knew that may have thought it was better seen than legal, but just like the other common sailboat lighting error of running deck lights and tri-color (also illegal) these lights can get fined.

I know of a case from many years ago where a fine for running both tricolor and deck lights was $50. Modern exposure is higher: “A civil penalty of up to $500 may be imposed by the Coast Guard for failure to comply with equipment requirements...” from a USCG Special Notices to Mariners.
Frankly, it is unlikely to be fined for just wrong lights, but in the event of a collision at night, illegal lights guarantees you a large part of the liability, regardless of who was at fault.

Tech developments that may have impacted this decision are bright LED lights, which even the smallest boat can run for long periods on a small battery.  This new rule also now makes the kayaker with a white light on his hat legal as well, which in the past it was not. Granted, you will look like a sternlight, but in principle that would not be a problem. Anyone approaching a sternlight should go around it.

At this point we can add that one of the USCG’s stated motivations of the changes (“more public awareness”) has just been achieved—you have read an article on the US Inland Rules! To know where they apply, check 33 CFR Part 80, in the back of the new Navigation Rules Handbook.


See a preview of our Starpath Pocket Navigation Rules Handbook for a very convenient presentation of the new rules and all related documents.... or better still, open it in your phone and bookmark it.

Friday, July 3, 2015

Direction of the Winds Aloft from Mare’s Tails — Time to Practice What we Preach

Mackerel sky and mare’s tails make tall ships set low sails.  That is a well known evaluation of a rather extended cloud sequence. It does not apply to the sighting of either cloud type on its own—as we show here very clearly, though not the topic at hand.

In other words that is a very specialized seat of the pants forecast tool that must be interpreted carefully and only as part of an ongoing sequence of cloud changes.

On the other hand, mare’s tails or fall streaks (cirrus uncinus ) do often tell us something else that can indeed be more universally valuable. They can tell us the direction of the winds aloft, meaning up at some 18,000 ft. We care about this direction as these are the winds that pull the surface weather systems across the globe. Thus if the winds aloft are from the SW, then we expect storms to approach from the SW, and with that information and the Buys-Ballot law we can watch the surface winds shift and from these two pieces of information know where the storm will pass relative to us.

The main reference for looking into the details of that analysis (sometimes called the cross winds rule) please see Modern Marine Weather. The section is shipboard forecasting without radio contact or any other form of official forecasts… all on your own.

This wind direction can also be used as a rough steering guide without a compass available or other natural aids, many of which are actually better than this one.  For that approach see Emergency Navigation. These winds are more valuable than surface winds as they might persist in direction for several days in a row, even while surface winds are going around the clock.

We have devoted much discussion of this over the years, but yesterday had a dynamic example hit suddenly with all info needed to evaluate it, the key one being an iPhone!  Namely on a walk through Discovery Park in Seattle, which from the high plain it is, offers a broad view of the sky, much as one might get offshore. Also with a shadow clearly in place we can compute the directions we need… not to mention we were on a path that could be identified on Google Earth, so we get as good as we might on land of the cloud directions. 

Needless to say, on a boat at sea you would do things relative to the steering compass. But the goal of this write up is to show how you might practice this on land when you get the chance.  The key point to keep in mind is these cloud patterns are transient.  The dramatic sky full of fall streaks we saw were pretty much all gone but an isolated patch in a matter of 20 minutes or so.

Going back to our first old saying, this would be “clear sky and mare’s tails, makes tall ships do whatever they were doing.”  No info here on changing weather… which in fact did not change at all.

And we see here the key issue of this evaluation of wind direction. We have to work with a linear perspective into some form of a vanishing point on the horizon which we assume is the wind direction. Just looking at one patch of clouds in one direction might not do the job.

This is the idea, but we are looking at clouds (turn this picture over). What I really needed and did not think of it, was the panorama option of the iPhone camera. That would be the ideal way to do this. We have here just a few snap shots and a movie that helps some.

So here are some pictures followed by the what was learned… and ways to improve it…. and i have to admit at this point that i hope it works out to be right!  I do not know the answer at this stage.  In any event it will be a good exercise, as it will show limits on the methods.  The principle is a sound one, so any big discrepancy will have to be sorted out.

We start with a few pictures of the sky.

Single picture taken, and shadow noted as a recorded reference.
The view above is misleading as it looks like the clouds are crossing the shadow line and trail at a steeper angle than one gets from seeing the full perspective. The shadow line is about 10º to the left (north) of the path line down by the tree on the right. The best bet at this point would be put a vertical stick in the ground and get its shadow.  Compiled here is just what was thought of at the moment. A planned measurement would be better—but we have to essentially plan for the unexpected as these are transient views. The panorama pic is the best thing to remember when a case presents itself.  Below we see shadow was 107 T and path near tree 114 T. So we see here the tricky part in interpreting a single view photo.

Below is a movie that helps some, but what we really need here is the panorama option that almost all phones have now.

It is frankly difficult to tell from what we have here, but my noted conclusion was the winds aloft direction was about 30º to the left of the path, the direction of which we can determine several ways.

Now we need two things, the direction of the shadow or path, and what was indeed the true direction of the winds aloft at this moment. We can get the direction of the sun from a neat free app called Astrolabe. I think there is an Andorid version as well.  There are many options for this.

Not sure this was the precise time of the sighting or the precise location, as we were moving around a bit, but not at all off enough to make any difference to any numbers. Az is what is normally called Zn, the azimuth of the sun. Alt is normally called Hc, the calculated (angular) height of the sun above the horizon.
Here we see the bearing to the sun was 287, so the shadow pointed toward 107 T.  We can also get this information from using time and location.

So from my records of the vanishing point being some 20º left of the shadow, we get winds aloft flowing toward 087, but this has to be clearly some ± 20º at least.  (My actual records had 30 left of the path, and later adjusted it to left of the shadow, thinking that would be a more accurate reference, but it turned out we could find the path direction easily. See later satellite image of the park.)

Now for wind direction we need to look at the 500 mb maps. This observation was at 1900 PDT = +7= 26z = 02z July 3.  Below are the 00z and 12z winds.

We do not have a clean example here, as the main flow was north of us. But we do see that the trend from 00z to 12z was toward more westerly, that is toward 090, and there is a trend to strengthen, that is, the contour lines (these are isohypses not isobars on this type of map—that is, contours that mark the same altitude of the 500 mb surface) are more parallel and closer over the Puget Sound. As a rule we do need pretty firm winds aloft to see these nice fall streaks.

So that is the exercise. In this case we are within 30º accuracy for sure (numerically closer, but that must be large part luck),  which is more than needed to make the weather forecasting judgements of which way the storms are approaching.  More generally we are just trying to distinguish between: from the SW, from the W, or from the NW, and this exercise told us from the West.  In rare cases, the winds aloft can distort enough to be more northerly or southerly, but not often. For example, had we done this at about 150W, we would have seen very different results, and learned that we had an unusual case of the storms coming up from the S-SW... that would be what is called around here the Pineapple Express, which has brought the very worst storms to the NW over the years.

Below is the Google Earth image of the location to show how you can use that as well to check for reference directions without relying on celestial navigation.

The pictures were taken just beyond the left end of the red line. The circle marks the tree referred to.

Add questions or comments below if you like to follow up on this example or your own.

Just thought of something else—illustrating the live nature of this write up.  There are phenomenal cloud images online.  Do a Google images search for cirrius uncinus for example and find tons... after a few seconds I found the following. There are certainly better examples, but we learn a lot from just these three.

These are not strictly the same clouds we were looking at above, or rather their relation to the winds aloft was slightly different, but the principle is the same, and we get the same info, namely direction of the winds aloft that are (most likely) marking the storm track.  Without a reference line or point, however, we do not know what it was. But this would be the kind of picture that would make the analysis more precise when practicing on land.

Notice that if you draw a line directly to the vanishing point, it will appear that the ones off to the side are crossing at a steep angle, so we do need to be careful about single picture or single view interpretation of the direction.

The main thing to learn from this is, I pointed the camera in the wrong direction. I was aligning with the reference line, which left the vanishing point off to the side. It seems it would be best to point the camera or center the panorama on the vanishing point, and let the reference line or point on the horizon fall where it may.  We would get better data.... or just turn on the compass app on the phone and hope it was right (not guaranteed.... need to check how to calibrate it, usually fast and easy, and hold it very level during the measurement.... maybe include a few landmarks as well to double check).

Again, this issue does not come up at sea, which is why this is a good trick to know. At sea you spot the vanishing point, take a known correct bearing to it relative to steering compass, and you are done.  In some cases, you might even be able to turn toward it and check the bearing uncertainly by swinging the heading around about the main direction.

And even more confidence is gained by doing it a bit later. The present pattern will likely go away and then maybe later come back.  Check it again and you learn more.

Friday, June 19, 2015

Atlantic and Pacific Weather Briefings

The Atlantic and Pacific weather briefing webpages are a unique service of the Ocean Prediction Center that are not as well known as they might be. These links are effectively the latest versions of all (or nearly all) of the maps available for each ocean displayed on a single webpage, in sequence.  See for example these links to the OPC versions:
Atlantic Briefing             Pacific Briefing.

The files they provide are gif format that show up in one long page of sequential pictures. Very nice for quick view on land but not useful underway in this presentation because of the large file sizes and low expensive bandwidth at sea.  Furthermore, we rarely need all of them at once

Needless to say, we can request any one of them individually by email request using the NWS's FTPmail or from Saildocs, but to do that, we need a custom request format and we need to know the actual file name for each one—there is also a unique file name for the latest version of each one, as opposed to, say, the file name for the map valid at 18z.

As it turns out, the NWS stores these maps in at least three different locations, and the file size for an identical map could be 300 kb one place, 85 kb another, and 27 kb in another. They also differ on the same site in size for a .gif file versus a .TIF file. As far as we can tell, the smallest ones are the .TIF files located at  so we have used these.  We can find theses files of <30 kb for all of them, which is convenient as some comm sources have limits of 30 kb attachments.

One price we pay for minimum file size is the use of the .TIF format, which requires a graphics viewer, but these days all computers should have this. Also for some reason, some are stored in the wrong orientation, so we have to rotate them ourselves. Not a big deal.  (Also just a side note, that the .TIF format is not inherently the smallest file size from a technical point of view; it just happens that of the files we have access to, the smallest are in this format. We will ask the NWS to see if they might consider a directory with reduced file sizes that will be better suited for direct transmission via HF radio or Sat phone.)

Thus we have made a shortcut index to the briefing products that gives the user the opportunity to ask for individual products bypassing the need to first open an online index.  Our custom index is in the from of a pdf with active links within it. Thus with this pdf in your computer, you can just open the pdf and then select the link you want.... or mail the pdf to yourself and open it in your tablet or smartphone to get the products in those devices.

Another change we had to make from the standard Briefing presentation is to break up the surface analysis into two parts. This is done to keep the file size down. And the important adjustment is to ask for these from Saildocs, which offers the wonderful service of taking these 30 kb files and reducing them by 50% so we can get them at about 15 kb each, sometimes 10 kb.

You have a choice to download the image directly when using an Internet link, or choose the other link for each one that prepares the proper request from Saildocs to be send by your email.

The Internet request is straightforward (these will come directly from the NWS server), but there are nuances to the email link. Namely, when the link generates the email for you, you must be sure that there is no signature or anything else in the email. It must be a blank email with just the send command in the body.  The subject line does not matter.

Also, for this to work seamlessly, you will need to have your sat phone or pactor modem connected to the computer as well as being sure that the default email program for your computer is the same as you are using for high seas communications—google something like "setting default email on win7" for instructions on that. Note that we include a link to the list of file names, so you can request any of them directly from Saildocs as shown in any of the live links.

A sample element of the Briefings pdf looks like this:

This one is actually 13kb. The pic is outdated.
Our Briefings pdfs are about 2 MB, but they only have to be downloaded once to your computer from a land line. They are large so you can zoom into the outdated thumbnails to see what each map actually looks like.

We also include the update times and approximate file size. The sample shown above, for example, exists for both 00z and 12z valid times, but you would not get the latest 00z map till about 05z or 06z. The latest 12z map would first be available at about 17z to 18z.  Update times vary within these limits, but you might be able to pin it down more precisely. The file sizes may vary a few kb as well.

You can get the pdfs here:

Please give it a try and let us know how this works. This is a new idea, so we will need some in practice feedback to learn how to improve it. Please post your comments or questions in the comments section below.

The above document was edited with new information at 2100, June 30, 2015. Since these documents are undergoing changes we keep this record here of the latest.

Pacific v3. 6/30/15. Incorporates the compressed files links from Saildocs on all but 500 mb 24h 500 mb. We will add these when they become available. For now these two are ~25 kb, when compressed they will be ~13 kb, as are the others.

Atlantic v3. 7/1/15. Updated to have downloads from Saildocs be ~13 kb each... all, except for 48h seas sate which is still twice that size.

Tuesday, May 26, 2015

The National Digital Forecast Database

The primary method of weather routing underway these days is based on vector wind forecasts in GRIB format displayed in echart navigation programs. Navigators who do not display the data directly in an echart program still use this data viewed in separate GRIB viewer software. One popular free program and data source is the WinFax program from Saildocs that includes both the GRIB viewer and a convenient link to download the data via email using HF radio or satellite phone when underway, or by Internet when available. Commercial GRIB viewers and data sources from Ocens, Expedition, and others offer expanded features and convenience.

The primary source of free data used in these programs is the numerical prediction forecast of the Global Forecast System (GFS) model from the National Weather Service (NWS). This is free, worldwide data. There are other (regional) model output options for selected regions that can sometimes improve on the GFS forecasts in some coastal waters, but once offshore we must rely on a global model.  There are global model data from other nations that are as good or nominally better than the GFS, but these are not free data. There is also free global data from the US Navy (NAVGEM, formerly NOGAPS), but this would rarely be an improvement over the GFS. The regional model from NWS is called NAM; the one from the Navy is called COAMPS.

The GFS model offers new wind forecasts every 6h, in 3-h steps, out to 10d, but none of the models are dependable on the level a sailing navigator might want beyond about 4 days (96h). The latest GFS model computes wind at grid points of 0.12º, but this high resolution is so far not typically available to sailors at sea. The most common sources of GFS data provides a wind vector every 0.5º, or about every 30 nmi, depending on Lat. The racing tactics program Expedition offers GFS data down to a 0.25º grid.

But here is the ongoing issue for those who want to do the best they can in wind forecasting. The GFS data are pure computer model output. The data you get this way has not been vetted in any way by human meteorologists once it left the computer. That is not to say it is wrong.  The computational science of the atmosphere is improving every year and is remarkably good artificial intelligence. In fact, the GFS winds will be very close more often than very wrong, but they can indeed be wrong, and can indeed be close, but not right, especially as we get beyond 48h.

So what do we do about this to optimize the forecasts? First we must remember that we do indeed have the actual forecasts from the human intelligence of professional meteorologists. We get these forecasts from the folks at the Ocean Prediction Center (OPC) and the National Hurricane Center (NHC). We also have regional forecast offices in Honolulu (HFO) and three in Alaska (ARH) that contribute to Pacific marine forecasts.

They provide (via Internet links or HF retransmission via USCG) surface analysis maps every 6h, as well as a 24h forecast and 48h forecast every 12h, and a 96h forecast every 24h. Thus we can compare their forecasts with what we see in the GFS forecasts as a way to judge if the GFS has withstood the scrutiny of the professionals. These meteorologists have, in preparing their own forecasts, studied all of the global models available, several of which are ranked higher than the GFS in overall performance skill. To make their judgments they include various ensemble studies of the models, which include looking at the output at a specific time based on different initialization times, as well as different input data. They can also vary the physics parameters of the solutions. The extent to which the results are independent of these variations in the calculations directs them to the best model to use for the situation at hand.

They also fold into this their experience with the climatic behavior in various regions as well as past performance of the models in particular circumstances. They can also use their experience to evaluate the ship and station observations that have seeded the models for the latest run, and they may have access to satellite wind and cloud image data that did not happen to show up in time to be assimilated into the last computation. In short, the background that goes into the forecasts they make is far more than just the output of one particular computer model, so we have every reason to believe that they could provide a better forecast.

With that said, we must add that the difference between what the meteorologists provide and what the GFS model alone provides depends on what part of the world you are in and when. Our recent study of this, for example, for the Transpac route, LA to Honolulu, during the summer, with a nicely formed and in place Pacific High, showed very little difference between human forecasts and pure GFS model predictions, and in such cases the NWS will indeed simply use the pure GFS themselves to create the isobars and subsequent wind fields.

But when things are not so benignly climatic, or at higher latitudes much of the time, the differences can be significant and it is our job as prudent navigators to make the comparison before relying on the GFS alone. Also we know the global models are not dependable near the coast, whereas it would seem the NDFD would be more reliable there as these are the same folks making the coastal forecasts.

Up until late last year the process of comparing the two forecasts when underway was rather tedious, because the NWS forecasts were only available as graphic images of the weather maps. Thus we had to plot various positions on these and then extract the wind and pressure data using special tables. These data could then be compared to the GFS values for the same forecast times. The process and required tables are in the textbook Modern Marine Weather. But now we have a new digital solution that for many parts of the world not only offers an easy solution but in effect diminishes the requirement for the comparison in some cases.

The answer is called NDFD, the National Digital Forecast Database, which is the vectorized versions of the NWS forecasts, which are now in a GRIB format that all mariners can download and run in their standard GRIB viewing software—the system is actually some years old; it is just our access underway that is new.  The raw data are readily available online, but in a complex format that requires special programs to interpret, and then it must be converted to the .grb format that mariners are used to. There may be other sources, but the only one I know of is an email request to Saildocs, and we are very grateful to them for providing this service. This option is not included within the WinFax Get Data option,  but it can be obtained by email request to with this in the body of the message: 

SEND NDFD:50N,45N,130W,120W|0.12,0.12|0,6..120

or vary this with the standard SailDocs conventions for region, and forecast span. The forecasts are available every 3h out to 72h, and then every 6h out to 168h (7d); however accuracy beyond 96h cannot be counted on regardless of who made it or how.  The longer run forecasts are still useful on some level for weather routing programs that must look ahead at something to make proposals for earlier times.

The resolution of the data is very high at 0.12º, which corresponds to roughly one wind arrow every 7 nmi, which is about four times finer grid than typical GFS data. A drawback for barometer oriented navigators like myself, however, is the absence of isobars. For now from the NDFD we just get wind speed and direction and significant wave height (SWH) for use at sea. The database itself includes air temp, humidity, and various other land oriented data.

What we do not have yet and very  much would like to see is that part of the NDFD that covers tropical cyclone surface wind forecasts of winds >34 kt, >50 kt, and >64 kt.  Unlike sea level pressure, these data are all in the NDFD already, but not yet available to mariners in conventional GRIB format.

The other limiting factor to use of NDFD is the data are not available worldwide.  Figure 1 shows the regions now covered. The borders do not have to be spelled out in the Saildocs request. That service will simply provide what is there within whatever you ask for.  I have been told by the NWS that it is in the planned expansion to extend the coverage from 140W on over to HI, but with such tight budgets these days we don’t know when that will happen. The first step is we need more vessels using it to learn its value.

Figure 1. Coverage of the NDFD wind and SWH data.  Wind arrows in this global view do not reflect what we obtain by download, which is one arrow every 0.12º.

This data set is a major breakthrough for sailors. It now covers a large extent of our sailing waters, and if the planned expansions go through it could be the main source of data for cruising and racing sailors in trans Pacific and trans Atlantic sailing. The key point here is that in principle the NDFD forecasts includes the skill of the ECMWF and UK Met models, which are ranked the top two numerical prediction programs. As we read in the official Forecast Discussions that accompany every forecast made by the NWS, oftentimes the NWS defers to one of these models for their forecasts. Whenever this happens we get that benefit in the NDFD. Normally only the boats with expensive contracts with private agencies have access to that data. (For a quick coarse comparison of the models see from weatheronline in the UK.)

A rough comparison of the NDFD and GFS is shown in Figure 2; but this underestimates the actual differences in cases where GFS was not the basis of the NWS forecast.

Figure 2.  Mean Average Error (MAE) for winds above 8 kts from land stations.  The GMOS values reflect the GFS data, but these Model Output Statistics (MOS) have been calibrated to climatic averages. The actual difference between raw GFS wind predictions and the corresponding NDFD predictions are notably higher.  This shows that even forcing a normalization to the GFS model output the NDFD still out performs the GFS above 72h. The plots give insight into the accuracy of the forecasts however they are made, which is remarkably good considering that the wind measurements themselves must be some ± (2 kts, 5º) at least.  It is not clear how ocean data would compare to these land data.

Also on the horizon is a new program called National Blend of Global Models (NBM). This will be an improved forecast system using both NWS and non-NWS models, along with state of the art ways to evaluate input observations plus enhanced ongoing verification to produce a top of the line forecast product. This work is for now focused on land based forecasts, but it will be extended to the ocean as well, and we will then have digital access to through the NDFD. The US did after all invent the concept of numerical weather prediction, so it looks like we may be working toward
regaining that leading role.

Looking at this one and the Tehujuanepec one at the end show that the real challenge here is having the model forecasts in hand when we get a good scatterometer wind field to test


Below are a few comparisons of the model predictions for several regions.  I will come back and discuss these shortly.

The above is GFS (purple) compared to NDFD (black) for a TC centered at abut 20N, 125W. The forecasts were made 18h earlier and the agreement of both of them with the observed satellite winds is very good, though both underestimate the actual wind speeds near the storm—the red feathers are observed winds of 30 kts or more. The contour lines mark the wind speed boundaries, shown from Expedition, which allows the forecasts to be scaled to the precise time of the satellite pass, a very nice feature for these tests. These were based on the h18 forecast for both data sets. We still have to overlay these by hand and georeference then all to be the same.

It seems that with the wind speeds so very close in the two data sets that the NDFD must have been either pure GFS or it was in any event very close to what was used in the NDFD.

Looking at this one and the last one below for Tehujuanepec show that the main challenge in making this type of true test of the forecast is having model forecast data for the times of the satellite passes.  Thus we have started a new program of automatically downloading the GFS and NDFD forecasts once a day and then when we see a nice display of scatterometer winds showing interesting behavior, we can go back to see which model did the best in forecasting it.  This is an ongoing project now.