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.


Sunday, May 17, 2015

Transpac Weather and Tactics by Stan Honey

Transpac Weather and Tactics
Stan Honey

Stan Honey has navigated in nineteen transpacific races and has won eight times.  As navigator, Stan has held the single-handed, double-handed, Pacific Cup, and Transpac passage records for monohulls to Hawaii.

Overall race structure and necessary decisions

The primary feature that dominates the Transpac is the Pacific High.  Typically there is no wind in the center of the high, and increasing wind as you get farther south, up to a limit.  The central question of the Transpac is how close to sail to the high, or how many extra miles to sail to get farther from the high.  In years when the Pacific High is weak (or weakening) and positioned well south, there can be strikingly more wind to the south.  There have been Transpacs where yachts that are 10 miles to the south of competitors can experience 1 knot more wind.  A sled, in 1 knot more wind will sail 1/2 knot faster, and therefore would gain 12 miles per day on the northern competitor.  This condition can persist for the entire middle third of the race, eliminating any chance of recovery for the yachts that are positioned too far north.  Note that all yachts in this middle third of the race are nearly fetching the finish on starboard pole, so the boats caught too far north cannot gybe out of their predicament without sailing a dramatically unfavored angle,  passing far astern of the competitors to the south.  This condition, dominates the results of most Transpacs.

Occasionally, however, the Pacific High will be strong (or strengthening), and located far to the north.  In these conditions, it IS possible to be too far south. The boats that sail closer to the high will not only get more wind, but will sail the shorter distance.  Typically in these sorts of years, the wind stays "reachy" throughout the middle third of the race, so the boats that paid extra distance to get south cannot even "cash in" the southing and reach up in front of the northern boats, because everyone is reaching fast.

The beat to the West End of Catalina

Generally, tack up the Palos Verdes coastline until the Westerly has filled in, and you can at least lay the Isthmus.  When you tack onto starboard to cross the channel, continue all the way across.  Do not tack on the shifts in mid-channel. There is substantial adverse current and lighter wind in mid-channel.  It is better to get right across into the accelerated wind and reduced current at Catalina.

Think of the Transpac in three sections:

1.    The windy reach to the ridge;
2.    "Slotcars" through the middle third; and
3.    The run for the last third.

The Pacific High nearly always has a ridge extending off of its southeast corner. On the weather map this is visible as "U" shape of the isobars on the southeast corner of the high.  After rounding the West End, you will have a windy reach for a couple of days, depending on your yacht's speed, but when you get to the ridge, the wind will lighten and veer very quickly.  For this reason you will find that after reaching in lots of wind for two days, when you finally get the spinnaker up, and are struggling to carry it, within 6 hours or so, the pole is back and you're running on your downwind vmg angles in much lighter air; you just crossed the ridge.

The most critical decision of the Transpac is where to cross the ridge.  The reason this is critical is, once you get to the ridge and the wind veers, you can not get farther south.  You are already sailing as low as you can on your polars, and you can not gybe without huge penalty.  That is why the middle third of the race is called "slotcars."  You have to stay in your slot, on starboard pole, until the wind eventually veers enough so that you can gybe out on port, if you choose.

The middle third of the race begins as soon as you cross the ridge, and the pole comes aft.  Throughout this part of the race, every yacht sails as low as it can (e.g. sails its downwind polars).   At the west end of Catalina you made your decision where you wanted to cross the ridge, you sailed there, and now you have to live with it for three days or so.  If you are too far to the north, you will be slowly destroyed by the yachts to the south of you, and there is nothing that you can do about it; you cannot gybe, you cannot sail lower.  As the wind gets lighter, your polars force you to sail higher and higher, until you "spin out" up into the high.  When you eventually gybe to avoid starvation, your angle on port pole has you heading due south, far behind your competitor's transoms.  The "slotcars" leg ends when the wind eventually veers far enough so that both gybes are symmetrical around the course to the finish, allowing you to sail either gybe.

The final third of the race is "the run."  This is why we sail Transpacs, the wind picks up as you approach the Islands, and you are surfing in tradewind swells. Generally the right hand side of the course is favored in the final third of the race, because the wind slowly veers as you sail further west.  Therefore the best course is to favor starboard pole until the last gybe to the vicinity of the Islands,
and come in on port pole to approach Molokai at Kalaupapa.  Be sure to account for the fact that the wind will continue to veer, and do not overstand Kalaupapa. In the final third of the race the wind speed is generally even across the course. Oddly, those boats that get too far north in the middle of the race and stew about it for 3-4 days, gybe onto port as soon as they can and sail to the south after
there is no longer a windspeed advantage to the South.  These boats then miss the right shift in the last third of the race and lose even more.

Approaching the Finish

Pick your approach to come into Molokai at Kalaupapa on port pole.  Gybe close to Kalaupapa and sail along Molokai in the accelerated wind.  When you get to the west end of Molokai, if you have been lifted away from shore, gybe back on port to get close to Ilio Point, where there is accelerated wind.  Gybe onto starboard off Ilio Point and cross the channel.  Never approach Oahu much above Koko Head, take another hitch on port in mid-channel if you have to.  It is fine to sail close to Koko Head, and from Koko Head sail a straight line to the finish. As you approach the finish, plot your track on the chart, and take GPS fixes as well as periodic bearings with your hand bearing compass.  The finish line is deceptive, and many yachts get too close to shore when they can not see the red buoy.  The best technique is to plot your position and navigate to the buoy, rather than expect to see it.  With spectator boats around, the buoy often cannot be seen until it is within 100 yards.


In contrast to popular perception, squalls do not always work the way "catspaws" do.  Catspaws have diverging wind in front of them.  Surprisingly, some tradewind squalls can have converging winds at their leading edge.  The wind converges because there is an updraft in front of the squall.  In addition,
the average wind in the squall is veered about 15 degrees or so to the right of the prevailing surface wind, and the squall itself moves about 15 degrees to the right of the path of the surface wind.  Behind squalls the wind is light, particularly near dawn.

Heavy boats:   As the squall approaches, gybe to port pole, stay on port pole right through the squall, and then gybe back when the squall has past completely over you and your wind speed and angle has returned to the prevailing conditions.  If you gybe back to starboard pole too early, you run the risk of crossing behind the squall and getting into the light air in the wake of the squall.

Light (fast) boats:  Gybe to get in front of any squall within reach.  Gybe back and forth in front of the squall for as long as you can.  Each gybe "back" towards the squall will be at a horrible angle, because of the way the wind "toes-in" in front of the squall, but do it anyway; the velocity makes up for the horrible
angle.  When the squall finally passes you, exit on port pole and get away from the squall to avoid getting becalmed behind it.  Port pole is more effective to avoid the calm behind a squall because the squall itself is moving to the right of the path of the surface wind, so port pole allows you to diverge rapidly from the light air area behind the squall.  It is perilous to exit a squall on starboard pole because of the risk of getting becalmed behind the squall, particularly near dawn.

Weather Information

The best source of information about the future position and strength of the high comes from the 500 mb progs, but they take some practice to interpret.  Zonal flow, or a straight E-W path of the jetstream is characteristic of sourtherly and weaker surface highs.  A jetstream with a large “omega block” is characteristic of a strong high.  The next best source of data is the surface data, either from weatherfax or grib files.   Satellite imagery via NOAA APT satellites is fun, but not really of much use in a race in the tropics other than to monitor the position of tropical depressions

Author's Disclaimers

All of the above comments are relevant to typical Transpacs.  There are unusual races in which you have to break the above rules to win.

Pay attention to your boat's polars.  If you are racing a sled, it is worth sailing extra miles to get extra wind, because no matter how hard it blows, a sled will sail still faster if you get more wind.  On the other hand, if you are racing a moderate displacement boat, do not sail any extra miles in order to get more wind than necessary to reach hull speed.  If you sail farther to get more wind,
you will have more fun, but your average speed won't increase enough to pay for the extra distance.

Watch for tropical depressions.  The inverted troughs that extend north of a tropical depression can cause the tradewind direction to shift from normal.  This can make a huge difference as you are picking your approach to the Islands.

Monday, May 4, 2015

Weather Models for Transpac Sailors by Angeline Pendergrass

Weather Models for Transpac Sailors
Angeline Pendergrass

There are a variety of weather and ocean forecast models whose data can be acquired for free.  Below are descriptions of some you are likely to encounter, which will hopefully demystify them a bit. 

1.  Global weather models

GFS (Global Forecast System)
The Global Forecast System, or GFS, is the US’s primary global weather forecast model.  It is global in the sense that it calculates the state of the atmosphere and how it changes everywhere on the planet at every time step, and in that it focuses on large scales.  Its forecasts are used by weather forecasters at the National Weather Service and disseminated in many ways.  It is an old-school model in that it represents the atmosphere in an abstract way, as sine and cosine waves, rather than on a grid as the modern global weather models do and as all regional models do, when it makes calculations.  Of course this is obscured to the user, but it influences the forecasts made by the model.

By some measures, the GFS isn’t as good as some of the flagship weather forecast models from other countries, like Europe, the UK, and Canada (see Cliff Mass’s blog  Hopefully we can turn it around with more computing power and focus on improving the model. 

The latest version, 12.0, has a horizontal resolution around 13 km (7 nmi, the exact resolution varies with latitude) out to the 10 day forecast, and then 35 km (20 nmi) from 10 to 16 days.  In previous versions, the switch to lower resolution happened at a week instead of 10 days.  It uses a much higher resolution SST observational dataset, 5 minutes instead of 1 degree, but because they are observations they are only available before the forecast time.

GFS Ensemble Forecast System (GEFS) and North American Ensemble Forecast System (NAEFS)

An ensemble forecast is a set of model integrations with the same model, each with slightly different initial conditions or model physics.  The goal of an ensemble system is to provide a measure of how certain the forecast is.   The more similar the simulations are, the more confidence we should put in the forecast.   The range or spread across the model integrations is how you can accomplish this (see NCEP’s GEFS-MNSPRD).  It can also visualized with spaghetti plots (see NCEP’s GEFS-SPAG), where one contour is chosen and plotted for each integration.  The messier the spaghetti looks, the less certain the forecast is.  It is a fun exercise to step through a loop of spaghetti plots.  It will start out very clean and smooth and get messier into the future, as the ensemble members diverge and what we can know about the future state of the atmosphere diminishes because of the chaotic nature of atmospheric motions.  This extra information about forecast uncertainty comes at a cost, and that cost is resolution, since NOAA has limited computational power.

The GEFS is an ensemble of 20 runs of the GFS model. The resolution of the GEFS is 55 km (30 nmi) for the first 8 days, and then reduced to about 80 km (40 nmi) out to 16 days. 

The NAEFS is a joint venture between the meteorological services of the US, Canada, and Mexico which began in 2004.  Two ensembles of 20 members each, the GEFS and an ensemble run by the Canadian model, are combined along with statistical adjustment incorporating observations to produce forecasts out to 14 days.


Nearly every national weather service runs its own global weather forecast model.  Since it’s crucial to make accurate weather forecasts, most of these models ingest similar observations, but the way that they do it is slightly different, and each model’s physics and other details differ as well.  One might be inclined to ask which is “the best” and rely on it, but this is probably not the best approach.  All of these models can hold their own against the US models, at least in many situations, so it’s difficult to say which is the best in general.  On any given day, it is worth comparing the analysis (which is in the past, so it can be verified) to see which model integration does the best job at capturing what we know has already happened. Aside from that, weather forecasters often treat the different models as an ensemble (see above) to get an idea of the range of possibilities of how the weather will unfold, and how much certainty to have in the forecast.

The US Navy runs its own global weather forecast system, called NAVGEM (US NAVy Global Environmental Model), formerly NOGAPS (Navy Operational Global Atmospheric Prediction System), at the Naval Research Laboratory.  This model can be used just like the other global models.  Since it’s run by the Navy, it also includes some ocean surface fields that are neglected by strictly atmospheric models like the GFS.

2. Regional weather models

NAM – North American Mesoscale Forecast System

The NAM is the flagship regional weather forecast model run by NCEP/NOAA.  It is regional in the sense that the atmospheric state is only calculated on a subset of the whole globe, rather than for the whole global, like the GFS.  Also in contrast to the GFS, the NAM is built so that it can explicitly calculate smaller-scale phenomena that produce large vertical motions (that is, it does not assume all motions are hydrostatic).

The NAM has nested grids.  That means it has one coarse grid covering its entire domain, but then finer grids focusing on regions of interest.  The coarsest grid, covering the whole domain, is at 35 km resolution. There are grids covering the Pacific and the continent at 12 km resolution, and higher resolution grids over some land regions.  When hurricanes develop, nested grids focusing on the hurricanes also run.

There are other regional atmospheric models.  In the US, the main regional model is WRF (Weather Research and Forecasting model), run mostly for research, rather than operationally at a variety of institutions.  WRF replaced the MM5 a few years ago. 

3. Models with more of an oceanic focus

COAMPS (Coupled Ocean/Atmosphere Mesoscale Prediction System)

Another regional model, which also incorporates the ocean state, is COAMPS, run by the US Navy.  It is initialized by NOGAPS/NAVGEM.

WW3 – NOAA WaveWatch III model
Unlike the other models, which calculate the state of the atmosphere, the WaveWatch model calculates ocean waves. As an input, it takes the near-surface winds from the GFS model.   The plain WW3 model is global. 

WW3-ENP WaveWatch III regional Eastern North Pacific model
This is a regional implementation of the WW3 model focused on the eastern north Pacific waters.







Competitors may only utilize weather information that is routinely available throughout the year to the general public without charge, and whose availability is publicly indexed. For example: Competitors may NOT arrange for routers or meteorologists to provide them with advice, custom data, or compilations of
public data during the race, no matter how that information is communicated. Competitors may receive regularly scheduled weather broadcasts or weather fax transmissions (e.g. from NOAA, USCG, WWV, NMC, KVM70). Competitors may receive imagery from satellites (e.g. NOAA, APT satellites).

Competitors may use any means to retrieve data from the Internet (e.g. from the web, from ftp sites, from email responders), provided that those data are intended for public use without charge, are routinely available for free throughout the year, and are publicly indexed (e.g. can be found via Google).

Prior to their preparatory signal, there is no limitation on private services or any other source of data or consulting, except that a competitor that has started may not provide weather information to another competitor that has started, or to a competitor that has not yet started except through the information provided to or from Transpac Race Communications.

This amends and clarifies RRS 41 (c), which states:

A boat shall not receive help from any outside source, except
(c) help in the form of information freely available to all boats.

Friday, April 10, 2015

TransPac Live Weather Resources

TransPac Weather Resources are now online at, designed to update at each page refresh. Check out the link to the unique scatterometer wind data we have presented.

Starpath will be presenting the weather seminar for the 2015 TransPac Race in LA on May 2 and 3.

Wednesday, April 1, 2015

Ways to Get Accurate GMT (UTC)

Celestial navigation is one of the few human endeavors that requires us to know the time accurate to the second. In earlier days of celestial navigation—which for the purposes at hand we can say means more than thirty years ago—this was more or less easily accomplished by HF radio broadcasts, but in these modern days of the Internet, cell phone networks, and ubiquitous GPS, it is now very much easier.

That does not distract, by the way, from the high value of having a good old fashioned watch on board whose rate we monitor frequently. A modern justification for learning cel nav after all is to be independent of electronics for ocean navigation, and we need to know the time to do cel nav well... or at least efficiently. Put another way, you can sail around the world fairly efficiently from port to port with nothing at all but a watch (and some books and knowledge), but take away any time piece and it will be difficult to DR for 100 miles. Our Starpath textbook Celestial Navigation has extended sections on time keeping in navigation.

Thus if you have a watch for navigation, you will need some way to check it frequently so you can establish its rate, ie how many seconds it gains or loses every week or so.  Another motivation for this note is our new electronic barometer (Starpath Mintaka Duo), which has in it a very accurate clock. At a maximum drift of 5s per month (and likely better than that) it could well be the most accurate stand alone clock on the boat.  A typical quartz watch is 15 to 20s/month and they are not as well temperature compensated. But we need some way to test that this is really true, so we show below here four independent ways to get accurate GMT.

In principle any one method would do, and one could just list what the methods are, but unless you see them side by side, then that would have to be taken on faith—a type of justification we try to avoid in navigation whenever possible.

Here are the methods

(1) Tune in an HF (SW) radio to one of the international frequencies that broadcast time tics. These are listed in Radio Aids to Navigation, the applicable chapter we have online at this link.  The best known and most often used of these is WWV and WWVH at 5, 10, 15, 20 MHz.

(2) Call this phone number to hear the WWV broadcast on your phone: (303) 499-7111. This is a great trick, and it would seem that navigators might want to have this number in their contacts list.

(3) Logon to and select UTC and see the time presented for you. You will see their note that the displayed time is "Corrected for network delay."

(4) Use any GPS that is connected to satellites and giving an active location to find the display that will also show the UTC.  Note that the GPS will turn on without satellite connections and indeed might even tell you the time, but this is not dependable without the actual connection.

(5) Read your cell phone time.  When you are connected to a network the phone should give you the correct time. Note that strangely enough, the iPhones do not have a native display of time accurate to the second, but there are numerous free apps that read it and then show the time to the second. I should also note that i have seen rare instances when the cell phone time was off a few seconds over a period of several minutes, but I do not know what might be the source of this.  The primary source of time in the phones is the network providers, which are in principle getting the time from GPS.

(6) Most modern computers are designed to show the network time whenever you are logged on to the Internet.  If you are some period of time off line, then the computer could drift, but if you have a wireless connection, your computer should be showing the right time.

Here is a video showing the whole band playing at once... in keeping with our totally non-professional standards of production.

Here is a snapshot of the Mintaka Duo in the time display mode. It always shows GMT as well as the time zone selected for display, which is shown in the top right.

Thursday, March 19, 2015

Marine Weather Services Chart — How to Make Your Own.

For many years the NWS published Marine Weather Services Charts (MSC) that listed crucial information for mariners using their services. There were fifteen charts that spanned all US Waters. The page size was 13”x 21”, printed both sides, with an annotated great-circle chart of the region on one side and all text on the other. The last printed versions of these are still to be found online from unofficial sources, but they are outdated. The NWS no longer supports them nor makes them available—the only exception is Alaska, MSC-15, which is still available from NWS, although they have trimmed down parts of the original content.

Nevertheless, the concept of the MSC remains crucial to good weather work underway. Environment Canada still offers their counterparts called Mariner’s Guide to Marine Weather Services, which are equally valuable for their waters. One approach to the missing MSC is just to print a copy of the last known version and then make pen and ink updates as needed on that copy. On some charts, the changes are few, or not relevant to your needs, and once updated their value remains high.

In these modern post-MSC days, the latest data are readily available online, but the challenge is finding it and putting it together into a useful format.  There is often too much data! We are faced with the same information in multiple formats, with some parts more convenient than others. Or some seemingly obvious thing we would want at hand underway turns out to be difficult to find online. Remember, the goal is not to provide actual resources, but to provide the information we need to use the resources we have access to underway.

So as a temporary solution—hoping the NWS eventually brings them back—we offer here a way to gather together the same data that were on the MSC charts, which you can then combine into some convenient format of your choosing. A sample section of one of the older charts is shown in Fig 1. Then in the following figures are examples of recent equivalent data found online.

Table 1 shows the data that were typically on an MSC along with links to where you can get this data to make your own compilation. For regions you plan to sail in, you can download, print and combine into a thin binder of what was in the MSC. Having this information at hand is fundamental to taking advantage of the wonderful resources we have available. The exercise will show clearly why we miss the MSC so much.

The latest word from the NWS is they do hope to re-issue some of the MSC as online pdfs, but they do not know when. This might be up to the local NWS Offices.

Fig 1. Section of now-defunct MSC-1, Eastport, ME to Montauk Point, NY, showing forecast zones and VHF NOAA Weather Radio transmitters. Blue-green is VHF weather coverage. Notice the indent in the coverage approaching Rhode Island Sound, in forecast zone ANZ235. You could get data in this region from the USCG broadcasts of Item (13). Some of these zones (Fig. 2) have changed.

Table 1. Make Your Own Marine Weather Services Chart
       Historic MSC content
            Links to online sources
FORECAST ZONES labeled and outlined on the chart

Coastal zone maps (including Great Lakes):
Offshore zone maps:
NOAA Weather Radio BROADCAST STATIONS and reception ranges

Start with this index map:
then click to state, then click the station, then click the map for an excellent pdf.
OBSERVATION STATIONS (light houses, buoys, etc) used in NOAA Weather Radio reports.

This is the place we miss the MSC the most, as we have to recreate these plots on our own. The best approach we have found is start here:
then click a region, then zoom in for a plot of the stations to print, then click each one to get the name of the station to transfer to your print.
TERMINOLOGY used in weather reports and forecasts.

LOCAL NWS OFFICES responsible for each of the forecast zones.
NAVTEX broadcasts.
USCG HF VOICE high seas and coastal broadcasts.
WWV and WWVH Storm warnings
USCG HF RADIOFAX high seas broadcasts


NAUTICAL CHARTS, how to order.
by email
You can get not just live buoy reports you can get essentially every NWS product available by email request through their program called FTPmail. See:
by telephone

This is the NWS longstanding Dial-a-Buoy program, which remains a very slick system, although smart phones offer even more options. See:

USCG VHF weather broadcasts
These are the repeats and relays of weather information on VHF 22a, which reach out farther than NOAA Weather Radio.
CANADIAN weather broadcasts when applicable.

Here is the overview of Canadian marine services:
And here is Canadian Weatheradio (note spelling):

The main list of NOAA/NWS Internet sites is at:
A shortcut url to all marine weather resources (which we hope will not change) is:
for mariners, unique to the region.

The closest we could find in the same spirit:
PORTS — Physical Oceanographic Real-Time System.

An amazing resource for quite a few places around the country:
NOAA Weather Radio by telephone

Covered well in AK on MSC-15, but this seems to be a service of the local NWS Offices, so you will need to check with your local NWS Office. See Item (5).
General and special information about the local forecast zones covered.

Start by finding the main link to the local NWS Office from here:
Then there is much information about specific regions. The unique channel winds map on the back of MSC-13 for HI is one good example. (See Fig. 6)
Definitions of VHF WX channels by frequency

Most resources define the VHF broadcast products by frequency, but on the boat we may only have channel names, wx1, wx2, wx3... so this can be useful data:
Unified Analysis Maps
Not cited on historic MSC, but new valuable resources

Fig. 2. Coastal forecast zone maps available online, from Item (1). Coastal forecasts in the outer coastal zones (we outlined in red) offer only warnings. They overlap the Offshore zones on the East Coast north of Charleston. Full forecasts in these outer zones come from the offshore forecasts.

Fig. 3. NOAA Weather Radio coverage (white areas). The online data shows the coverage gap as well. See Fig 4.
Fig 4. Detailed coverage map of WXJ39 Providence (WX2, 162.400 MHz) showing why there is a gap in the coastal coverage. It is an inland station and there are is no overlapping coastal coverage.
Fig 5. Locations of the observations stations reported on NOAA Weather Radio, from Item (3). We must then click each online to ID the station and make a list. These are the places we get recent observations from (updated every 3h) in the continuous NOAA Weather Radio broadcasts.

Fig. 6. Back of the out of print MSC-13 for Hawaii.
Fig. 7. Section of a Canadian Marine Weather Services Chart. Their offshore zones have names, ie "Explorer."

Monday, March 2, 2015

Telling Time by the Stars

As is the case with tricks for finding directions from the stars, there is no exclusive way to tell time from the stars, so we are free to make up whatever method works. To make up generalized star clocks that work on any arbitrary day of the year, however, does require some background, to be reviewed here. It is much easier to make up specific clocks on the spot, using a correct watch to calibrate it for the present date, and then use it on following nights by applying a simple daily correction. This does not require special reference books and calculations.

To tell time from the Big Dipper, as one example of a generalized star clock, imagine its pointers as the end of clock hands whose pivot point is Polaris and imagine a 24-hour clock face printed backwards on the sky around Polaris as shown in Figure 1. Midnight (0000 hours or 2400 hours) is straight up from Polaris; 0600 hours is to the west of Polaris and 1800 hours to the east. In 24 hours, the pointers sweep counterclockwise once around this clock face.

When the clock hand points straight up from the horizon, the clock reads midnight; when the hands point east with the pointers lying parallel to the horizon the clock reads 1800, and so forth. To read the clock at any time of the night, estimate the hour and fraction of an hour from the relative orientation of the pointers on the imaginary clock face. That’s all there would be to it if the sun kept pace with the stars. But the sun does not keep pace with the stars, and our daily time keeping is based on the sun so we must make a correction for this.

 All star clocks are fast; they gain 4 minutes each day because we keep track of time relative to the location of the sun, and we are moving around the sun relative to the stars at a rate of about 1º per day (360º/365d). Thus when we make our daily 24h rotation from noon to noon (relative to the sun) we are then 1º farther along our orbit, so we have passed any stars overhead by 1º. At our daily rotation rate of 360º/24h this 1º is equivalent to 4 minutes.

If you look at the same star on successive nights at the same time, it will be 1º farther (more westward) along its path across the sky. Thus if you want to see it at the same place on successive nights, you have to look 4 min earlier. This is basically how new stars appear on the eastern horizon at sunset as the seasons progress—although that is a bit more complicated because the time of sunrise is also changing. (We learn star positions relative to Aries, so check out the value of GHA Aries on successive days at the same time and you will see it increases by about 1º.)

At a gain of 4 minutes per day, star clocks gain a whole day in one year, so all star clocks reset themselves on a particular date that depends on the particular star clock in use—and by star clock we mean any two stars with the same SHA so the line between them rotates around the pole. The Big Dipper star clock resets itself on March 8th so all corrections must be reckoned from that date. (Official scientific star time used by astronomers resets on the Vernal Equinox, March 21st; the shift to March 8th comes about because scientific star time does not use the Big Dipper pointers for a reference line.)

To tell time from the Big Dipper, we need to know how many days have passed since March 8th. The time we read directly from the star clock is then fast by 4 minutes for each of these days. As an example, suppose the date was September 22nd and the stars looked as they do in Figure 1, with the star clock reading 0830. September 22nd is 198 days past March 8th, so the clock is fast by 198 × 4 minutes, which equals 792 minutes, or 13 hours and 12 minutes. The first 12 hours of the correction just switches the time from AM to PM, so the correct time of night is 2030 - 0112, which equals 1918, or 7:18 local time.

Figuring the correction is a bit involved, but this preparation need only be done once, after which the results can be rearranged to be more convenient. On September 22nd, for example, you could make an equivalent new rule for reading this star clock: change the star clock time from AM to PM (or vice versa, later in the night) and then subtract 1 hour and 12 minutes. Each subsequent night, you would subtract an extra 4 minutes, because the clock is still gaining time each night.

The time you figure from the corrected star clock will be the proper standard time for your time zone to within, at worst, some 30 minutes. It would be exact only if you happened to be located right in the middle of a time zone, each of which is about 1 hour wide according to star time. Star clocks also do not know about daylight saving time, so when daylight saving time is in effect, you must add 1 hour to the final result. Corrections for both longitude (the time zone correction) and for daylight saving time can be made simultaneously if you calibrate the star clock with a known time. In the last example, if the uncorrected star clock read 0830 AM at a time you knew was 8:10 Pacific Daylight Time, the rule becomes much simpler: subtract 20 minutes tonight, and then 4 minutes less each subsequent night.

The final accuracy of the time obviously depends on how accurately the star clock itself is read, which requires an estimate of the angle between the clock hand and the horizon—similar to reading a stylish watch with no numbers on the dial. Sticks held in one line with the Pointers and one with the horizon can help with this. The angle found this way can then be transferred to a sketch of the clock or to the compass rose of a chart. Reading the clock by eye alone, however, is usually adequate. Note that in normal circumstances most people have an adequate sense of time even without a watch, but under a great deal of stress this is not the case at all. During long storms at sea, it is possible to even lose track of how many days have passed. This is not likely to happen in a routine cruising, but one could imagine getting caught in coastal waters at night without a safe harbor nearby. If the wind and seas began to build on top of this, one could easily muster enough stress to lose track of time. Without a watch, you could monitor the duration of the adventure with the stars.

(Note: A star clock resets when the common SHA of the two stars making up the clock hand leads to GHA = 0º 0' at 00 UTC. For the Big Dipper clock, Dubhe and Merak have SHA = 194º 4.2'±14.3', so we need the nearest date when  GHA Aries = 360º - 194º 4.2' = 165º 55.8' at 00 UTC. You can get rough estimate from the Planet Diagram, or interpolate the Almanac to find that this is March 8.)

Stargazing for orientation in time and space clearly requires some hands-on practice. It is not like learning the combination to a lock, that once memorized can be opened at will. It is more like learning to play a kazoo. You start by learning to play a few notes well, and pretty soon you are playing a fine tune. And the enjoyment to be had from exercising this skill can be just as rewarding. It is one way to get in a little more in tune with a dependable part of the environment.

The above is adapted from our book Celestial Navigation: A Complete Home Study Course.

Friday, February 27, 2015

Boxing the Compass

This name for the process of listing or reciting the points of a compass card arose after 1851 and before 1911. In the 1851 Bowditch “boxing” was a verb meaning to back wind the jib. In 1911 edition it was used as is done today. On the other hand, the size and concept of a compass point (11.25º) dates to the earliest navigation records from the 16th century.

The call for this note came from reading the 1851 edition of Bowditch; in particular the log of his voyage from Boston to Maderia that he made in 1836. It is a fascinating document that reminds us of many of the fundamentals of marine navigation. One of which is the procedure of taking a departure on ocean voyages. Taking a departure means simply recording the bearing to the last land you see as it slips out of sight, and adding to this an estimate of its distance off. We bring this important concept back into practice in our textbook Celestial Navigation: A Complete Home-study Course.

Modern navigators have mostly forgotten about this step in their navigation routine, and to the extent that happens we lose one more of the good procedures established over many years by our seafaring forefathers. Even in the age of GPS, we should take and record our departure. As we sail out of sight of land, it is in a sense the last thing we know for sure!  

The first thing you run across in the Boston to Maderia log book is “At 8 PM, Cape Cod Light-house bore S by E 3/4 E, distant 14 miles; from which I take my departure.” 

To a modern reader, the first job is to figure out what bearing this really is. He is speaking in terms of compass points. There is a point on the compass called “South by East,” and from this point you turn three quarters of a point to the east, and you are facing the lighthouse. 

The general procedure of converting compass points to azimuthal degrees is called boxing the compass. There are 32 points in a circle, thus each point is 11.25°. Easy enough it would seem, but nevertheless, boxing the compass is no simple matter. And it was at this point I realized that this question comes up to modern navigators more often than we might guess—usually in the context of reading an older book, but sometimes part of navigation tests that choose to hang on to some older traditions. Not to mention that compass points are still marked on compass roses of most US charts and magnetic compass cards, so an instructor is obligated to give some level of explanation. Compass points are also referred to in the Navigation Rules in that, for example, sidelights show from straight ahead to two points abaft of the beam.

Compass points date from our earliest record of navigation. They are shown, for example, in the famous John Davis work from the late 1500’s. Figure 1 shows this and also gives a hint of where the term "compass rose" might have come from. Though not named as points, modern compasses often mark the cardinal and inter cardinal points in the same style as used on older compasses.

Figure 1. Compass rose from Seaman's Secrets by John Davis. Note the center has a rose in it!  Also note that East is marked with a cross, which in those days marked the direction to Jerusalem, where the crusaders were all headed. Even poor ole Columbus had the vision of making enough money from his ventures to finance his own crusade to the East. It seems modern charts might have to start using that symbol again.

But when it comes to looking up how to box a compass we quickly learned that this is not easy to find. It has long been dropped from modern textbooks, and if you go back to the days when it was commonly used for bearings and courses (1800’s) you find it was then presumed a known basic, and so not covered there as well. Thus the best source is a text from early 1900s.

Referring to the figure of the annotated Kelvin compass card (Figure 2), we see that each point is named relative to the nearest cardinal or inter-cardinal point. Thus the name of the third point to the right of north is NE by N and not NNE by S. The word “by” means the point next to the reference point. It is sometimes abbreviated with an “x” such as NE x N.

Figure 2. This compass rose is from a drawing submitted with American Patent No 4,923 in 1889 by William Thomson, known also as “Lord Kelvin.” In small print in the fleur-de-lys are the words “Sir W. Thomson’s Patent”. It is marked off in quarter points and degrees. We have added the numbering of the points and we added the markings outside of the azimuth ring of degrees, else it is as he presented it. The inside shows what is presumably his proposed design for the compass needles. A sample of a modern version is shown above. Thomson was one of the leading physicists of the 19th century, but also worked on many practical matters, which brought him great wealth. Besides fundamental physics he (and his large staff of assistants) also worked on such mundane maritime matters as optimizing the design of a compass card and the creation of mechanical machines for tide prediction.
The motivation for the dominant use of compass points for courses and headings throughout the 18th and 19th century in place of actual degrees is not clear to me. We see that degrees were on the compass roses back in the 16th century, and all the reasons we use them now rather than compass points would seem to be true then as well.

The finest divisions used are quarter points (11.25/4 = 2.8125°).  The labeling of the quarter points is where all the fun begins. Fractional points are referred to the nearest whole point, but which one do you use. For example, the bearing one quarter point N of NE could be called NE 1/4 N or NE x N 3/4 S. Only one is right, however. 

The convention used is to box from the North toward the East and West, and from the South toward the East and West, except that the points adjacent to the cardinal and inter-cardinal points are always referenced to these points. Thus in the example given, the right answer is NE 1/4 N. 

The full compass is shown the table below. There is some rough analogy here with the use of roman numerals, which proceed upward for a period then back one then upward again: i, ii, iii, iv, V, vi, vii, viii, ix, X, xi, xii etc. Thus we count up to a reference point and the adjacent points to it are referenced to it and not in an ongoing sequence. But we all recognize this as a convoluted way to count. Movie makers even put the date in this format so we can’t figure it out as it flashes by. Could it be the early mariners used this convoluted system to protect the captain from mutiny by untrained crew in the sense that it is said they did with the practice of celestial navigation?