Sunday, June 7, 2020

Introduction to Vessel Running Lights (Navigation Lights)

Navigation lights help identify vessel type and its direction of motion.  The required lights are specified in the Navigation Rules, Part C, Rules 20 to 31.  Power vessels in Rule 23 and sailing vessels in Rule 25 are the two classes we focus on here. These two basic vessel classes have simple rules.  Other vessel classes (towing, fishing, restricted motion, etc) are covered in other Rules. Rules for towing vessels are complex.  An important part of these rules is  included in Rule 20b, namely that no other lights than those specified by the rules should be shown if they could possibly interfere with the identification of the required lights.

The lights are required (must be shown) from sunset to sunrise, but may be shown any time deemed appropriate. Note that automobile lights are required from evening twilight to morning twilight, which is a longer period, but less precisely defined.

For now we look at the three basic nav lights categories: sidelights (red and green), masthead lights (white), and sternlight  (white). These are defined in Rule 23a. These rules establish the terminology as well: sternlight and sidelight are written as one word.

Sidelights are seen from dead ahead to 22.5º abaft the beam. Sternlight is shown in the aft gap between those two, and the  masthead lights are seen from everywhere except the stern sector, ie masthead lights and sidelights are seen from the same sectors.

The seemingly strange choice of 22.5º is a historic accident as this equals two points on the compass and the original Rules from the 1800s were written in terms of compass points. See our note on boxing the compass.

For now we consider only power driven vessels (PDV) and sailing vessels (SV). Note that a sailboat is only an SV when it is actually sailing, no engine engaged, else it is a PDV. This fact applies not just to lights but to all aspects of the rules. A sailboat under power is a powerboat.

We consider PDV in only two categories for now: less than 50 m (164 ft), and 50 m or more in length.

Larger vessels require two masthead lights, the forward one lower than the aft one. Thus seeing them on the horizon we know which way the vessel is headed. The above and below pics are from the USCG Navigation Rules Handbook.  There is also a printed edition.

For our Radar Course quizzes, we have to determine which lights we see based on the relative motion track of the vessel on the radar screen—but a first step in that analysis is knowing what the lights look like from various perspectives in plain sight.

Referring back to the first figure, from anywhere in the red sector you would see only one red light and one or two white masthead lights; same from the green sector, one green and one or two whites depending on the size of the vessel.

In the aft sector, you would  see only one white light, the sternlight.  A 15 ft runabout and the Queen Mary look the same from astern.

From dead ahead you would see red and green sidelights. In this case "dead ahead"—as determined by seeing the other vessel's sidelights—is tied to the technical specs of the lights. According to the Rules, they must fade to zero intensity within 3º of dead ahead. In other words, in principle you should never see green from left of the bow, but to account for technical limitations, you should never see green at all beyond 3º to the left of the bow. 

The borders at the aft end of the range are ± 5º, meaning the specified intensity (visual range) must apply to at least within 5º of the aft limit, and then must attenuate to 0 by 5º beyond that. See:

COLREGS Annex I— Positioning and Technical Details of Lights and Shapes,
Section 9. Horizontal sectors
(a) (i) In the forward direction, sidelights as fitted on the vessel shall show the minimum required intensities. The intensities shall decrease to reach practical cut-off between 1 degree and 3 degrees outside the prescribed sectors.
(ii) For sternlights and masthead lights and at 22.5 degrees abaft the beam for sidelights, the minimum required intensities shall be maintained over the arc of the horizon up to 5 degrees within the limits of the sectors prescribed in Rule 21. From 5 degrees within the prescribed sectors the intensity may decrease by 50 percent up to the prescribed limits; it shall decrease steadily to reach practical cut-off at not more than 5 degrees outside the prescribed sectors.
This means that a head-on course could vary from reciprocal by as much as 6º and in practice even more due to improper shades on the lights and the yawing about of the headings. Understanding the head-on encounter is crucial to safe navigation. Not maneuvering before you are certain of the situation is also crucial. 

The aft boundary (two points abaft the beam) that separates the crossing and overtaking courses as determined by lights has an uncertainty of ± 5º.  We can often determine relative courses to well within this accuracy by solving the relative motion diagram on our radar screen. 

We can also learn the other vessel's course (COG) from its AIS signal if available. Vessels with a heading sensor programmed into the AIS will also tell us their heading, which is what determines the lights we should see.  With no current or leeway, the heading and COG would be the same.

(In the formative days of the Rules, the sidelights were specified to extend 4º across the bow, but that proved dangerous and was ended in the 1890s. Side lights was two words in those days.)

Masthead lights can be seen from farther off than sidelights.  Below are the required ranges given in Rule 22, for the three lights we consider here:

(a) In vessels of 50 meters or more in length:
  • (i) a masthead light, 6 miles;
  • (ii) a sidelight, 3 miles;
  • (iii) a sternlight, 3 miles;

(b) In vessels of 12 meters or more in length but less than 50 meters in length;
  • (i) a masthead light, 5 miles; except that where the length of the vessel is less than 20
  • meters, 3 miles;
  • (ii) a sidelight, 2 miles;
  • (iii) a sternlight, 2 miles;

(c) In vessels of less than 12 meters (39.4 ft) in length:
  • (i) a masthead light, 2 miles;
  • (ii) a sidelight, 1 miles;
  • (iii) a sternlight, 2 miles;

It is clear why we can see a ship from farther off than a smaller vessel. The lights are higher and indeed brighter. In all cases, we see the masthead lights before the sidelights.

Below are samples of how the aspect of the lights tell us how the vessel is moving.

These two pictures are from our book Fundamentals of Kayak Navigation. All vessels care about understanding navigation lights.

The mariner at location A sees a red turn into red and green, and then into green alone, as the vessel turns toward them and then white as they passed. Vessel B sees a red light the whole time, except possibly at the very last location where it would go white. Study this figure to be sure both views are clear.

Masthead lights on a ship are often referred to as its "range lights,"  because we can think of them as a navigational range, and use that concept to decide where we are relative to that range. When navigating in the vicinity of a charted navigational range, when the two lights are lined up from our perspective (the more inland one is always higher), we know we are precisely on the charted line that the two lights define. In the case of a vessel, this means we are looking directly at its bow.  Recall we do not see masthead lights astern; only on the sides and ahead.

As the vessel turns toward us at the top of the pic, we see the range lights "closing"; as it turns away from us, we see the range lights "opening."  This is common terminology in nighttime navigation:  Are the masthead lights opening or closing?  When the two lights are lined up, one above the other, the ship is headed straight toward us.  We often can see these lights with binoculars and identify its heading long before we see its sidelights.

Sailboat lights are summarized in the figure below

When sailing, there are sidelights and a sternlight, but no masthead light. Under 20m (65.6 ft) all underway sailing lights can be in a combined unit at the masthead—but vessels choosing that option must also have deck level sidelights because the tricolor is not legal under power. Thus a sailing vessel can likely have both masthead sidelights and deck side lights but these cannot be run at the same time. There is a rarely used optional all-round red and green masthead light for when sailing.

Under power, sailboat lights are easy; they are same as  PDV of the same length.  The above figure is from our textbook Inland and Coastal Navigation.

As you study navigation lights, keep in mind the salient fact that every collision involves the violation of at least one of the Navigation Rules by both vessels. For collisions at night, illegal lights are almost guaranteed to buy you some fraction of the liability, even if they or you were not the primary cause of the collision.

Also it takes an effort to ensure that sailboat lights are legal. It is worth taking the time to view your boat at night from another boat at all aspects—or have a friend take a cellphone video. Sometimes sidelights on a bow can reflect from the bow pulpit or a sail on the bow and be seen in many directions. The light used to illuminate the masthead windvane (Windex) can often be seen as a white light, and so on.

A version of the Navigation Rules that is easy to search is our Pocket Navigation Rules Handbook. You can view it online and then save as a PDF and the cross links should work. Then mail it to yourself and open in your phone and save into the library of your favorite ebook reader.

Samples of running lights.

Vessel under sail is very simple. Sidelight or sternligbht.  If we see only a sidelight with no masthead, that is likely a sailboat undersail.

A sailboat sternlight, but we only know that because we can see the boat.  Seeing only the light, we do not know what kind of vessel it is... which in a sense does not matter if we are overtaking, we have to stay clear.

A ship's sternlight.

Cruise ships are mixed bag when it comes to lights.  The positive is you cannot miss them, they are a great smear of lights. The negative part is it is very difficult to see any of the official lights in the maze of other lights so it can be hard to tell which way they are going... if they are going at all.  They could be just drifting gambling casinos.

Here we can see the two sidelights as it turns left in front of us (red, red and green, and green), but note the illegal red light on the starboard side. It can be difficult to identify their masthead lights—but this is not a fair evaluation of that because we are so close.

A cleaner view of a ship's green sidelight and masthead lights. Many ships that do show distracting lighting when near port, do not have these extra lights on at sea. This is true of cargo and tanker vessels, not cruise ships.

Here is a case of many distracting lights but the mastheads (range lights) are still clear, as is the port sidelight.

This screencap is from—believe it or not—a video about using the proper lights. It is recorded from a speaker on the vessel we see here, filmed by another vessel following it around. This is an example mentioned above of unwanted reflections of your sidelights... the strange part is they could watch this and not edit this out.

Monday, May 11, 2020

How to Delete Browser Cookies

Our online courses use cookies to determine when you are logged in. In principle, when you close a browser or shut off your computer, these login cookies should be removed. Periodically this does not work as intended, perhaps due to the use of multiple tabs and multiple browsers for the same course session.

In any event, if you get notice that you cannot enter a page because you are already logged in, then this is likely an issue with these login cookies. The easy way to avoid this is to log out when done with a course session before closing the browser or before just leaving your browser open to move on to other computer projects.

After you delete your cookies, then you can login and then logout and that should put you back to normal.

There are two ways to remove cookies from your browser. One is more of a shotgun, deleting all cookies from all sites in the browser;  the other method deletes cookies from just a specific website, leaving others undisturbed.

Chrome Shotgun (v81, May 11, 2020)

Top left menu Chrome / Clear Browsing Data... / check Cookies and other site data / press Clear Data

Surgical Delete in Chrome

(1) Go to website of interest and right click a page and choose Inspect.
(2) In the top menu bar choose Application.
(3) On the left, expand Cookies menu and highlight website.
(4) Right click the highlighted website and choose Clear.

Firefox Shotgun
Top menu History / Clear Recent History... / check Cookies / press OK

Surgical Delete in Firefox (v76 May 10, 2020)

(1) Go to website of interest and right click a page and choose Inspect Element.
(2) In the top menu bar choose Storage.
(3) On the left, expand Cookies menu and highlight website.
(4) Right click the highlighted website and choose Delete All.

Safari Shotgun
Top left menu Safari / Clear History... / select time frame / press Clear History

Surgical Delete in Safari (v13, May 10, 2020)

(1) Go to website of interest and right click a page and choose Inspect Element.
(2) In the top menu bar choose Storage.
(3) On the left, expand Cookies menu click to highlight website.
(4) On the right select any one cookie, then Cmd+A or Ctrl+A to select all, then right click and choose Delete.

Edge Shotgun
Top right 3-dot menu / History / top right link Clear history / check Cookies and saved website data / press Clear

Surgical Delete in MS Edge (Win10 v1903, May 10, 2020)

(1) Go to website of interest and right click a page and choose Inspect Element.
(2) In the top menu bar choose Storage.
(3) On the left, expand Cookies menu click to highlight web page in view.
(4) On the right select any one cookie, then right click and choose Delete.All Cookies.

Tuesday, April 28, 2020

Social Distancing and the Navigation Rules

These sad times have introduced a new terminology to the public called social distancing, but this is not a new concept to mariners familiar with the Navigation Rules; it is called close-quarters. Its goal is precisely the same vessel-to-vessel as it is person-to-person: to prevent harm by not getting too close to each other.

The definition is the same in both applications. We want to define a space around us within which our own safety is under our own control. If we let the other get closer, vessel or human, we are not protected against a sudden, unexpected maneuver of the other.  Our close quarters or social distance is our safety zone wherein  we can control our fate with our own maneuver and not be dependent on the other vessel or human.

The Navigation Rules effectively instructs us not to let any approaching vessel into our close quarters. The term appears explicitly in Rule 8, Action to Avoid a Collision, and in Rule 19, Conduct of Vessels in Restricted Visibility, but it is also implied in Rule 17, Action of the Stand-on Vessel that instructs us to maneuver if the other vessel is "not taking appropriate action," which means, among other things, is getting so close we could not avoid a collision by our own maneuver.

The dimensions and shape of close quarters amongst vessels is not defined in the Navigation Rules but has been addressed in numerous court cases. Going slow in a narrow channel, it could be yards; at high speeds in the open ocean, it is more often thought of in miles. And it depends on the vessels involved. It is the knowledge and prudence of the skipper to determine the extent of their own close quarters in various circumstances.

The size of the social distancing range is clearly oversimplified in the government specified distance of 6 ft, which, even worse, is sometimes specified as 3 ft—close enough that someone could spit on you and grab your phone.

Six feet is likely chosen because it is easy to think of. We can picture 6 ft; it is a nice round number—half a dozen. But you can smell someone's perfume at 6 ft off, which quite literally means molecules coming off of their body have entered your body. But these social matters are more complex that vessel traffic.  It all depends on what you want to protect against. Dr Anthony Fauci demonstrated in  a TV interview what a 24-ft sneeze looks like... and it did not seem so unusual at all.  The social distance for a shady character on a dark street is going to be larger than 6 ft.

But my point for now is simply that these are the same concepts, and it might on some occasions help to keep that in mind, both on the water and on the dock.

Here is how this term appears in the Rules... with colored text added

Action to Avoid a Collision, Rule 8 (c).  If there is sufficient sea room, alteration of course alone may be the most effective action to avoid a close-quarters situation provided that it is made in good time, is substantial and does not result in another close-quarters situation.


Conduct of Vessels in Restricted VisibilityRule 19 (d). A vessel which detects by radar alone the presence of another vessel shall determine if a close-quarters situation is developing and/or risk of collision exists. If so, she shall take avoiding action in ample time, provided that when such action consists of an alteration in course, so far as possible the following shall be avoided:
      (i) An alteration of course to port for a vessel forward of the beam, other than for a vessel being               overtaken;

      (ii) An alteration of course toward a vessel abeam or abaft the beam.

Rule 19 (e). Except where it has been determined that a risk of collision does not exist, every vessel which hears apparently forward of her beam the fog signal of another vessel, or which cannot avoid a close-quarters situation with another vessel forward of her beam, shall reduce her speed to be the minimum at which she can be kept on her course. She shall if necessary take all her way off and in any event navigate with extreme caution until danger of collision is over.

Action by Stand-on Vessel,  Rule 17 (a) (i) Where one of two vessels is to keep out of the way, the other shall keep her course and speed. (ii) The latter vessel may, however, take action to avoid collision by her maneuver alone, as soon as it becomes apparent to her that the vessel required to keep out of the way is not taking appropriate action in compliance with these Rules. of which is avoiding close quarters.

Note that Rule 19d is a much stronger rule than others in this regard. Action to avoid a collision and related rules in clear weather refer to actions that "avoid close quarters," whereas Rule 19d in restricted visibility instruct us to maneuver to prevent the development of close quarters.  This calls for earlier maneuvers.

Thursday, April 16, 2020

Accuracy of Backup Cel Nav using a Mark 3 Sextant

In early printings of our textbook GPS Backup with a Mark 3 Sextant we discussed the accuracy of the shortcut procedures using our custom sun almanac.  These notes did not account for the type of sextant being used, but now that we have a dedicated online course for Backup Cel Nav that includes a Mark 3 sextant, it is now assumed that we would be using that type of sextant, so we need to fold in its own inherent uncertainties.

We have numerous examples that show that our custom long-term sun almanac can usually provide declination to within about ±1' and GHA to within ±2', and then we have approximations to the sun's semi-diameter (we call it 16' always) and we have abbreviated Refraction and Dip tables, so on the Latitude reckoning we have about a ± 2' uncertainty, with more like ± 3' on the Longitude reckonings.

We have a sextant (the Mark 3) that can only be read to ± 2’ and we have to effectively do this twice counting the index error measurement. Independent uncertainties add as the square root (sqrt) of the sum of the squares, so this sextant uncertainty is about sqrt (4 + 4) = 2.8'.

So for a Latitude sight we have a total uncertainty of about sqrt(2.8^2 + 2^2) or about 3.4'.  We thus estimate that we can find latitude as:

Latitude ± (3 to 5) nmi

This is to be compared to best possible with a good metal sextant and full almanac and a 1000-sight experience of ± 0.5’ for either a Lat or an actual fix of both Latitude and Longitude.

For Lon we can assume the watch is right, so sextant error is not really a factor, and we are left with finding peak time of the LAN curve of sextant heights versus UTC, which we combine with our GHA uncertainty of ± 3'.  

We can typically find the center of that curve to within  about 1 minute if we have good sight data. Sometimes half that, but for safety let's call this a 1 minute uncertainty, which in turn leads to a Lon uncertainty of ± 15'.  We combine this with the GHA uncertainty of ± 3' for a final value of sqrt(15^2 + 3^2) = 15.3.  Thus for longitude sights at LAN we have:

Longitude ± (10 to 20) nmi.

We expand the 15' a bit as there can be other uncertainties if we are moving very much in the N-S direction during the time we are measuring the curve of Hs vs UTC. This distorts the Hs curve, and the center does not reflect the actual location of the sun crossing our meridian.  For typical yachts this is usually a small factor.

This backup method has a large Lon uncertainty, but we must remember that this does not mean the Lon is wrong by that much. It most likely is not, but it is just uncertain by that much and we have to keep that in mind during decision making.  Indeed if we do several LAN fixes over a few days and our DR is agreeing with these fixes, then we have learned that we are likely doing better than this large uncertainty.

Sunday, April 5, 2020

Stargazing for Mariners: Northern Sky

We have a new Regiment of the North Star that can be used for finding latitude from Polaris without the official USNO annual nautical almanac. It can be used anytime you see Polaris, on any date or year. The method relies on identifying one of two stars, Alkaid at the tip of the handle of the Big Dipper and Segin, the trailing star of Cassiopeia on the other side of Polaris.

Both stars are trailing stars, meaning as the sky rotates, these are the stern of the constellation thought of as a boat sailing around the pole, once a day. They are also about equal distance from the pole and opposite to each other.

They are not bright stars. Alkaid at is the brightest of the Big Dipper stars, which as a group represent typical Magnitude 2 stars.  The leading 3 stars of Cassiopeia are magnitude 2 and the trailing 3 are magnitude 3.  Segin is the weakest of the lot at 3.3.

Because these stars are faint, we have to rely a lot on the pattern of the constellation. Alkaid is very easy, at the tip of the handle of the Big Dipper, but we need more help with segin.

Cassiopeia looks like an "M" from the Middle... meaning standing at the pole of the sky where Polaris is. Keeping that in mind, you can find this group, located about the same distance from the Pole as Polaris is on the other side.

Also the trailing loop of the "M" Cassiopeia is lazy, meaning not as tall, which also helps find this weak member of the group.

Other tips that help are these:

(1) Often the entire sky between Cassiopeia and the Big Dipper is blank except for two stars, and they will be Polaris and Kochab, one the Guards on the tip of the cup of the Little Dipper.  Kochab, Polaris, and Alkaid are about the same brightness.

(2) The line between Segin and Alkaid passes very closely through the the pole of the sky, and if you zoomed in you would see that Polaris is on the Cassiopeia side.

(3) We are talking about a large span of the sky between Alkaid and Segin. At middle latitudes they span from about NW to NE in bearing.

(4) Since the stars are faint, when any part of either constellation is within some 10º of the horizon, they are likely to be extinguished by the (clear weather) atmosphere.

Wednesday, March 18, 2020

SCATSat — The Indian Scatterometer

At present we have 6 satellites providing scatterometer data, led by Metop-A, B, and C with their ASCAT Scatterometers. There is also the USN satellite Coriolis with the WindSAT instrument. Here I want to note the one from the Indian Space Research Organization (ISRO) called Oceansat 2, with its scatterometer called ScatSat. Valuable wind data from all five instruments are available in graphic format from the STAR Center  (Google "ASCAT"; it will be the first link.) Graphic scatterometer data are also presented by KNMI at the EUMETSAT's Ocean and Sea Ice Satellite Application Facility (OSI SAF).

Figure 1.  SCAT stats with satellite id info needed for tracking. Data from China's National Satellite Ocean Application Service (NSOAS) instrument HY-2B is only available at KNMI/OSI-SAF.

The data index pages from the STAR Center for five instruments are shown below.

The ASCAT-A, B, and C instruments have the same pattern; they just come by at different times.

We see immediately that the wide swaths of the scatsat data are very attractive, but we need some confirmation that the data are dependable.  One easy check is to see if they agree with the ASCAT data where there is overlap. The NWS highly regards the ASCAT data, which is used as input to model forecasts. A couple samples are below.

These two passes are about an hour apart, which is fairly close for this type of comparison. We see here very good agreement—also keep in mind the ScatSat is 12.5 km resolution compared to 25 for the ASCAT, so the details we see in ScatSat SW of Islas Marias could be real.

Another example:

These passes are at 0424 and 0451 respectively, which we read from the small purple times at the base of the diagrams.

Again we see good agreement and indeed we see the great value of the much larger data swath. Check out the Low in the bottom right, just missed in the ASCAT pass. We might even guess that this Low is moving north because of the stronger winds on the right-hand side.

And one more:

These passes are 0604 and 0451 respectively, 1 hr and 13 min apart. Not too bad for a rough comparison. We see wind directions good throughout and wind speeds seem consistent. The lower resolution ASCAT wind speeds are easier to read.

We can do a much broader comparison using a unique display tool at OSI SAF called Multi-platform Product viewer.  This tool does exactly what we want. We choose a location of interest where there is ScatSat data and then with the tool we can look at all passes at about that time from the ASCAT instruments. Below is just one comparison off the coast of Perth, Australia, taken about 30 min apart. I will try to make a short video demo of the tool and post it here.

Wind speeds are a bit harder to read in the KNMI data as they include a cloud overlay and the color codes are tied to the Beaufort Force scale. The green arrows are the ECMWF model forecasts for the times of the satellite data, which is another neat feature of this display. We should not be too surprised by the general agreement observed as the ASCAT winds are used to seed the model runs.  

In summary, this ScatSat data seems a boon to ocean navigation. You can get it the same way we get the ASCAT data underway with an email request to Saildocs. The key is knowing the file name to ask for. We have a graphic index to the file names in our textbook Modern Marine Weather.  You can figure these on your own from the main index page at STAR Center.  Just hover over a location to read the link. Then make a list.

The data images that cover the first 700 mi on the way from San Francisco to Hawaii would be this request to


This gets you the ascending and descending passes. When the images are full of data, the email is about 100 kB each.  When empty of data, about 15 kB.

A good way to learn the value of this data is compare these images to the GFS or other models multiple times a day for several days.  You will then be wanting to make your file list so you can ask for them at any time.

You can predict where the satellite will be using this link:

There are mobile apps and even screensavers that will track satellites that can be helpful.

Thank you India Space Research. Recall that we benefited from their OSCAT program for several years (2009 to 2014) when the amazing US QuikSCAT failed after a long and successful career.

[And a note of thanks to navigator Jeff Feehan, for directing me to the new Multi-Platform Product Viewer at KNMI, which was added here after the original was posted.]


Note added Apr 7.

Since publication of this note, we have had two emails like the one below, indicating that the data are offline, and sure enough when this notice comes we do not see data at the STAR Center.  In each case, we get another email the same day or next informing that the data are back online.  So this is just a heads up that this is not an official source of data.

For what it is worth, the ASCAT data itself as obtained from the STAR Center is also described after all these years as still Experimental.  But we have not seen outages of that data.

OSI SAF service message #2041
Sent on Tue, 07/04/2020 - 06:39 UTC

Title : ScatSat-1 winds unavailable

Message :
We are currently not receiving ScatSat-1 input data. The OSI SAF ScatSat-1 
winds (OSI-112-a, OSI-112-b) are therefore not available. Note that you can
check the current processing status on [1].

This service message has been published on OSI SAF web site :

This email has been sent to people who chose to subscribe to wind service
messages on OSI SAF web site.
You can update your subscriptions in your profile on


Saturday, February 29, 2020

Evaluating a Weather Forecast — Slides and Notes Only

These are the slides with short notes and references for the video talk given at the  Cruising Club of America Seminar,  Feb 29, 2020.  You can see the slides in the video, but they do not have the notes and live links, and are not in high res like here. (Disregard the first line below from the first slide; you are looking at that link!)
• This talk is largely highlighting topics whose details are important, which are covered in     
  the book.
• Link to

• I used to say “but they are not marked good or bad," but this is no longer true as we now  
   have probabilistic forecasts for marine weather, as we shall see.
• We have lots of ways to check the timing of a system, mainly with pressure.
• If a forecast is in doubt then just do "half of what you want" until the next forecast in 6 hr
• We are not going to just download a GFS grib and make a decision.

• Print out each of the maps viewed side by side for quick overview 
• Both available underway by email request to Saildocs
• Learn something about the map AND something about the text 
• Here we learn the speed of the high moving E 
• We sail around Highs, so this is crucial info 
• First 3 days of GFS is usually good, but we need routes farther, so need extended forecasts. 
• Text forecasts (Storm Advisories) are especially important when dealing with tropical 
   storms. They are available from Saildocs.

• GFS overlaid onto OPC analysis viewed in Expedition compared to official Forecast  
• We want to use the digital data, so we use OPC maps to check them. 
• Compare at several forecasts. ie compare 06h forecast made at 12z with the 0h 
  forecast made at 18z
• Timing is everything. GFS 0-96h is about 4h, longer forecasts add an hour 
• Discussion tells which models were used and why 
• Both available underway by email request to saildocs 
• Auto load maps in Expedition and OpenCPN. One button click to get georeferenced maps 
   right on your nav screen 
• Let’s look at the zoomed area 

• Two GFS isobar parameters — needs a talk of its own! 
• Viewed here in LuckGrib, which has a tutorial with references on MSLET
• Saildocs switched to MSLET as has LuckGrib. Both still call it PRMSL to minimize  
  confusion in display options. WRF models also use MSLET.
• With MSLET we can better understand wind flow relative to isobars (gradient and 
• Standard PRMSL is effectively an average over ~80 nmi 
• Especially valuable around tropical stormsbut GFS is still not dependable there. 

• Accounts for patches of model wind not consistent with PRMSL isobars 
• Set isobar spacing to 0.25 mb and often see the trof of a front, or some indication of a trof       preceding a front (squall line?) 
• We can set the GRIB viewer or nav software to interpolate the model forecasts to
   the time at hand.
• This wind check process will be illustrated as we proceed

• New automated way to compare measurements to models and buoys
• Many videos and info online at
• Calibrated instruments are crucial, as is corrections for wind heights
• Designed for analyzing Expedition log files… but could be other sources.
• N2k gateways provide ways to log wind data from any system
• Our example only with a buoy 
• We added the error bars and redid the base figures
• Automated for buoys with internet connection. We have a note on how to use the buoy
   function underway. Buoy reports by email.

• Models and forecasters do not use all ship reports. Many subtle tests applied to the reports 
• Can get either map by email request. Only BW version can we get at 50% size reduction
   from saildocs.
• This bottom right corner or ridge is crucial to Hawaii races, rare to have reports here 
• Either one can be downloaded automatically, georeferenced in opencpn or Expedition. 
• We look at the ship reports on the surface analysis for consistency.  

• Recall nominal wind barbs are ± 2.5 kts
• SST > 80F for hurricane.
• Stable air would make it lower; very unstable only up 10% or so, not 14 to 25.
• Curvature could raise is about 10%. But not in this case
• Ship reports can distort the isobars... ie the forecast adjusts to account for the report
• Something to think about when not in agreement.
• Cannot rely on “wind&waves” maps. Sometimes lots on barbs; more often just
   those from the reports.
• Strange format to emphasize we use 80% and not the more standard 60%
• Practice with GFS wind and pressure to see how it works out.

• One click download of all buoy data in Expedition... super nice feature
• Mouse over shows what you would read at NDBC
• Procedure, download the buoys then set model time to match the buoy times,
   which may vary from one to the other.
• Expedition brings in latest set of buoy data but saves it when you re-load
• This will be a feature of the S-412 weather overlay to S-100 ENCs.

• NDBC stopped sending them to NWS. No reason given. No time for comments.
• No other NOAA source, hence moving weather  data delivery to commercial third parties
• Still in FTP instructions
• We can get the data from Saildocs,  one line per buoy
• Or subscribe specifying days to send, starting time UTC, and hourly interval
• Saildocs has a shortcut: send buoy51000.cur, which works for most buoys but not all. 
  The above link covers all sources as well as land stations such as wpow1.txt

• All reports within past 6hr within 300 nmi.
• Server checked on the whole minute and mail sent.

• Provides text report plus an attached GPX file of the data
• Includes range and bearing to report.
• Pressure in inches but we convert to mb in the GPX file

• Inset shows ship report compared to GFS at the same time
• Can load the GPX reports into any nav program. This example is OpenCPN
• Need to adjust GFS model time to match each report
• We can ID ships to projects when we get later reports
• Can spot isobar shifts. Use red data to figure where that isobar should be.
• Ships with full reports likely have better data… i.e., they are more careful.
• Isobars are moving west at this time.

• GRIB ASCAT available from Expedition and LuckGrib.
• Timing the main issue: GRIB versions are 15+ hours old; graphic index is updated
   hourly, and latest pass to show up will be ~ 2.5 hr old
• Latest useful pass could be 6 to 10 hr old.
• Generally get useful data about 3 times a day, since it does not have to be where
   you are to be useful.
• Bottom right is overlay with GFS in Expedition. ASCAT red, GFS black. Shows good  
   agreement east and west of Cabo, but very poor to the SE.

• File name index in Modern Marine Weather, the main reference for ASCAT
• Also ASCAT B and C… A plus B makes a somewhat broader swath. Mixing data
   about an hour apart.
• This is a most powerful tool, but it comes with a learning curve. See textbook.
• There is also another set of sat data from Navy WindSat, but it is not often used by NWS.

• Text discusses satellite passage apps to help predict when you get the next useful data
• This is rough sketch
• Added so you can read from the slides.
• Can plan out coverage before the race or voyage, so you know specifically when new data
   will be available and you can add it to your sources timetable.

• In the NH we sail around Highs
• Omega block can last 10 days
• If a blocking High, the map will look like this tomorrow.
• Else it will likely change
• We can get these 500 mb maps from Saildocs 

• Main reference on role of 500 mb maps is from Joe and Lee article. Online at the Mariners
   Weather Log and the book by Chen and Chesneau Heavy Weather Avoidance
• Centers of Action discussed in Modern Marine Weather

• See text for more discussion
• These general rules discerned from reading a lot of forecast discussions.
• Operative rule: there is no dependable rule; just guidelines.
• BUT now we have much more to work with… as we shall see.

• We will follow up on this example
• Let’s see what happens at about 72 hr out along a Pacific sailing route
• SD shows the variations between the ensemble members at each grid point
• A small variation means forecast not as sensitive to input variations and thus it is more
• Large SD means forecast  is not as dependable. Small input changes lead to large forecast changes.
• Now back to last slide on 500 mb indicating maybe weak forecast about 72 hr out

• SD of 2 kts is more or less consistent, ie ± 2 kts or so is agreement.
• At 44h, 68% of all winds will be within 12 to 16 kts
• Compared to 68% within 0 and 28 kts at 90h with divergence starting about 72h
• Out 84 hours out the SD blows up. Average is about same as control, but member 
   runs diverge a lot, meaning forecast is fragile.
• In short, after about 80 hr the forecast is likely in question.

• In stable winds we go out 5 days and still have small SD

• Another way to look at forecast dependability
• NBM Oceanic covers most of the globe to 20S
• Probably best extended forecast, meaning more than 3 days or so
• Maybe best on the ocean for all times. This is not clear yet. Could be GFS is best for first 3 days.
• Now look at another example first GFS only, then the blend, then the GEFS

• With GFS we see the wind and pressure over time. This is a determinist model. We learn the values, 
  but no uncertainties.
• We do see a veer when the trof  the the nW crosses at 66h
 BUT that is all we see!
• Now let's look at this same forecast with NBM

• From h0 to h41 wind is ± 2 kts, which is as good as it gets
• Then it blows up
• Forecast is weak after this point
• Now let's compare this same forecast with GEFS (ensemble)

• Here we see mean and control winds along with SD values
• When we see both GEFS and NBM indicate weakness or strength at about the same time
   and place, we are likely safe  in believing it.
• Next we look are some really new resource, just one week old

• This forecast (a blend of model outputs) added standard deviation (SD) just last week.
• Extends about 400 to 600 nmi offshore.
• We see notable light patch running NE of Guadalupe Island… but no more details.
• Could be best regional model… includes HRRR and NDFD. We need to study this one in
  comparison to HRRR, which we know works well most of the time to learn more about
  local  forecasting.  We have a note online on this topic. Use of Regional Models

• Another example of very new data, i.e., SD In NBM conus just last week
• SD in NBM v3.2 new as of Feb 20
• The light patch has good SD
• But before and after are very uncertain ± 5 kts of wind
• Next add lower valley marker

• Let’s look at this point on Google Earth
• Here we see what likely leads to the uncertainty in the forecast
• About 1,200 ft each side of 150 to 200 ft
• Channeling is very sensitive to upstream wind direction
• In short, SD has shown us where the forecast is weak… which we do not see from
  any deterministic model

• New resources that help us evaluate a forecast.
• Note.  In this study we found forecasts that differ quite notably can still yield about the same optimum routes, with different finish times.  So an eye to the polar diagrams can show how sensitive the route is to the actual wind direction. But as stressed many places, the polars have to be right, or the routing has little meaning, if not a distraction. Also we obviously do not want to take risks for small gains, even though the optimum route will do so, namely go for whatever makes it faster. Thus again the value of the statistical forecasts.

Speaker's related sites:

Frank Bohlen's articles on the Gulf Stream

Ocean Prediction Center (, Jos Sienkiewicz, Branch Chief)

Locus Weather (Ken Mckinley's weather services) (Stan Honey and Sally Lindsay Honey's 's articles and videos)

Seattle Forecast Office (Kirby Cook, Science and Operations Officer) (David Burch HQ)

References cited in discussions:

Use of Regional Models

Inverse Barometer Effect

Free Marine Barometer app (iOS and Android)






NOMADS (NCEP source of model data)

How to Obtain Custom Grib Files

How to Combine Grib Files