TYPES of forecasts:
“EarlyBriefing” is sent each morning (90%+ of the time) in order to get you the information you need to make decisions about today.
Normal forecast usually arrives mid-day.
“InterimTropical” is issued for ALL REGIONS when significant Tropical weather is possible.
Other “Interim” emails may be issued for Earthquakes/Tsunamis or significant weather events or emergencies.
“Schedule” emails alert you to changes to Marine Weather Center operations.
Normal Forecasts are in 5 parts:
1.) Recent observational data (ASCAT, sometimes Microwave satellite winds, BUOY observations).
2.) Analysis of current satellite / RADAR imagery.
3.) Synopsis of weather features, sometimes with a general discussion of predicted conditions in some areas of the region. Synopsis is either by-area or day-by-day, depending on how I can best generalize the pattern.
SE Caribbean typically Martinique-Trinidad.
NE Caribbean typically Dominica-Leewards-VI-PR (always S Coast of PR unless specified)
DR is always N Coast of DR unless specified
Venezuela is offshore Islands near 11N-12N (LosTestigos-LosRoques) unless specified
ABCs is Aruba-Bonaire-Curacao
NOTE: for locations near the boundary between areas, you should average conditions in the 2 areas. For instance, if you’re transiting waters between StLucia-Guadeloupe, and I only give a forecast for SE Caribbean / NE Caribbean, then you’ll average the 2 forecasts.
When conditions vary within an area, I’ll usually break that area into smaller pieces.
4.) Outlook for 5-10 days, or as far into the future as I can make a guess.
5.) Specific forecasts for Precipitation (and squalls) / Winds / Seas.
This is normally divided by weather-parameter (Precipitation first, then Wind, then Seas). One reason for by=area: if the forecast is wrong, you may see conditions similar to adjacent areas…so it’s useful for you to be aware of the forecast for adjacent areas. Occasionally (during significant weather events), I’ll break this section down by LOCATION, with each parameter (Precip, Wind, Seas) discussed by location.
PRECIP – I usually discuss coverage (in order of increasing coverage:
isolated / scattered / numerous or widespread). I also discuss character of precip (convergence can be nasty but not severe, while convective has potential to be severe), and try to offer a guess as to wind anomalies in the precip – sometimes as wind speed you should add to gradient wind (i.e. “5k enhanced wind”), sometimes as total wind in squalls (i.e. “to 30k”).
WIND – speed & direction, as follows:
Direction: I usually give this in degrees TRUE. The forecast is NOT so accurate that you should really expect wind of 070-degrees to be from 070-degrees. My main reason for specifying 070-degrees is to offer you insight into the TREND in wind direction. 070-degrees is just about ENE. 060-degrees is ENE. A change from 070 to 060 suggests a BACKING TREND to the wind direction, rather than a forecast for those numbers specifically.
You may notice the < sign. You should read “<” as the word “becoming” – it describes a trend. For instance 090@12<15 means 090-degree wind building from 12k to 15k during the applicable period.
Speed: in kts. I usually only give a specific number, such as 18k. You should mentally “bracket” this ± 20%…in the case of 18k, you should interpret this as a forecast for 15-21k. “g” means “gusty”. Forecast 18g23k means 18k sustained, gusting 23k. Of course, you’ll bracket this, so you’ll expect 15-21k sustained, gusting 19-27k.
Two reasons I give specific wind velocity, rather than brackets: 16k & 18k are both “15-20k”. 18k today & 16k tomorrow suggests a TREND of decreasing wind. TRENDS are very important in your decision-making. Also, it saves several characters – many clients receive forecasts on slow email connections, and every character we can shave off we do.
During significant weather events, when I feel it’s more important to convey the overall wind possibilities than to convey coverage of squalls, I’ll sometimes use the letter “s” (for wind in squalls) in the wind forecast. For instance, 30g37s50k means 30k sustained wind, gusting 37k, squalls to 50k.
Sometimes, I give wind direction in “cardinal” units (N or NW or SSE, etc.), or wind speed in brackets – I do this when I’m so uncertain that I can’t get more specific.
REMEMBER TO CONSIDER WIND IN SQUALLS! My “wind” forecast is generally for wind in the absence of significant squalls. ALWAYS refer to the “PRECIP” forecast for wind in squalls!
Seas are a bit messy to interpret. Forecast for seas assumes you’re in deep water, and in an area completely exposed to the Ocean. Seas within the Islands of the Virgins will be lower – the extent to which they’re lower depends on your location. Seas (and wind) between Islands in the Windward/Leewards are typically higher than over open waters (away from Islands)…and the Islands also bend wind & sea directions locally.
Again, my forecasts for wind & seas assumes you’re in exposed areas, and away from the local effects imparted by Islands…
What you really care about is the ACCELERATION of motion of your vessel due to seas. By acceleration, I mean any change in motion versus what you’d have with flat calm seas.
6′ seas with a 7-sec interval from the E will have a different impact on your vessel (seas will cause different acceleration) on a 30′ sailboat versus a 50′ sailboat…and different for any given boat if you’re heading in a different direction, etc., etc.
But, IN GENERAL, seas with an interval much greater than 1-second-per-foot of height are swells. And swells are less-bad than wind-driven chop.
For instance, 6′ seas with 6-sec interval is a wind-chop. 6′ seas with 8-sec interval is a short-interval swells. 6′ seas with a 12-sec interval are long-interval swells. Swells are less-steep, and, therefore, impart less acceleration on the vessel for a given height.
As for adding height…unless different wave trains PHASE, there’s no need to add height. However, individual waves from each wave train DO PHASE. Think of it this way:
If you encounter the crest of an individual 6′ NE swell at the same as you encounter the crest of an individual 6′ E wind-driven wave, then the total height of the perfectly-phased wave will be close to 8-10′ (about 50% larger).
Let’s say you have a 6′ wind-chop and no swell. The 6′ wind-chop causes height of the ocean on which your vessel is floating at one instant in time to be 3′ higher or 3′ lower than the mean sea-level.
But if the crest of the 6′ wind-chop occurs where the height of the sea is 3′ above mean sea-level (because you’re at the top of a swell)…then, at the top of the phased swell-and-chop, you’re 6′ above mean sea-level.
Since the wind-chop and swell are of different intervals, they are not likely to remain in-phase at the trough of that wave, so you’re not likely to be lower than about 3-4′ below mean sea-level at the trough…for a total height of 9-10′ from crest-to-trough in the above example.
So, as a rule, when there is a swell and a wind-chop, I would add up-to-50% of wind-chop height to the swell to arrive at the total height of seas.
NOT all seas you encounter will be this high…
When there’s only 1 wave-train, as many as 1-in-200 waves can be 50% higher than the significant sea height.
When there’s a combination of wind-chop and swell, and especially when they’re from different directions, SOME waves will phase (at-least partially) and cause many many more waves to be 50%+/- higher than the swell height.
Here’s how I’d use my sea-state forecast in decision making:
Assess whether each wave train, by itself, is acceptable.
If so, then assess whether adding 50% of wind-chop height to the swell would be acceptable.
If so, then assess whether wave trains are from different directions, and, if so, assess whether the acceleration from DIFFERENT DIRECTIONS imparted on your vessel is acceptable (you’re pushed in 1 direction by 1 wave, then immediately pushed in another direction by a wave from the other train).
When making decisions, bear in mind the direction you’re traveling, and how your motion influences the apparent interval of seas (if you’re traveling into the seas, you’ll shorten their apparent interval / if traveling with seas, you’ll lengthen their apparent interval (and thus lessen acceleration)).
Weather Forecasting & Vessel Routing
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