Denis Webser,

by Denis Webster, Tiger Lily II

Denis Webster is a meteorologist who just completed over 20 seasons cruising the Caribbean aboard his Hylas 47, Tiger Lily II. During the winter sailing season he can be heard on the SSB at 4420 USB at 7:45 AM AST.

Introduction

The internet contains a multitude of sites to obtain weather information and forecasts. They range from information for marine interests through aviation. All of these sites are useful when preparing a weather forecast. This document is an attempt to organize a few websites in a logical format to provide steps to a final marine forecast. One may also use a completely digital output i.e. GRIBS, however the outcomes may be very different depending on the weather situation. As a professional meteorologist, I always like to take a very organized approach to maximize the use of available websites.

I use the following websites and suggest these steps when you get there:

1. National Hurricane Center   (steps)
2. The Atlantic and Caribbean Tropical Satellite Imagery   (steps)
3. ASCAT (The Advanced Scatterometer)    (steps)
4. Animation Mosaique Antilles    (steps)
5. NWS Radar Images from Puerto Rico/Virgin Islands   (steps)
6. Jenifer Clark's Gulfstream   (steps)
7. NHC Tropical Prediction Center Marine Weather Discussion   (steps)
8. Passage Weather: Computer Model Information   (steps)
9. SFC Obs   (steps)

National Hurricane Center

Click to open the Marine Forecast section

Scroll down to Graphical Products and open Tropical North Atlantic map

Click on the map to magnify

The map will show major weather systems over your area and provide a framework for looking at all of the subsequent websites. You may want to note the positions of major fronts and highs that will affect your area.

Atlantic and Caribbean Tropical Satellite Imagery

Click on Atlantic Wide View and scroll down to GOES imagery

Find the Flash column and click on Rainbow. If your browser doesn't support Flash, you should try the link for the gif instead but you will not have all the options below.

When the picture opens, it will be in motion. Click to include LatLon, HDW-low and NCEP fronts across the top

The lower level winds are displayed in red.

The cloud coloration runs from white in the lower levels to red depending on cloud height.

Use the cloud motion and speed to get an estimate of current conditions.

The wind speeds are rough estimates of surface winds.

ASCAT (The Advanced Scatterometer)

There are 2 forms of orbit – ascending pass (S->N) and descending pass (N->S). If your area is not under the ascending pass at the top of the page, scroll down to the descending pass just below.

Choose an orbit close to your area and click on it.

Note the time of the pass to be sure it's not old

The surface winds are displayed by color according to the scale at the top

Look at the wind direction and speed for your area.

Animation Mosaique Antilles

This site displays the Martinique and Guadeloupe radar images of showers passing through your area. Intensity can be related to the scale on the right.

NWS Radar Images from Puerto Rico/Virgin Islands

You can also open NWS Radar Images from Puerto Rico/Virgin Islands if you need information for that area. Click on Composite Loop, to set the image in motion. (Flash plugin required) . All radar beams radiate in a straight path from the station and do not follow the curvature of the earth. Thus the beam will only paint the tops of the highest clouds at the extreme end of the range (in this case - out at Anegada.)

Note

At this point you should have a good picture of weather conditions for, area including clouds, the wind speed, direction and approaching showers. However before we start into the forecasting part, check 2 more internet sites to ground truth the information.

Jennifer Clark's Gulfstream

Open 'Ocean and Weather Sites", and then open Interactive Marine Observations

If you are not interested in the information she carries, you can go directly to the National Data Buoy Center

Now you can click on the box for the Eastern Caribbean

The closest buoy to the island chain is 42060. You can also look at buoy 41052, which is in the Virgin Islands These buoys provide wind direction, speed, sea height and if you scroll down, a detailed wave summary.

NHC Tropical Prediction Center Marine Weather Discussion

Open the National Hurricane Center Tropical Prediction Center and click on "Atlantic & E Pacific" (under "Marine Forecasts") and scroll down to 'Text Products' .

Under the 'Atlantic' column, click on 'Marine Weather Discussion,'Atlantic'

This bulletin is designed for forecaster to forecaster information. There are a couple of nuggets of information. The bulletin is divided into regions. Scan the section under Caribbean Sea and Tropical North Atlantic. Note the model preference and the confidence level. Read the section to see if there is any information pertaining to your area.

Now return to the 'Text Products' page and click on the 'Atlantic Interactive Zone Map'.

Hover the cursor over your area in the Caribbean chain. Now is the time to evaluate the 5 day forecast with the information you've gathered so far. Check to make sure that the wind direction and speed, the sea heights and the cloud cover and showers agree with the NOAA forecast. If you have previously downloaded GRIB files, check to see how NOAA has modified them in their forecast.

Computer Model Information

Open Passage Weather

This is an interactive program which provides one model, the GFS model in a gridded form. It's the basis for most GRIB files.

Open the West Indies large box.

The first map is the 'wind' output. You have previously looked at the current satellite and radar imagery and ground truthed it with buoy information. You have also looked at the 5 day forecast for wind, seas and precipitation. Now it is time to compare that information with this GFS raw output.

Note that passage weather is raw output from the WW3 and GFS models. The professionals will let you know how well these models are handling current conditions at the marine weather discussion.

Step the model forward on a day by day basis to see whether it differs from the NOAA forecast.

Now scroll down to the last map, Wave Height and Direction

This is based on the WW3 – wave watch 3 model . This model is notoriously inaccurate, particularly for swells in the tropical North Atlantic. Animate the map to get a broad picture of what the wave direction and height is forecast for the next week.

Lastly, there is another map called Visibility and Precipitation which you can also animate. It is a somewhat broad picture.

If you have limited internet, you can save these files by clicking on "download all ...(.zip file)" for replay when you are away from the internet.

At this point, you should formulate your own forecast, making adjustments for variations from the information that you've just collected. I encourage you to note down your opinion and look at it over a period of days to verify whether you interpreted the information correctly.

Just before you leave on your passage, one final site can be visited:

SFC Obs

Open "SFC Obs"

This site is designed for aviation but provides a useful look at current weather conditions at airports. You can check wind direction and speed temperature and precipitation and set your barometer. Bear in mind that airports are normally built in sheltered locations and you can expect higher windspeeds at sea.