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Old 08-31-1998
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Weather Forecasts for Sailors

Where do marine weather forecasts come from? Basically, they originate from the National Weather Services Marine Prediction Center in Camp Springs, Maryland, and local weather service offices where meteorologists prepare analysis and forecast. But how do they really begin?

Analysis and forecasts begin with the gathering and processing of weather observations, which include: cloud height amount and type, visibility, wind speed and direction, air temperature, pressure, sea surface temperature, sea and swell height and direction, ice conditions (if any).

Data comes from ship observations, weather buoys, satellite imagery, airplane sensors, weather radar, coastal stations, individual reports and weather balloons. The data, which is gathered every six hours worldwide, is electronically sent to collection centers and then used to run a variety of computer database models. These take initial conditions and project anticipated conditions. The data collection, overseen by the World Meteorological Organization, is then sent to Cray super computers, which run numerous weather models simultaneously, chugging through millions of data points on pressure, temperature, wind, etc. Within three to four hours of receiving data, forecasts are prepared.

Analysis and forecasting
Analyses gives snapshots of current weather, but does provide limited information on future conditions. On the other hand, using short, medium and long-range models, forecasts provide insight to future weather. Short-term forecasts generally go out to 24 hours, medium range to 120 hours (5 days), and long range out to 240 hours (10 days) and beyond.

There are eight regional models, five global models, three ocean wave models and three reanalysis models employed by the Marine Prediction Center. Models use a three-dimensional grid. In the United States, warnings and forecasts for offshore Atlantic and Pacific waters originate at the Marine Prediction Center while forecasts for coastal waters out to 60 miles originate from local forecast offices in Taunton, Massachusetts; Sterling, Virginia; Seattle, Washington; and Monterey, California.

Marine Prediction Center meteorologists analyze and forecast surface and upper air conditions, including winds, waves, fog and ice formation. In addition, the center issues high seas warnings and monitors incoming consults with the U.S. Coast Guard and other U.S. agencies.

Ensemble forecasting, a technique now being refined by the MPC, combines output from several models to arrive at a forecast. Ensemble forecasting is valuable in predicting precipitation and extreme weather events, such as blizzards and rapidly intensifying lows. In future years global models, with far-improved resolution, will be run out as far as seven months, with lower resolution runs made for one month. Ozone and atmospheric contaminants are also being incorporated into forecasts, as are ocean currents.

Most models are run continuously, with outputs produced from once a day on up to hourly. As computer processing power improves, models are improving, running more frequently. The hurricane forecast models, for example, have improved accuracy 1% each year.

When models are more accurate than reported data, forecasts tend to be wrong. But as more accurate initial data is received, errors in final analysis and forecasts are being reduced. Other reasons for forecasts to be off can be due to the user being in a different location than expected. The boat's position, for example, is on opposite side of a front from where thought. Other inaccuracy occurs when the expected weather (such as the arrival of a cold front) is several hours ahead or behind its predicted arrival.

Physical location has a great effect on forecasts since areas with stable patterns, such as the tropics, can be predicted for longer periods than higher latitudes where weather is more dynamic. Fall and spring are seasons also play into this instability when mid-latitude weather is most dynamic.

Voice, text and charts weather information is delivered to U.S. Coast Guard and to the National Weather Service radio stations where every six hours they are broadcast over VHF and SSB voice frequencies and put out charts on weather facsimile frequencies. Weather information is also delivered via the International Maritime Satellite System (Inmarsat) to vessels with satellite communication systems.

Weather resources
Forecasts areas are listed in the U.S. Government publication entitled Selected Worldwide Marine Weather Broadcasts. In this book, charts, text descriptions and a weather station listing are provided for all ocean basins. A list of weather facsimile stations is available on the Web at: http://our world.CompuServe.com/home pages/HFFAX.

Tropical weather information for Atlantic, Pacific and Gulf of Mexico is prepared by the National Hurricane Center located outside Miami, Florida, and broadcast via the Coast Guard stations in New Orleans, Louisiana. This information is also found at www.nhc.noaa.gov/index.html. A complete listing of National Weather Service analysis and forecast information is located at:www.nws.fsu.edu/wxhwy.html.

Information on the Marine Prediction Center is found the Web: www.ncep.noaa.gov/MPC.

Weather information outside the U.S.
In countries other than the United States, forecasts and analysis are performed generally in the same manner as here, with modifications based on local needs, geographical location and weather service capabilities. Many countries now offer telephone consultation service, in addition to traditional methods. In Canada, for example, you can talk with a professional meteorologist for $1.95 per minute (2-minute minimum) by calling (900) 565-5555. For inquiries particular to Nova Scotia the number is (900) 565-6565. There are also free-recorded forecasts, a list of which can be obtained by calling Environment Canada at (902) 426-4926.

On the other side of the equator, Australian weather forecasts and analysis for can be found on the WWW at: www.bom.gov.au, and for New Zealand by contacting the meteorological service at this e-mail address: service@met.co.nz. Included at these addresses are satellite photos, weather radar images, forecasts, warnings and educational material.

Weather analysis and forecasting is improving faster than ever and will continue as computers, communication systems and computer models grow and develop. But forecasts and analysis will always be subject to the whims of nature, something which sailors appreciate.

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