An abundance of weather information is available these days, requiring a navigator to be thoughtful and careful in selection. Weather information must be used correctly and efficiently, since too much information and mis-interpretation can often cause as many difficulties as insufficient information.
Good use of weather information first requires a voyage to be considered either "coastwise" or "open ocean" and then divided into three phases: Pre-departure, en route and pre-arrival.
A coastwise voyage is one where a vessel will be within 25 miles of land, while an offshore voyage involves being more than 25 miles from land during a substantial portion of the trip
Why is this distinction needed? For two reasons: Weather occurring along coast -where land and water meet -can be quite different than weather in the open ocean. Secondly, marine weather forecasts are divided into distinct categories based on geography and accuracy, which define how they should be used.
A pre-departure phase centers around go/no-go criteria such as; wind and sea conditions at a harbor entrance, combined effects of wind and seas, tides and currents, ability to gain sea room, type and number of other vessels-especially large commercial ships.
For Example; departure from Portland Maine when winds are blowing from the east at 25 knots would be difficult due to large swells that converge on the harbor's east facing entrance. Large tankers regularly transit this entrance, leaving little room to maneuver for vessels approaching each other. However, once out at sea 25 knot easterly winds could be easily handled.
During the en-route phase weather concerns center on keeping a vessel sailing on the most efficient and comfortable track. Weather patterns hundreds and thousands of miles away need to be analyzed to determine the type of wind and seas they will bring. A key word during the en-route phase is "big picture", focusing on weather patterns during the overall voyage, not just day to day events.
When a destination is a day or two away then weather analysis shifts to a pre-arrival phase. Concerns now involve more local variables such as; interaction of wind and waves with coastal variables such as; interaction of wind and waves with coastal geography, tides and currents, shoaling, swells, funneling or converging of winds, and possibility of fog.
SHORT TERM VHF VOICE FORECASTS
Short term VHF voice forecasts are used just before departure and arrival to determine prevailing local conditions. These broadcasts cover a 6 to 24 hour window of time and are designed to give mariners information on immediate or daily conditions, but are not intended for use in making long term plans.
They are updated every few hours, or more frequently if needed, and are intentionally restricted to a broadcast length of 4-6 minutes. Discussion of long term weather features are normally not included, though significant features which may not be within the forecast area, such as hurricanes, are often highlighted.
NOAA operates 400 transmitters, each having a range of 40 miles, broadcasting short term VHF forecasts. Seven frequencies, from 162.400 TO 162.550 MHZ, are used to provide overlapping coverage. Details on VHF broadcasts can be obtained by contacting NOAA at: National Weather Service, warning and forecast branch (ATTN:W/OM11), 1325 East-West Highway, Silver Spring, MD 20910.
A chinese proverb reads "one picture (weather chart) is worth more than a thousand words" and this certainly applies in weather forecasting. Every six hours the marine forecast branch at the US National Meteorological Center produces synoptic (large area) surface weather analysis and prognosis charts for the Atlantic, Gulf and Pacific oceans.
Analysis charts are a weather snapshot, showing weather features at a specified (valid) time. Prognosis charts give a view into the future, at twelve and twenty four hour intervals, out to five days (120 hours). These charts provide a wealth of information; displaying high and low pressure centers (with their 24 hour track), isobars, fronts, tropical cyclone position, wind direction and strength, cloud cover, and ship observations.
Charts are obtained in a number of ways: Dedicated weather fax receiver, computer and modem connected to SSB receiver, computer connected to satellite reception system, computer and modem connected to National Weather Service via phone (cellular or landline), or through a private weather forecaster.
Connecting to the National Weather Service (NWS) via the Internet using a laptop computer, modem and phone is an expeditious and low cost method of obtaining charts. Using either HTTP or FTP protocol charts for Atlantic, Gulf and Pacific areas can be downloaded and printed. There is no cost for this service other than normal phone charges.
Charts obtained from the NWS via the Internet are the same as those received via Wefax, with an advantage there is no limitation on when charts can be obtained due to a broadcast schedule, as is found with SSB Wefax Broadcasts.
NWS marine radiofacsimile charts can be found on the Internet at:
SOURCES OF USEFUL WEATHER INFORMATION:
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