Planning a strategy to deal with thunderstorms should begin well before the day you launch a flight. The Weather Channel, on television or on the internet (weather.com), is a great source for long-range planning. Watching a couple of days in advance of a trip can provide a pretty good idea of whether the weather's going to be benign or demanding. Another source of information a couple of days out is the NOAA's Storm Prediction Center at spc.noaa.gov. When you click on "conv outlooks" you're presented with maps and text detailing the potential for thunderstorms during the next three days. The current day's convective outlook is available for several different times, in both text and graphics.
Today most of us are using Direct User Access Terminal Systems (DUATS) or other online weather services to do preflight planning. When you enter your flight plan in DUATS or detail it to a Flight Service Specialist at 800/WX-BRIEF you have the option of three types of preflight briefings: the "outlook" (for flights more than six hours away), "standard" (for most flights) and "abbreviated" (to update specific weather items after you received a standard briefing).
Among other weather products you'll get during a briefing are meteorological aerodrome reports (metars), terminal aerodrome forecasts (TAFs), area forecasts (FAs) and, perhaps the most important for scoping out where storms are expected, convective sigmets.
Metars, the hourly reports of conditions at airports, can give you an idea of what's currently happening at airports along your route. If there are thunderstorms at an airport, a "special" observation is broadcast and the next metar will indicate the times the thunderstorm or storms began and when they ended. If there are separate storms, the beginning and ending times will be reported in a metar only if the intervening time exceeds 15 minutes. The storm will be described as "occurring at the station" when it's within five statute miles, "in the vicinity of the station" when it's between five and 10 statute miles from the observer and "distant from the station" when beyond 10 statute miles.
TAFs usually cover 24 hour periods. The main body of the TAF describes the weather that is expected to exist for at least half the time and have a greater than 50 percent chance of happening within five miles around the airport. The TAF forecasts significant weather including thunderstorms, wind shear, freezing precipitation, moderate or greater rain, accumulating snow, winds, and wind changes and gusts.
FAs provide an 18-hour synopsis (a 12-hour forecast plus a six-hour outlook) of expected weather conditions in each of six regions of contiguous states. The area forecast is a good source for en route weather and can help determine the conditions at airports that don't provide terminal forecasts.
In preparing for a flight, the most serious indications of potential problems with thunderstorms come in convective sigmets. Convective sigmets are significant meteorological messages that contain information specifically about thunderstorms. The convective sigmet contains three parts that provide advisories for the eastern, central and western sections of the country. Convective sigmet bulletins are disseminated every hour at 55 minutes past the hour and are valid for two hours. Each of the sigmets is defined using VORs on the In-flight Advisory Plotting Chart. (In-flight Advisory Plotting Charts can be downloaded from the NOAA website at aviationweather.gov/static/info/advsry/.) By connecting the "dots" on the In-flight Advisory Plotting Chart you can outline the area of convective activity. A convective sigmet is issued when any of the following minimum criteria are met:
• There is a severe thunderstorm containing hail 3/4 inches in diameter with winds of 50 knots and/or a tornado; • There are embedded thunderstorms; • There are lines of thunderstorms greater than 60 nautical miles long with greater than 40 percent coverage of significant radar echoes; • There are areas of thunderstorms greater than 3,000-square nautical miles in size with greater than 40 percent coverage of significant echoes; or • In the judgment of the forecaster the thunderstorms pose a threat to aviation operations.
It's important to know that if there are no thunderstorms that meet the criteria, the sigmet bulletin is still transmitted, but indicates there is no convective sigmet. But the lack of a convective sigmet doesn't mean there are no thunderstorms, only that any that do exist don't meet the criteria.
Another excellent source of weather information is the Aviation Digital Data Service (ADDS), a joint effort of NOAA Forecast Systems Laboratory, NCAR Research Applications Program (RAP), and the National Centers for Environmental Prediction (NCEP) and the Aviation Weather Center (AWC). Available at adds.aviationweather.noaa.gov, ADDS combines information from National Weather Service (NWS) aviation observations and forecasts, and makes them available on the internet along with a number of visualization tools to help us use the information for flight planning.