Page 30 - Volume 15 Number 8

With a 5-statute-mile radius, the NWS considers a TAF to be a point forecast and not a zone or area forecast. The red circle denotes the size of the terminal area for Charlotte/Douglas International Airport (KCLT). While this “area” may look kind of large on this zoomed-in image, this circle would be no larger than a dot from a pencil point if shown on a map of the entire United States, hence a point forecast.
to quantify their uncertainty. Of course, this approach tends to frustrate many pilots.
So, is a TAF a probabilistic forecast? The simple answer is yes. If a forecaster places TSRA in a FM (from) group such as FM221800 22014G21KT P6SM -TSRA BKN025CB, they are suggesting to the pilot that there’s a 50 percent or greater chance for light rain and thunderstorms to impact the terminal area beginning at 1800 UTC in this particular example. If the forecaster believes there’s less than a 50 percent chance of thunder in the terminal area, you won’t see
28 • TWIN & TURBINE
TSRA – but the forecaster has other options that I’ll discuss in just a bit.
Terminal aerodrome forecasts (TAFs) are one of the most detailed aviation forecasts available to pilots. Issued by forecasters at your local weather forecast offices (WFOs), essentially they describe the meteorological conditions at an airport on an hour-by-hour basis over a specified period, normally 24 or 30 hours. Consequently, all pilots tend to utilize TAFs on every flight they plan. TAFs are also one of the most difficult forecasts any meteorologist must make especially when it comes to convection.
Here’s the issue. The terminal area is defined as the cylindrical volume of airspace that is within a 5-statute-mile (SM) radius from the center of the airport’s runway complex. That’s such as small area of real estate that the NWS considers it a “point” forecast. In other words, it’s not a zone or area forecast and should never be used as such. When thunderstorms are a threat, can the forecaster be absolutely confident that a thunderstorm will develop within or roll through this tiny forecast area six hours from now or even three hours from now? Rarely can a forecaster be that certain.
AUGUST 2011