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Using TAFs

Source: www.pilotworkshop.com/tips/aviation_weather_services.htm, Featuring Scott Dennstaedt

I teach my instrument students that all weather products have important limitations. In other words, they are created with very specific assumptions and guidelines in mind. If you are not aware of these limitations, you will mostly likely misuse the product some day.

Terminal Aerodrome Forecasts, better known as TAFs, have some important limitations especially when associated with forecasts for thunderstorms. TAFs along your route of flight show up on DUATs briefings and will also be provided to you when you call Flight Service for a standard briefing.

Despite these limitations, it is very common for a pilot to use TAFs as if they were an area forecast. They simply bounce through all of the TAFs along their route of flight looking for a forecast for thunderstorms. We'll see why this isn't such a good idea for determining thunderstorm potential along your route.

One of the first words that should come to mind when using a TAF is the word "tiny." The terminal area is defined as the cylindrical volume of airspace from the center of the airport's runway complex to a radius of five statute miles. It is basically like a forecast for a college campus.

Even the terminal area's vicinity is also very small representing the donut-shaped ring ranging from 5 to 10 statute miles excluding the 5 statute mile terminal area. This includes a forecast for thunderstorms in the vicinity, showers in the vicinity or fog in the vicinity.

The moral is that TAFs are not intended to forecast weather beyond 10 statute miles; they are a "point forecast" and not a zone or area forecast. Therefore, it is not a good idea just to "scan" the TAFs and assume that the lack of a forecast for thunderstorms in the TAFs mean an absence of thunderstorms en route. The area forecast, on the other hand, will have an indication when pulse or air mass thunderstorms are possible even when the TAFs are indeed silent on the issue. As a result, it is very common in the summer for terminal forecasts to be void of thunderstorms when the area forecast has a clear indication of isolated or widely scattered thunderstorms.