How much can you trust TAFs?

Learning about weather is one of the most important things you can do as a student pilot (or as an experienced pilot, for that matter). And one of the first weather products you’ll encounter is the Terminal Aerodrome Forecast (TAF), which predicts the weather at specific times for many larger airports. This forecast includes wind, ceiling and visibility–sort of like a METAR in the future.

But just like any weather product, TAFs take some interpreting to get the most value out of them. With that in mind, here are some tips to keep in mind when reading a TAF.

• TAFs are valid only for the airport area. An airport 25 miles away is pretty close in flying terms, but can be a million miles away in weather terms. Be careful not to extrapolate too much when reading a TAF–terrain or other local conditions may mean that nearby TAF is worthless for your flight. This is especially true with localized weather events, like ground fog or scattered thunderstorms. Technically, a TAF is only valid for the area within a 5 statute mile radius of the airport.
• Focus on the trend more than the exact times. The TAF says the ceiling will go from 800 feet to 5000 feet at precisely 10am. Does that mean that a 10:05 departure is a good idea? Probably not. It’s likely that the forecaster will get the trend right (i.e., improving ceilings), but nailing the timing is fiendishly difficult. Focus on the general trend you can expect, but be conservative with timing.
• Don’t pretend the weather is better than it is. Put another way, current weather information always trumps a forecast. So if the TAF suggested the wind wouldn’t pick up until later, but the latest METAR says it’s gusting to 25 knots, don’t ignore reality. It’s likely the forecast was wrong. Some pilots have been known to take off in poor weather conditions simply because the TAF (that could be hours old) promised it wouldn’t be that way. Be more skeptical.
• The age of a TAF matters. Related to the last point, the older a forecast, the less reliable it is. So the age of the TAF you’re reading matters (and is published right at the beginning of every one). Fortunately, TAFs are released on a regular schedule (0000Z, 0600Z, 1200Z, 1800Z), so you can plan when to check the weather. If it’s 1710Z, go ahead and read the TAF, but check back after the 1800Z forecast comes out. In particular, see what changes the forecaster made–is the trend getting better or worse since the last forecast?

Here’s one final tip–did you know that not all TAFs are created equal? Most TAFs forecast conditions for the 24 hours after issuance. But at certain large airports (usually commercial airports that get a lot of long-distance international flights), the forecast period is extended to 30 hours. This is needed for pre-flight planning when a flight could last 18 hours. Use this to your advantage–a longer range TAF can be helpful for planning the night before a flight.

Also, some airports–including Chicago, Atlanta and New York–update their TAFs every three hours instead of every six hours. This is because of the high volume of flights that use these airports, where a more precise and current forecast is essential for planning. Here again, as a general aviation pilot, you can use this to your advantage if you’re flying close to one of these airports.

TAFs are a valuable pre-flight planning tool, and they are created by experts who carefully consider a variety of different models and weather reports. But don’t be seduced by the precision of these forecasts. As you conduct your pre-flight weather briefing, consider the TAF a single data point, and nothing more. Also look at the weather synopsis, METARs, radar, satellite and Area Forecasts.

Too many pilots have learned the hard way that just because a TAF said it would be good weather, doesn’t mean it actually will be.