The TAF, the Terminal Aerodrome Forecast, and How to Read the FM, TEMPO, and BECMG Groups Before You Ever Trust the Sky

A Terminal Aerodrome Forecast (TAF) is a professional forecaster’s prediction of the weather at a single airport over the next 24 to 30 hours, covering roughly a 5 statute mile radius around the field. Reading it well comes down to mastering four change groups - FM, BECMG, TEMPO, and PROB - and always checking the valid period first. Once you can decode those, a TAF stops looking like random code and starts reading like plain English.

What Is a TAF and How Is It Different From a METAR?

A METAR is an observation - a snapshot of what the weather is doing right now, at the field, at the moment it was taken. A TAF is a forecast - a trained human forecaster’s best professional prediction of what the weather is going to do at a specific airport.

The two share a lot of the same shorthand, but they answer completely different questions. The METAR tells you what it is doing. The TAF tells you what it is going to do. On a checkride, an examiner wants to see that you know which tool you’re holding.

What Area Does a TAF Actually Cover?

A TAF is not a regional forecast. It covers roughly a 5 statute mile radius from the center of the runway complex. When the TAF for your destination says clear skies, that’s a statement about a small bubble over that one field.

The ridge you have to cross 40 miles out is not part of that picture. For the bigger view, you need other products - the Graphical Forecasts for Aviation (GFA), AIRMETs, and SIGMETs. The TAF is a specialist that knows one airport very well and doesn’t pretend to know anything else.

Does Every Airport Have a TAF?

No. Only about 700 locations in the United States have a TAF, and they tend to be busier fields with the equipment and traffic to justify one. A quiet home strip out in the country probably does not have its own forecast.

Part of using this product is finding the nearest airport that does have a TAF, then asking honestly whether that field is close enough - and similar enough in terrain and elevation - to tell you anything real about your actual destination. Sometimes the answer is yes. Sometimes it’s absolutely not, and you need to lean on other tools.

How Do I Read the Header of a TAF?

Every TAF opens with the same structure. Take a Denver forecast as our example:

• Airport identifier - the station the forecast belongs to.
• Issuance time - for example, the 12th of the month at 1120 Zulu. Zulu is Coordinated Universal Time, the one clock the whole aviation world agrees on.
• Valid period - written as two blocks, such as the 12th at 1200 Zulu through the 13th at 1800 Zulu. That’s 30 hours of coverage.

New pilots skip right past the valid period, and it matters enormously. If you fly at a time that falls outside that window, you are no longer reading a current forecast - you’re reading history. Always check the valid period first.

What Are the TAF Change Groups?

A TAF describes weather in blocks of time. Each block has a set of conditions, and special keywords tell you when the weather changes from one block to the next. Master the change groups and you’ve mastered the TAF.

The first line after the valid period, with no keyword, is your starting condition - what the forecaster expects right at the beginning of the window. Conditions always appear in the same order: wind, then visibility, then weather, then sky condition. For example, wind 240 at 12 means from 240 degrees at 12 knots; P6SM means greater than 6 statute miles visibility.

There are four change groups you need to know cold.

FM - “From” (a Rapid, Lasting Change)

FM means from. When you see FM followed by a time, everything listed after it replaces everything before it. So FM1800 - from 1800 Zulu - introduces a whole new line of conditions that takes over and stays until the forecast says otherwise.

FM groups are the backbone of the forecast. If you read nothing else, read the FM lines, because they carry the main story. Think of FM as flipping a switch.

Here’s a tip worth internalizing: don’t just find the FM block covering your departure. Find the one covering your arrival too, and read the whole span in between. Many pilots see beautiful conditions at takeoff and miss the FM group that drops the ceiling to 800 feet right during their arrival window. The weather didn’t ambush them - they stopped reading too soon.

BECMG - “Becoming” (a Gradual Change)

BECMG means becoming, and it’s more gradual than FM. It comes with a time range, usually two hours. BECMG 1416 means the transition happens between 1400 and 1600 Zulu, with the new conditions in place and persisting by the end of that window.

If FM is flipping a switch, BECMG is slowly turning a dial. The change is real and lasting - it just takes its time getting there.

TEMPO - “Temporary” (Brief Fluctuations)

TEMPO means temporary, and this is where pilots get into trouble. It also comes with a time range, but each fluctuation is expected to last less than an hour and to cover less than half the total period. So TEMPO 2000/2300 with 2 miles visibility in mist means the visibility drops to 2 miles on and off between 2000 and 2300 Zulu - but that’s not the steady state.

The safety trap: a TEMPO group is not a maybe. It’s a “this will happen, just not the whole time.” If a TEMPO calls for 2 miles visibility and you need 3 to get in legally under visual flight rules (VFR), you cannot plan on threading the good moments. Plan to the worst condition in your arrival window, TEMPO included - then carry an alternate and the fuel to reach it.

PROB - “Probability” (a Percentage Chance)

PROB means probability, and you’ll almost always see it as PROB30 or PROB40 - a 30% or 40% chance of the conditions that follow. It’s usually attached to the serious stuff: thunderstorms, low visibility, the weather that would ruin your day.

A 30% chance of a thunderstorm at your destination is not a green light. It means the forecaster looked at the setup and decided the odds were high enough to put in an official document. That’s a yellow flag with your name on it.

How Do I Use a TAF to Make a Go/No-Go Decision?

Here’s a full scenario. You’re planning a cross-country from Denver to the eastern plains, departing around 1500 Zulu and arriving around 1900 Zulu.

The opening conditions look great: light wind, visibility better than 6, high few clouds. But you keep reading. There’s an FM1800 group: from 1800 Zulu, wind picks up to 270 at 18 gusting 28, and the ceiling comes down to broken at 1,500 feet. Tucked under it is a TEMPO 1800/2200 with 3 miles in rain showers, plus a PROB30 for thunderstorms in the vicinity.

Your arrival is 1900 - squarely inside the FM1800 block and in the middle of that TEMPO window. So the picture that matters is not the pretty opening line. It’s gusts to 28, a 1,500-foot ceiling, visibility that can drop to 3 miles in showers, and a real chance of a thunderstorm near the field when you arrive.

That’s a completely different flight than the one you thought you were planning. Your good options: move your departure earlier to beat the change, pick a solid alternate to the north where the TAF stays clean, or decide it’s a good afternoon to stay on the ground. The one bad decision is the one made without reading past the first line.

How Do I Get Better at Reading TAFs?

Reading weather is genuinely hard, and it takes real repetition. Nobody looks at their fortieth TAF the way they looked at their first.

The drill is simple: even on days you’re not flying, pull the TAF for your home airport, read it out loud all the way through, then look outside and see if the sky agrees. Do that for a month and the code stops being code - it just becomes weather.

Everything here lines up with how the Federal Aviation Administration (FAA) and the National Weather Service (NWS) format these forecasts, and it’s all spelled out in the Aviation Weather Handbook if you want to go deeper.

Key Takeaways

• A TAF is a forecast, not an observation - it predicts weather for a single airport within about a 5 statute mile radius for 24 to 30 hours.
• Read the valid period first and make sure your entire flight, wheels up to shutdown, fits inside it.
• Learn the four change groups: FM (rapid, lasting change), BECMG (gradual change), TEMPO (brief fluctuations under an hour), and PROB (a 30–40% chance of significant weather).
• Read every change group between departure and arrival, not just the ones at the ends.
• When a TEMPO or PROB shows an ugly number, plan to that number - the forecast gave you the bad news on purpose.