The TAF and the Change Groups, What BECMG, TEMPO, and PROB Thirty Really Promise You When You Read a Terminal Forecast

A Terminal Aerodrome Forecast (TAF) is a weather forecast for a small area - five statute miles around the center of a single airport - and its change groups tell you when and how the base conditions will shift. BECMG signals a gradual, permanent change; TEMPO signals brief fluctuations that come and go; and PROB30/PROB40 flags a 30 or 40 percent chance of conditions the forecaster isn’t confident enough to commit to. Once you know which change group covers your arrival time, you know exactly what to plan for.

What Is a TAF and What Does It Cover?

The letters stand for Terminal Aerodrome Forecast - a forecast for an airport. The word “terminal” matters more than most students realize.

A TAF is not a regional forecast. It is a promise about the air over a specific runway, covering a circle just five statute miles around the airport’s center. When a TAF says the ceiling will drop, it’s talking about the field you care about, not the whole county.

Most TAFs cover a 24-hour or 30-hour window, and busy ones are amended throughout the day as forecasters watch the weather refuse to follow the models.

How Is a TAF Structured?

A TAF is written in blocks. The forecaster starts with a base condition - wind, visibility, weather, and sky - which represents what they expect for the entire period unless they tell you otherwise.

The way they tell you otherwise is with change groups. Think of the base line as “here’s the plan,” and each change group as “except right about here, something else happens.”

Your entire job when reading a TAF is to figure out which block of time you’ll be sitting in when your wheels touch down - and what that block promises.

What Does FM (FROM) Mean in a TAF?

FM (FROM) is a hard line in the sand. At the stated time, the weather flips completely and stays flipped. Everything after a FROM group replaces everything before it - a brand-new base condition with a clean break.

For example, a base line might read calm wind, visibility better than six miles, sky clear. Then: FROM 1800Z, wind 240 at 15 gusting 22, visibility 3 miles in rain, ceiling broken at 1,000.

Before 1800Z, you plan for the nice stuff. At 1800Z and after, you plan for wind, rain, and a low ceiling - all the way until the next FROM or the end of the forecast. There is no ambiguity in a FROM group. It’s the forecaster being decisive.

What Does BECMG (Becoming) Mean in a TAF?

BECMG describes a gradual change over a window of time, usually a two-hour stretch. Somewhere in that window, conditions transition to the new values - and once they arrive, they stay.

You’ll see it written as BECMG followed by two times, like 1618, meaning the change unfolds between 1600Z and 1800Z.

Here’s how to treat a BECMG group for a real go/no-go decision: be conservative and assume the new conditions arrive at the beginning of the window, not the end. If fog is becoming a 400-foot ceiling between 1600Z and 1800Z and you planned to land at 1700Z, don’t gamble it will hold off. Plan as if that ceiling is already down when you get there.

Weather doesn’t read the TAF. The forecaster gives you a window because they’re being honest about not knowing the exact minute. Respect that honesty by planning for the worse edge of the window.

What Does TEMPO (Temporary) Mean in a TAF?

This is the one everybody misreads. TEMPO describes conditions expected to come and go - temporary fluctuations, each lasting less than an hour, and in total covering less than half of the window they’re attached to.

Critically, TEMPO is not a new base condition. It does not replace anything. It’s layered on top of whatever base is already in effect. The forecaster is saying the general picture is one thing, but every so often expect it to briefly get worse, then bounce back.

Rain showers are the classic example. Say the base is visibility better than six with few clouds at 4,000, and then TEMPO 2000-2400Z, visibility 2 miles in a thunderstorm with heavy rain, ceiling broken at 2,500. Most of that four-hour window is fine - but somewhere in there a cell rolls through, visibility craters for under an hour, and then it moves on. Maybe it happens again.

Here’s the trap: students see the good base numbers and relax. But the whole reason the forecaster added the TEMPO is that the bad stuff is likely enough to bet on. For planning purposes - especially for a required alternate under instrument rules - treat those TEMPO conditions as if they’ll absolutely be happening the moment you arrive.

What’s the Difference Between BECMG and TEMPO?

If you take one thing away, take this: BECMG changes and stays. TEMPO flickers and returns.

Becoming is a one-way door. Temporary is a swinging door.

What Does PROB30 or PROB40 Mean in a TAF?

PROB is the wild card. You’ll almost always see PROB30, occasionally PROB40. It means the forecaster believes there’s a 30 or 40 percent chance of the conditions that follow - not confident enough to commit with a straight TEMPO or FROM, but worried enough not to leave it off the page.

You almost never see PROB below 30 (not worth mentioning) and never at 50 or above (at that point the forecaster just commits to the change outright).

So PROB30 in a thunderstorm is the forecaster saying there’s a roughly one-in-three chance a storm parks on the field during that window. One-in-three odds of a thunderstorm over the field is not a bet a careful pilot ignores.

How Do I Read a TAF for a Real Cross-Country?

Picture this: you’re departing your home field in the late morning and arriving at your destination around 2100Z. You pull the destination TAF:

• Base line: calm winds, better than 6 miles, sky clear.
• BECMG 1800-2000Z: wind south at 12, visibility 5 miles in haze, ceiling broken at 4,000.
• TEMPO 2000-2400Z: visibility 2 miles, thunderstorm, ceiling broken at 2,000.
• PROB30 2100-2400Z: visibility 1 mile, heavy thunderstorm, ceiling overcast at 1,000.

A student who reads only the base line sees “sky clear” and files with a smile. Walk it forward instead.

You’re arriving at 2100Z. By then the BECMG has already happened, so your real base is south wind, 5 miles in haze, 4,000-foot ceiling - not clear skies. On top of that, you’re landing in the middle of a TEMPO calling for a 2-mile thunderstorm, so plan as though a storm is over the field. And the forecaster has flagged a PROB30 for a 1-mile, 1,000-foot overcast heavy thunderstorm during your exact arrival window.

None of this means “don’t go.” It means go smart: carry more fuel, pick a solid alternate nowhere near the line of storms, brief where you’ll divert, and set a personal hard deck. If you arrive and the TEMPO is real, you go to your alternate with fuel to spare rather than circling and hoping. The TAF didn’t tell you to stay home - it told you exactly what to be ready for.

How Does the ACS Expect Me to Use a TAF?

The Airman Certification Standards (ACS) expect more than reading a TAF out loud. Under the weather information area, the examiner wants you to interpret it and apply it to your specific flight - say the words, then say what they mean for your airplane, on your day, at your arrival time. Don’t just translate the code; make the decision out loud.

Two more habits worth building:

Find your arrival window first, then read backward to figure out which base condition and change groups you’re actually sitting in. Don’t read a TAF top to bottom like a novel - jump to your time and work out the picture.

When the TAF and current conditions disagree, believe the current report. A forecast is a best guess. The weather at the field right now is a fact.

The definitions here come straight from the National Weather Service and the FAA’s Aviation Weather Handbook, and the interpretation standard lives in the Airman Certification Standards. The change-group section is about a page and a half - worth reading for yourself.

Key Takeaways

• A TAF forecasts weather for just five statute miles around a single airport, typically over a 24- or 30-hour window.
• FM (FROM) is a clean, permanent change; BECMG is a gradual, permanent change - plan for the conditions arriving at the start of the window.
• TEMPO conditions come and go (each under an hour, less than half the window) and layer on top of the base - plan as if they’ll be present when you arrive.
• PROB30/PROB40 means a 30 or 40 percent chance; treat a PROB30 thunderstorm with serious respect.
• Read a TAF like a pilot: find your arrival time first, apply it to your specific flight, and trust current conditions over the forecast when they disagree.