Decoding the TAF, the FM, BECMG, TEMPO and PROB30 Groups, and the Cross-Country Where the Line Between Legal and Grounded Lived in One Change Group

A Terminal Aerodrome Forecast (TAF) is the National Weather Service’s forecast of the weather expected within five statute miles of an airport, valid for 24 or 30 hours and reissued four times a day. The skill that keeps you safe is reading its change groups - FM, BECMG, TEMPO, and PROB - and lining them up against your departure and arrival times. Done right, a single six-character change group can be the difference between a legal, comfortable arrival and getting stuck two hundred miles from home.

What Is a TAF, and What Is It Not?

A TAF is a forecast, not an observation. The METAR tells you what the weather is doing right now; the TAF tells you what an experienced human forecaster expects it to do next. Confusing the two is how pilots get hurt.

It covers a small bubble - five statute miles around the airport - and runs for 24 or 30 hours depending on the station, with a fresh forecast issued roughly every six hours.

Only certain airports get a TAF: larger, towered fields with enough traffic to justify one. Your local grass strip won’t have one, so part of the job is borrowing the forecast from the nearest reporting airport and applying judgment to bridge the gap.

How Do I Read the Valid Period and Zulu Time?

Every TAF opens with the station identifier, the issue time, and a six-digit valid-period block introduced by the letter K. The first two digits are the day of the month, the next two are the start hour in Zulu (UTC), and after a slash comes the end. Our sample forecast is valid from the 15th at 1800Z, running 30 hours ahead.

The entire weather system speaks Zulu time, so if your time-zone math is wrong, everything downstream is wrong. Convert the valid period - and your own departure and arrival times - to your local clock before you read another line.

Do the conversion once, calmly, on the ground. Write your local offset on a sticky note on your kneeboard rather than doing subtraction under pressure in the cockpit.

What Does the Opening Forecast Line Mean?

The first weather line is the prevailing forecast at the start of the valid period. In our TAF it reads: wind 240 at 8, visibility greater than six miles (written P6SM, where the P means “greater than”), and few clouds at 12,000.

That’s a bluebird afternoon. If the whole TAF looked like that, there’d be nothing to brief. But after the opening line, the forecast starts to change - and how it signals those changes is the whole skill.

What’s the Difference Between FM, BECMG, TEMPO, and PROB?

These four change groups look similar but mean very different things. Blurring them together is where good pilots make bad calls.

FM - “From”: A Clean Break

FM followed by a six-digit time means: at this time, throw out everything above and start fresh. It signals a rapid, significant, essentially permanent change to a whole new set of conditions - wind, visibility, and sky all replaced at once.

Our TAF has FM 160000Z: wind shifts to 160 at 12 gusting 20, visibility 5 miles in mist, sky broken at 800 feet. Right at 0000Z on the 16th, the gorgeous afternoon flips to an 800-foot ceiling with a gusty southerly wind. That’s the front arriving.

BECMG - “Becoming”: A Gradual Transition

BECMG comes with a time window and tells you the conditions will transition to a new value sometime during that window and then stay there. It doesn’t snap - it settles in.

Our TAF has a BECMG group from 161000Z to 161200Z: ceiling becoming broken at 3,000. Sometime between 1000Z and 1200Z, the low deck is forecast to lift and break up. That’s the morning improvement.

TEMPO - “Temporary”: Brief Fluctuations

TEMPO also comes with a time window, but it means something different. It warns that during that window, the stated conditions will appear temporarily, in bursts - each lasting less than an hour and, all told, covering less than half the period. It is not a permanent change.

Our TAF has TEMPO 160000Z–160400Z: visibility 2 miles in mist and light rain, ceiling 300 overcast. Between 0000Z and 0400Z, on top of the FM picture, expect it to periodically drop to 2 miles and 300 feet.

Here’s the trap: many pilots read TEMPO as “probably won’t happen.” Flip it around. For planning - minimums and alternates - treat the TEMPO condition as if it will happen, exactly when you’re on final. The forecaster included it because it’s a real, expected possibility.

PROB - Probability: A Real but Less-Than-Even Chance

PROB is followed by a number, usually PROB30 or PROB40 - a 30% or 40% chance. You’ll see it on threats like thunderstorms, where confidence isn’t high enough to commit but the hazard is too serious to omit.

Our TAF carries a PROB30 from 0200Z to 0600Z: thunderstorms with visibility 1 mile. Not likely - but if it happens, it’s a very bad night to be shooting an approach.

How Do I Turn the TAF Into a Story?

A real briefing isn’t decoding symbols - it’s telling yourself the story of the weather.

Here’s the story of our TAF: a gorgeous afternoon, then right at 0000Z a front slides through - ceilings crash to 800 feet, the wind swings around and gets gusty. For the first four hours after the front, it bounces down to 2 miles and 300 overcast in bursts, with a 30% shot at thunderstorms mixed in. Through the small hours it stays ugly and low. Finally, between 1000Z and 1200Z, it lifts and breaks to 3,000 feet. A cold, wet, low night sandwiched between a nice afternoon and a decent morning.

The Cross-Country Where One Change Group Decided Everything

The mission was a 200-mile leg to bring an airplane home, departing from our TAF airport. The plan was an afternoon departure - no problem - but the temptation was to run errands, have dinner, and push the launch to late evening instead. What’s a few hours?

Look at the TAF. An afternoon departure is clear, comfortable, and legal in every direction. But push to a late-evening launch and you slide right up against the FM group at 0000Z - and even before that, the TEMPO window opens at 0000Z with its 2 miles and 300 overcast.

This pilot was VFR only, with no instrument rating. For a VFR pilot, an 800-foot ceiling and five miles is already a hard no across most of the airspace they’d cross, and 300-and-2 in a TEMPO burst is simply not survivable weather to scud-run toward.

The whole flight - legal versus grounded - lived in the FM group and the TEMPO right behind it. Depart at 4 p.m. and you land in daylight under a bluebird sky with two hours to spare. Depart at 9 p.m. to save a second trip and you launch into the teeth of a front you were warned about, in plain language, hours earlier. Same TAF, same airplane. The only variable was whether the pilot read the change groups and believed them.

That pilot did. They flew in the afternoon, landed easy, and were home for dinner while the front did exactly what the forecaster promised. That’s not luck - that’s a TAF, read correctly, turned into a decision.

What Are the Practical Steps for My Next Briefing?

1. Read the valid period first and convert it to local time before reading another word. Almost every TAF mistake starts with a time-zone offset error that puts the bad weather right at your planned arrival.

2. Line up your departure and arrival times against the change groups. Ask explicitly: which line am I taking off under, and which line am I landing under? Weather at arrival is the whole ballgame.

3. Respect TEMPO and PROB as if they’ll happen to you on final. If a TEMPO or PROB30 drops you below your personal minimums or below approach minimums, that’s your cue to consider an alternate - or not going.

4. Feed the forecast into your alternate planning. For IFR, this reading directly drives the alternate requirement. Even VFR, set a personal alternate and a hard turnaround time. The forecast is your evidence.

5. Build the story on your checkride. The examiner doesn’t want you to parrot definitions of FM and TEMPO. The Airman Certification Standards expect you to retrieve, interpret, and apply the data and correlate it to your specific flight - then make a go/no-go decision you can defend.

The first fifty TAFs you decode will have you looking up the format and second-guessing the Zulu math. That’s normal - that’s the work. Eventually you’ll look at a string that used to be gibberish, see the whole night laid out in front of you, and make the quiet, correct decision to stay on the ground.

Key Takeaways

• A TAF is a forecast for weather within five statute miles of an airport, valid 24 or 30 hours and reissued four times daily - it predicts, while a METAR observes.
• FM is a clean, permanent break to new conditions; BECMG is a gradual transition over a window; TEMPO is brief sub-hourly fluctuations; PROB30/40 is a less-than-even chance of a serious hazard.
• Always convert the valid period and your own times to Zulu, then match your departure and arrival to the specific TAF lines they fall under.
• Plan for TEMPO and PROB conditions as if they will occur at your worst moment - they drive your minimums and alternate decisions.
• Read the TAF as a story and let it produce a defensible go/no-go call - the exact skill examiners test under the ACS.