What's the difference between BECMG and TEMPO in a TAF?

BECMG signals a gradual but permanent change that sets in during the window and stays. TEMPO signals brief, come-and-go conditions expected less than half the time in bursts under an hour, with the base forecast still in effect around them.

How large an area does a TAF cover?

A TAF forecasts only a five-statute-mile radius around the airport. It says nothing about the weather along your route between fields, so it can never replace a full briefing.

How often is a TAF issued and how long is it valid?

TAFs are issued four times a day at roughly 0000, 0600, 1200, and 1800 Zulu. Most are valid for 24 hours, while the busiest airports get 30-hour forecasts.

What does PROB30 or PROB40 mean in a TAF?

PROB30 means a 30% chance and PROB40 a 40% chance of the conditions that follow, usually thunderstorms or lower ceilings. Those odds are high enough to warrant a backup plan.

How do I read the valid period like 1218/1318?

Read it as day-and-hour, day-and-hour. 1218/1318 means the forecast is valid from the 12th at 1800Z until the 13th at 1800Z - a full 24-hour window.

The Terminal Aerodrome Forecast, the TAF, and How FM, BECMG, and TEMPO Tell You When the Weather Is Actually Going to Change

SUMMARY: Learn to read a TAF and decode FM, BECMG, and TEMPO so you know exactly when the weather will change before you fly.

A Terminal Aerodrome Forecast (TAF) is a forecaster’s prediction of the weather in a five-statute-mile radius around an airport, telling you what conditions to expect over the next 24 to 30 hours. While a METAR reports what the weather is doing right now, the TAF tells you what it will do later - when you’re airborne and it’s too late to change your mind on the ground. The three change groups FM (from), BECMG (becoming), and TEMPO (temporary) are the codes that reveal exactly when those changes arrive.

What Is a TAF and Who Issues It?

The National Weather Service issues a TAF for thousands of airports, with a trained human forecaster reviewing the models and making a best call for each field. The forecast covers a five-statute-mile circle around the center of the airport - and nothing beyond it.

Most TAFs are valid for 24 hours. The busiest airports get 30 hours. They’re issued four times a day, at roughly 0000, 0600, 1200, and 1800 Zulu (Coordinated Universal Time, the clock all of aviation uses so everyone is talking about the same moment).

One detail worth burning into memory: TAFs get amended. If the weather goes sideways and the forecast is suddenly wrong, the forecaster issues a new one marked AMD for amended. The TAF you pulled at 6 a.m. may not be the one that’s live at 9 a.m. Check it again before every flight.

How Do You Read the Header of a TAF?

A TAF starts with the letters TAF, then the station identifier - let’s use a fictional field, KRAD. Next comes the issuance time: a six-digit group followed by Z. For example, 121730Z means the forecast was issued on the 12th day of the month at 1730 Zulu (day, then hour and minute).

Right after that comes the valid period, which trips people up. It looks like two chunks separated by a slash, such as 1218/1318. Read it as day-and-hour, day-and-hour: valid from the 12th at 1800Z until the 13th at 1800Z - a full 24-hour window. Everything that follows describes the weather inside that window.

How Do You Read the Base Forecast Line?

The first line of weather is the base forecast: the conditions expected at the start of the valid period. Say it reads wind 240 at 8 knots, visibility 6 statute miles, scattered at 2,500 feet, and broken at 8,000 feet.

So at 1800Z, the field expects a wind from 240 degrees at 8 knots, six miles of visibility, a scattered layer at 2,500 feet, and a broken layer at 8,000 feet - a decent VFR day.

Cloud heights in a TAF, just like in a METAR, are above ground level and written in hundreds of feet using three digits. That 8,000-foot layer is written 080: three digits, add two zeros.

What Do FM, BECMG, and TEMPO Mean in a TAF?

This is where the TAF earns its keep. Its whole purpose is to tell you not just what the weather is, but when it’s going to change - and it does that with three change groups. Learn these three and you’ve cracked the code.

FM (From): A Hard Cutoff

FM means “from,” and it’s a clean, hard switch. When you see FM followed by six digits (day, hour, minute), it means that at that exact time you throw out everything above and use everything below - a complete change of the forecast.

A line like FM130000 reads “from the 13th at 0000Z, brand-new conditions apply.” It might now say wind 310 at 15 gusting 22 knots, visibility greater than 6, and sky clear - the forecaster’s way of saying a front swings through around midnight Zulu, the wind clocks to the northwest and picks up, and the sky clears. After that moment, it’s a different day.

BECMG (Becoming): A Transition Over a Window

BECMG is a transition, not a switch. It comes with a time range - again, two chunks with a slash. A group like BECMG 1302/1304 reads “becoming, between the 13th at 0200Z and the 13th at 0400Z.” The change happens gradually somewhere in that two-hour window, and by the end the new conditions are in charge and they stay.

Forecasters use BECMG for slower, permanent shifts: fog burning off, a ceiling gradually lifting through the morning.

Here’s the conservative way to use it: assume the worse of the two conditions exists for the entire becoming window. If a ceiling is becoming lower, plan on the lower ceiling starting at the beginning of the window. If it’s becoming better, don’t count on the improvement until the end. That’s exactly the pessimistic-case thinking a checkride examiner wants to see.

TEMPO (Temporary): The Sneaky One

TEMPO also comes with a time window, but it describes conditions that pop up only briefly and don’t last. The National Weather Service rule of thumb: the temporary condition is expected to occur for less than half the time period, in bursts of under an hour each.

A line like TEMPO 1220/1300, visibility 2 statute miles in mist, broken at 800 feet, reads “temporary, between 2000Z on the 12th and 0000Z on the 13th, expect occasional drops to 2 miles in mist with an 800-foot ceiling.”

The base forecast might still say 6 miles and scattered clouds. The TEMPO is layered on top, warning that every now and then the bottom is going to fall out for 20 or 30 minutes at a time.

What Is the Most Dangerous Mistake When Reading a TAF?

The deadliest mistake is reading the base line, seeing nice VFR numbers, and skipping the TEMPO group underneath. A pilot launches, hits one of those temporary bursts - 2 miles in mist and an 800-foot ceiling - and is suddenly a VFR pilot scud-running under a ceiling the forecast described in plain text.

Remember the pattern: FM is a permanent switch at a moment. BECMG is a permanent change over a window. TEMPO is a temporary come-and-go over a window. Permanent, permanent, temporary. When you decide whether a flight is doable, the TEMPO groups are the ones that bite, because the good numbers are still sitting on the base line, ready to lie to you if you let them.

What Do PROB and WS Mean in a TAF?

You’ll sometimes see PROB followed by a number - usually PROB30 or PROB40. That’s probability: a 30% chance or 40% chance of the conditions that follow. It’s typically attached to thunderstorms or lower conditions the forecaster thinks are possible but not likely enough to bank on. Treat a PROB40 thunderstorm over your destination as a real maybe - 30 to 40% odds of a storm is plenty of reason to build a backup plan.

You’ll also see low-level wind shear written as WS, then a height, then the wind. A group like WS020/21045KT means non-convective low-level wind shear: at 2,000 feet the wind is from 210 at 45 knots, while the surface is calm or light. That’s a big shear layer, and it matters a lot on approach and departure. Don’t gloss over it.

Why Is a TAF Never a Complete Weather Briefing?

Because a TAF forecasts only the airport - a five-statute-mile radius - and nothing else. The weather 20 miles down your route, over a ridge, out where there’s no reporting station, is completely absent from the TAF. Your departure and destination fields can both have beautiful forecasts while the middle 60 miles hold the stuff that gets people hurt.

A TAF is one instrument in the panel, not the whole panel. Pair it with the winds aloft forecast, AIRMETs and SIGMETs, the graphical forecast, and pilot reports (PIREPs) from aircraft that are up there right now. The TAF tells you about the endpoints; the rest of your briefing tells you about everything in between.

A Practical TAF Workflow for Your Next Cross-Country

Run this on your next flight:

Pull the TAF for your departure field, your destination, and a good alternate along the way.
Read the valid period first and confirm it actually covers the hours you’ll be flying - a forecast that expires before you land is useless.
Read the base line for the starting conditions.
Read every change group underneath, all the way to the bottom - every FM, BECMG, TEMPO, and PROB. This is the step people skip.
Ask yourself: what’s the worst this forecast will let the weather be during my window? Plan for that number, not the pretty one on the top line.
Compare that worst case against your experience level and personal minimums, and make the go/no-go decision on the ground, where it’s safe and cheap to decide.

The weather that hurts pilots is usually the line they didn’t scroll down to read.

Key Takeaways

A TAF forecasts a five-statute-mile radius around an airport for 24 hours (30 at busy fields), is issued four times daily at 0000/0600/1200/1800Z, and can be amended (AMD) - always recheck before you fly.
FM is a hard, permanent switch at an exact time; BECMG is a permanent change over a window; TEMPO is a brief come-and-go over a window.
For BECMG, plan for the worse of the two conditions across the whole window - the conservative, checkride-ready mindset.
TEMPO groups are the most dangerous to overlook because good base-line numbers can hide brief but hazardous conditions underneath.
A TAF is never a complete briefing - pair it with winds aloft, AIRMETs/SIGMETs, graphical forecasts, and PIREPs to cover the route between your endpoints.