Reading the TAF, the Terminal Aerodrome Forecast, and the TEMPO and BECMG Groups That Tell You What the Weather Is About to Do

A Terminal Aerodrome Forecast (TAF) is a professional weather forecast for a five-statute-mile radius around a single airport, and reading it well means going past the base visibility line to the change groups - FM, BECMG, TEMPO, and PROB - that predict how conditions will shift during your arrival window. The base line might show good weather while a TEMPO group buried in the same forecast warns that visibility could briefly collapse right as you land. Learning to read the whole forecast, not just the first happy number, is the difference between a routine landing and an unplanned diversion.

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

A METAR is an observation - what the weather is actually doing right now at the field when the report was taken. A TAF is a forecast - a human forecaster at the National Weather Service predicting what the sky will do at one specific airport over a set period.

Here’s the detail that trips people up: a TAF is not for the whole state or even the whole city. It covers a five-statute-mile radius around the center of the airport. That’s it - one small bubble.

If your destination doesn’t have its own TAF (and many smaller fields don’t), you’ll be borrowing the forecast from the nearest reporting airport. Be honest with yourself about how far away that is and whether the terrain in between changes the picture.

Most TAFs are valid for 24 hours, and larger airports get 30 hours. They’re issued four times a day, at roughly 0000, 0600, 1200, and 1800 Zulu, and they’re amended when conditions aren’t doing what the forecaster predicted. When you see AMD (“amended”) at the top, pay attention - that’s the forecaster raising a hand to say the earlier forecast was wrong and here’s the correction.

How Do I Read the TAF Header?

The header tells you three things: who the forecast is for, when it was written, and the window it covers.

It starts with the airport identifier - for example, KDSM for Des Moines, with the “K” prefix the international format uses. Then a six-digit issuance number giving the day of the month and the time in Zulu. Then the valid period: a “from” day/time and a “to” day/time.

Don’t overthink the header. Confirm the airport is the one you actually want, confirm the valid period covers your arrival time, and move on.

One habit worth building: check the valid period against your ETA every single time. Pilots pull a forecast, feel great about it, and completely miss that their arrival is 40 minutes after the forecast expires. If the TAF doesn’t cover the time you care about, it told you nothing.

What Do the Base Conditions Tell Me?

After the header comes the first forecast line - the base conditions, the forecaster’s opening prediction for the start of the window. You read it in the same order as a METAR, on purpose: wind, then visibility, then weather, then sky condition.

Wind comes first. A reading of 31012 means wind from 310 degrees at 12 knots. A “G” would mean gusts, which matter for your crosswind math and for how rough the ride in will be.

Visibility in a U.S. TAF is in statute miles. P6SM means “plus six statute miles” - greater than six. Anything six or above gets capped at “plus six,” because beyond that it doesn’t change your decision much.

Weather appears next if there is any - rain, snow, mist, whatever is expected. If there’s nothing there, there’s nothing expected. No news is good news.

Sky condition comes last, and it’s the one that keeps you both legal and alive: few, scattered, broken, or overcast, with the height in hundreds of feet above ground. BKN008 means broken clouds at 800 feet. Only broken or overcast layers give you a ceiling - scattered and few do not. A ceiling of 800 feet with dropping visibility is exactly what turns a VFR flight into a bad afternoon, and it’s sitting in plain sight if you know to look.

What Do the TAF Change Groups Mean? (FM, BECMG, TEMPO, PROB)

The weather doesn’t hold still for 24 hours, so the forecaster uses four code words to say “starting now, expect something different.” These are what separate pilots who read a TAF from pilots who just glance at one.

FM (FROM) - a rapid, wholesale change. Written FM plus a six-digit day/hour/minute in Zulu. This is the big one. When you see FM1800, everything before that line is done - throw it out. At 1800 Zulu the picture flips cleanly to a new set of wind, visibility, and sky, and that becomes your new baseline until the next change group. A FROM group replaces the entire previous forecast. It’s not a maybe; it’s a committed prediction, usually tied to a front moving through or the wind shifting behind it.

BECMG (BECOMING) - a gradual, permanent change. Written BECMG plus a start and end time. Conditions shift gradually to the new description sometime during that window, and once they get there, they stay. Think of morning fog burning off - it doesn’t snap clear at a precise second; over a couple of hours the ceiling lifts. For planning, assume the worse of the two conditions exists for the entire window until the change is complete. If it’s becoming better, don’t count on the better weather until the end of the window. If it’s becoming worse, plan for the worse weather at the start. Plan conservative and the weather can only surprise you in your favor.

TEMPO (TEMPORARY) - brief fluctuations. This is the sneaky one. TEMPO means temporary fluctuations expected to last less than an hour each and, in total, less than half the time period. The base conditions are still basically in charge, but every now and then, briefly, expect it to get worse - a rain shower rolls through, visibility drops, then it recovers.

Here’s the trap. Your base line says P6SM, beautiful. But the TEMPO group says that during this window you should temporarily expect 2 statute miles in rain and a broken ceiling at 900 feet (BKN009). If you only read the base line, you planned for a clear approach. If you read the TEMPO, you know there’s a real chance that the exact moment you arrive, you’re looking at 2 miles and a 900-foot ceiling. Ask the honest question: is my airplane, my currency, my personal minimums ready for the temporary ugly conditions - not the nice ones? The weather doesn’t care which line you read.

PROB (PROBABILITY) - a flagged chance of serious weather. You’ll usually see PROB30 plus a time window, meaning a 30 to less than 50 percent chance of the described conditions - often thunderstorms or serious weather. It’s uncertain, but uncertain in a way the forecaster thought was worth writing down. A 30 percent chance of a thunderstorm at your destination is a 30 percent chance of a really bad day. Treat PROB groups as a signal to have a plan B ready and to check for updates before you commit.

How Do I Put the Whole TAF Together Into a Decision?

Reading each piece in isolation isn’t the skill. Reading the story is the skill.

A good TAF tells you a story about the day. Say the base line is decent VFR with a southerly wind. Then a BECMG in the late morning swings the wind to the northwest and lowers the ceiling. Then a TEMPO through early afternoon brings a couple of miles of visibility in rain showers. Then an FM group in the evening clears it back out behind the front.

Read together, that isn’t a pile of codes - it’s a cold front moving through your destination in the early afternoon, exactly when you planned to arrive. Now the forecast has done its job. It’s telling you to either move your departure earlier to beat the front or sit tight and go in the evening once it’s cleared.

This is exactly what the Airman Certification Standards (ACS) expect for both the private and instrument ratings. It’s not enough to read visibility off the line - the standard asks you to interpret the weather, correlate it with your route and airplane, and make a defensible go/no-go decision. On a checkride, the examiner wants you to point at a TEMPO group and say out loud, “here’s what could happen when I arrive, and here’s my plan if it does.” That’s the difference between a pilot who reads weather and a pilot who uses it.

Practical Steps for Your Next Flight

1. Check the valid period against your ETA first. Before you read another word, confirm the forecast actually covers your arrival time. If it doesn’t, it isn’t answering your question.

2. Read the base line, then hunt for change groups. Don’t stop at the first happy number. Find every FM, BECMG, TEMPO, and PROB, and read what each one does to your arrival window.

3. Plan for the bad end of a TEMPO or worsening BECMG. Assume it lands right when you do. If you can’t accept those conditions, you’ve learned something crucial before starting the engine instead of after.

4. Respect the five-mile bubble. A TAF covers one airport. If your destination is borrowing a forecast from a field 30 miles away with a ridge in between, get the area forecast discussion and look at the bigger picture. Don’t bet the flight on one narrow bubble.

5. Remember the TAF is a prediction, not a promise. Check for amendments and pull the actual METAR as you get closer. If the observation and forecast start disagreeing, believe your eyes and the current observation - and give the forecaster credit for raising the flag.

Every one of those groups is a person trying to warn you about something. The TEMPO is a warning. The PROB is a warning. The BECMG is a warning that the door is closing - or opening - over the next couple of hours. Your job is to listen. For deeper reference, the National Weather Service Aviation Weather Center and the FAA’s Aviation Weather Handbook lay all of this out in more detail.

Key Takeaways

• A TAF forecasts conditions for a five-statute-mile radius around one airport; a METAR only observes current conditions.
• TAFs are valid 24 hours (30 at larger fields), issued four times daily at 0000, 0600, 1200, and 1800 Zulu, and marked AMD when amended.
• The four change groups each mean something specific: FM = rapid wholesale change, BECMG = gradual permanent change, TEMPO = brief fluctuations under an hour, PROB = a 30–49% chance of significant weather.
• Always check the valid period against your ETA and plan for the worst conditions in any TEMPO or worsening BECMG, assuming they hit as you arrive.
• The TAF is a prediction, not a promise - verify with current METARs and amendments before you commit.