The TAF, the Terminal Aerodrome Forecast, and the FM, TEMPO, and PROB Groups That Tell You When the Weather Is Actually Going to Change
Learn to decode a TAF and its FM, TEMPO, and PROB change groups so weather never surprises you at your destination.
A Terminal Aerodrome Forecast (TAF) is a trained meteorologist’s prediction of the weather at one specific airport, covering a five-statute-mile radius around the field for a set valid period. The forecast’s change groups - FM (from), TEMPO (temporary), and PROB (probability) - are what tell you when conditions will actually shift while you’re flying. Master those three words and you can read what the sky is going to do at your estimated time of arrival, not just what it’s doing now.
What Is a TAF and How Is It Different From a METAR?
A METAR is an observation - a person or automated station recording what the sky is doing right now. A TAF is a forecast - a trained aviation meteorologist predicting what the weather will be at a single airport over a specific window of time.
Here’s the detail most pilots miss: a TAF is not a regional forecast. It describes only a five-statute-mile radius circle centered on the airport. When you pull the TAF for your destination, it’s telling you about that small bubble of sky over the runway - not the route you’ll fly to get there.
Most TAFs are issued four times a day, roughly every six hours, and cover either 24 or 30 hours into the future depending on the airport. In practice, a professional is re-forecasting that field for you four times daily.
How Do You Read the TAF Header Line?
The forecast opens with the airport identifier, then a six-digit issuance time (day and time it was issued), then the valid period.
The valid period is the single most important number on the line, and it’s the one pilots skip. It might read as the 8th day at 1800 Zulu running to the 9th day at 1800 Zulu. That’s your window - how long the forecast is good for. If your flight lands after the valid period ends, this TAF does not cover your arrival, and you need the next one.
Every time in aviation weather is in Zulu (Coordinated Universal Time). If you can’t convert your local time to Zulu in your head, make that tonight’s homework. A forecast you can’t place in time is a forecast you can’t use.
What Order Does the TAF Body Follow?
After the header, the body breaks into chunks, and each chunk lists elements in the same order every time: wind, visibility, weather, sky. Get that sequence in your bones and the code reads like a sentence.
- Wind - direction and speed
- Visibility - usually in statute miles
- Weather - rain, mist, and similar phenomena
- Sky condition - cloud coverage and height
Cloud height is given as a three-digit number multiplied by 100 to get feet above the ground. A broken layer written with the digits 040 means a broken ceiling at 4,000 feet above the field. Coverage runs few, scattered, broken, overcast.
The first chunk, right after the valid period, is the forecast for the start of the window.
What Do FM, TEMPO, and PROB Mean in a TAF?
These three change groups are the entire ballgame. Own them and you own the TAF.
FM (From) - A Clean Break to New Conditions
FM is the strongest, cleanest change indicator in the forecast. When you see FM followed by a day and time, the forecaster is saying: at that moment the weather changes, and everything before it is erased. The conditions after FM become the whole picture and stay that way until the next change group.
Think of FM as turning the page - a front moves through, the wind shifts, the marine layer burns off. If an FM group at 1600 Zulu shows the wind swinging northwest and visibility dropping, then from 1600 Zulu onward, that is your airport.
Practical step: To find your arrival weather, first locate which FM group your ETA falls inside. That FM block is your baseline. Start there.
TEMPO (Temporary) - Conditions That Come and Go
TEMPO does not turn the page. It forecasts temporary conditions expected to come and go during a period - for less than half of that period, generally in stretches under an hour.
Picture a main forecast of broken clouds at 3,000 feet and 6 miles visibility, with a TEMPO group from 1800 to 2200 Zulu calling for 2 miles in mist and an 800-foot ceiling. Most of that four-hour window you’ll have the good conditions, but briefly and repeatedly it will sag to 800 and 2.
Practical step: Plan for the bad number. You cannot predict which 20 minutes you’ll be arriving in, so treat the TEMPO condition as what you’ll get. If it would put you below your personal minimums or below the minimums for your planned approach, that’s a real possibility - not a maybe.
PROB (Probability) - Honest Odds on Uncertain Conditions
You’ll almost always see PROB30 or PROB40 - a 30% or 40% chance of the conditions in that group. You won’t see PROB50, because at 50% or higher the forecaster is confident enough to put it in the main forecast or use TEMPO. PROB is reserved for lower odds and is often used for thunderstorms and hard-to-pin-down overnight conditions.
Thirty percent may not sound like much - but you wouldn’t drive a car with a 30% chance the brakes failed. A PROB40 for thunderstorms near your arrival time means the ingredients are there. Respect it.
How Do You Apply a TAF to Your Actual Arrival Time?
Here’s a scenario that makes it click. You’re planning a cross country a couple of hours out. The destination TAF opens beautifully: wind calm, visibility better than 6, sky clear. But keep reading:
- FM at 2100 Zulu - wind out of the south at 15 gusting 22, broken layer at 2,500 feet
- TEMPO from 2200 to 0200 Zulu - 3 miles in light rain, ceiling 1,200 feet
- PROB40 from 0200 to 0600 Zulu - thunderstorms
Your ETA is 2130 Zulu.
The lazy read sees “sky clear” and goes flying. The real read finds where 2130 falls - after the FM group - so your true baseline is wind gusting to 22 and a ceiling at 2,500, not clear skies. You’re also arriving at the leading edge of a TEMPO window that could drop you to 1,200 and 3 miles in rain, with thunderstorms trying to organize behind it. Same TAF, completely different flight. The only difference is whether you read it or just looked at it.
The muscle to build: Don’t stop at the first line. Find your arrival time, walk the change groups until you land in the right block, and ask what’s the worst thing the forecast promises around your arrival. If you have the fuel, the outs, and the personal minimums to handle that worst case, go fly. If not, you’ve made a great decision on the ground, where decisions are cheap.
How Do Examiners Expect You to Interpret a TAF on a Checkride?
The Airman Certification Standards (ACS) for both the private and instrument ratings expect you to obtain and correctly analyze a weather briefing - and that includes interpreting TAFs.
Examiners look for two things: that you can decode the raw forecast (not just the plain-language translation your app produces), and that you can apply it. If they ask what the weather will be at your destination at your ETA and you point at the top line and say “clear” while an FM group and a TEMPO sit right there, that’s a problem.
How to show a good decode - narrate it out loud:
- “My valid period covers my flight.”
- “My arrival is at this Zulu time, which falls in this FM block, so my baseline is this.”
- “There’s a TEMPO here that could give me this, so my plan accounts for it.”
- “There’s a PROB40 for thunderstorms, so here’s my alternate and here’s my go/no-go.”
That narration is exactly the risk management the examiner wants to hear - it makes your invisible thinking visible.
Why Should Experienced Pilots Still Read the Raw TAF?
The skill gets rusty. Once you’re comfortable, it’s easy to let the app do the interpreting and slowly lose the ability to read the raw code yourself.
Pull the raw TAF sometimes - even when the app has already color-coded it - and read it cold to prove you still can. The day the internet is down at the FBO, or you’re copying it over the radio from Flight Service, the raw code is all there is.
Where Does the TAF Fit in the Full Weather Briefing?
When you get a standard briefing through Flight Service (Leidos, 1800wxbrief.com), the TAF is built in alongside current METARs, winds aloft, AIRMETs, SIGMETs, and NOTAMs.
Get the whole standard briefing - don’t cherry-pick. The TAF tells you what the forecaster expects at the field, but the AIRMETs, winds aloft, and adverse conditions section give you the why and the what else. A TAF read alone is a keyhole view.
Remember its two limits: it covers only a five-mile circle around the airport, and only its valid period. The two hours of sky you fly through to get there? The TAF says nothing about that - which is what graphical forecasts and AIRMETs are for. The TAF is one vital instrument in the panel, but just one.
Your Assignment for the Next Flight
- Pull the raw TAF for your destination and read it out loud.
- Find your arrival time in Zulu.
- Walk the FM groups until you’re in the right block.
- Look for a TEMPO or PROB that could bite you.
- Plan for the worst honest number in the forecast.
Do that every time and you’ll get surprised by weather far less often - and getting surprised less is the entire game. This guidance comes straight from the National Weather Service, the FAA’s aviation weather product guidance, and the Airman Certification Standards - all free and worth reading at the source.
Key Takeaways
- A TAF is a meteorologist’s forecast for a five-statute-mile radius around one airport, issued four times a day and valid for 24 or 30 hours - always in Zulu time.
- The TAF body always lists elements in the order wind, visibility, weather, sky, with cloud heights read as a three-digit number times 100.
- FM turns the page to entirely new conditions; TEMPO flags brief conditions that come and go for less than half the period; PROB30/PROB40 give the odds of uncertain conditions, often thunderstorms.
- To find your arrival weather, locate the FM block your ETA falls in, then plan for the worst number any TEMPO or PROB promises around that time.
- Read the raw code yourself as part of the full standard briefing - the TAF alone is a keyhole view of the weather.
Radio Hangar. Aviation talk, built by pilots. Listen live | More articles