Decoding the TAF, the FROM and TEMPO Groups, and Reading Wittman's Terminal Forecast the Night Before You Launch for Oshkosh

A Terminal Aerodrome Forecast (TAF) is a professional forecaster’s prediction of the weather within about 5 statute miles of an airport over roughly the next 24 to 30 hours. Unlike a METAR, which reports current conditions, a TAF tells you what’s coming - and if you can decode it yourself the night before a flight, you understand your own risk far better than any briefing summary can give you. The real skill lies in reading the change groups - FM, BECMG, TEMPO, and PROB - as a narrative that writes your whole flying day for you.

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

A METAR tells you what the weather is doing right now at an airport. A TAF tells you what a forecaster thinks it will do over the coming 24 to 30 hours, and it only covers a small bubble - about 5 statute miles around the field itself.

It is a forecast, not a promise. But it’s a professional human forecaster putting their reputation on the line, and that carries real weight.

The TAF is one of the six elements Flight Service reads in a standard briefing. You don’t have to wait for someone to read it to you, though - you can pull it up yourself tonight and interpret your own risk directly.

How Do I Read the Header and Valid Period?

A TAF starts with the station identifier followed by a long number: the issue time and the valid period. A forecast might be issued at 2320 Zulu with a valid window running from the 23rd at 1800Z through the 24th at 1800Z.

Everything in aviation weather runs on Zulu time (Coordinated Universal Time). It doesn’t care what time zone you’re standing in, so learn to convert it in your head. In summer, Central Daylight Time is Zulu minus 5, which means 1800Z is 1:00 p.m. local. Write it on a sticky note if you have to.

Right after the valid period comes the first line of weather - the prevailing forecast at the start of the window. You read the wind first: 210 at 10 means wind from 210 degrees at 10 knots. Then visibility: P6SM means “plus six statute miles,” or greater than six, which is as good as a TAF reports. Then the sky: for example, few clouds at 5,000 and scattered at 12,000.

That first line is your baseline. If the entire TAF looked that clean, you’d close the laptop and sleep well. But it rarely does - because underneath come the change groups, and that’s where the real skill lives.

What Do the FROM, BECMG, TEMPO, and PROB Groups Mean?

There are four change groups you need to know cold.

FROM (FM): Written as FM jammed against a time, like FM2100. It means “from 2100 Zulu, throw out everything above and use this instead.” A FROM group is a clean break - a front coming through, the wind swinging around, the whole picture changing. When you see FM, rewrite your mental weather from scratch at that moment.

BECOMING (BECMG): A gradual transition over a window, usually a couple of hours - for example, BECMG 0100 to 0300 Zulu followed by a new wind or ceiling. The change happens somewhere in that window, gradually, and once it’s done, it sticks. Think of fog slowly lifting or a ceiling settling in over the evening.

TEMPO: Temporary. It flags a condition that comes and goes, lasting less than an hour at a time and, in total, less than half the period it covers. This is the classic Wisconsin summer afternoon: a TEMPO group might read 3 statute miles, thunderstorm, rain. The field is basically flyable, but pop-up storms roll through, and when one’s overhead you get three miles and a downpour for twenty minutes before it passes.

PROBABILITY (PROB): You’ll see PROB30 or PROB40 - a 30% or 40% probability of the conditions that follow, usually thunderstorms. The forecaster isn’t confident enough to commit but is confident enough to warn you. Treat a PROB30 for thunderstorms the way you’d treat a friend saying “there’s a decent chance it storms this afternoon” - you wouldn’t ignore that before planning your day.

A word on TEMPO at Oshkosh: a TEMPO for thunderstorms is not a “maybe” you can shrug off. Afternoon convection is the number one weather story of a Midwest summer. When that group covers your arrival window, the forecaster is waving a flag - build a plan around it, don’t bet against it.

How Do I Read a Full TAF as a Story?

Decoding the pieces is one thing; reading the narrative is another. Say tonight’s Wittman Regional TAF reads like this in plain English:

• Start of the window: wind from the south at 10, better than six miles, scattered at 6,000. Nice.
• TEMPO group over the early afternoon (your exact arrival window): three miles in thunderstorms and rain, broken ceiling at 3,000.
• FROM group in the evening: wind swinging to the northwest at 15 gusting 22, sky clearing.

Now read it as a story. The morning is gorgeous - launch early and get in before lunch and you’re golden. By early afternoon, storms are firing, exactly when you don’t want to be holding over Ripon in a conga line of airplanes. By evening the front is through and it clears, but now you have a gusty crosswind for landing.

That TAF just wrote your whole day. It told you to get up early, that the sweet spot is morning, that the afternoon is a trap, and that an evening arrival demands sharp crosswind skills - all from one paragraph of letters and numbers, free, from the National Weather Service.

Why Does the Examiner Care About TAF Interpretation?

The Airman Certification Standards (ACS) has an entire task on weather information and lists the TAF by name. On your checkride, the examiner wants more than “yeah, I got a briefing.” They want you to interpret the forecast and tie it to a go, no-go, or divert decision.

They want to hear you say something like: “This TEMPO group for thunderstorms during my arrival window means I move my departure earlier, build in a delay, or plan an alternate.” They want to see the decision, not just the reading.

That interpretation is exactly what keeps you alive on a real cross-country to Oshkosh, where the airport itself may go no-radio and hold you out if the weather turns. You can’t count on an in-flight update the way you would on a normal flight, so the forecast you read the night before - and refresh at dawn - carries more weight than usual.

Practical Steps for Reading a TAF Before Any Cross-Country

1. Read your whole route, not just the destination. Go station by station: departure, a couple of fields along the way, destination, and alternate. The story that matters unfolds across your route in time, not as a snapshot at one field.

2. Always check the valid period against your actual flight time. Students confidently read forecasts that expired before they’d even land. If your arrival is past the end of the valid window, that TAF has nothing to say about your landing - go to the Graphical Forecasts for Aviation (GFA) to fill the gap.

3. When you see TEMPO or PROB for thunderstorms near your window, don’t ask “can I make it” - ask “what’s my out.” Where’s the good weather? Which nearby field stays VFR? For Oshkosh, that’s often Fond du Lac to the south. Having that in your pocket before launch is the difference between a calm diversion and a scramble.

4. Respect the forecast’s shelf life. A TAF gets shakier the further out you read. The first six hours are gold, twelve hours out is good, and 24 hours out it’s a planning tool, not a landing decision. Read it tonight to plan, again at dawn to decide - and keep looking out the windshield, because the sky is the final authority and it doesn’t file amendments.

Key Takeaways

• A METAR is current weather; a TAF forecasts the next 24–30 hours within about 5 statute miles of an airport.
• FM is a clean break, BECMG is a gradual transition, TEMPO is a come-and-go condition (under an hour at a time), and PROB30/40 is a probability warning - usually for thunderstorms.
• Read the change groups as a narrative that tells you when to launch, when to wait, and when to expect a crosswind.
• Always match the valid period to your actual arrival time, and know your diversion field (Fond du Lac for Oshkosh) before you launch.
• Examiners and survival both demand interpretation tied to a decision, not just a completed briefing.