The Terminal Aerodrome Forecast, the TEMPO and PROB Groups, and How a Two-Hour Window of IFR Can Hide Inside a Forecast That Reads VFR

A Terminal Aerodrome Forecast (TAF) can read as a beautiful VFR day at the top and bottom while hiding two hours of solid instrument conditions in the middle. That hidden window lives inside the change groups - specifically the TEMPO and PROB groups - which layer additional hazards on top of the baseline forecast rather than replacing it. The disciplined move is to find your arrival time, locate the change group sitting on top of it, and plan for the worse number every time.

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

A METAR is an observation - it reports what the weather is actually doing right now at the field, measured and confirmed. A TAF is a forecast - a trained aviation meteorologist stating what they expect the weather to do at that specific airport.

That word “specific” matters. A TAF covers the area within about five statute miles of the center of the runway complex - not the region, not the county, just the airport. That precision is exactly what makes it useful, and also what makes it easy to misread if you don’t respect how tightly it’s written.

Most TAFs are issued four times a day and cover a 24- or 30-hour period. They’re written in a fixed order using a dense shorthand that looks like keyboard noise until you learn to take it apart.

How Do You Read a TAF Line by Line?

Here’s a sample forecast for a destination airport, decoded slowly:

• Station Kilo, issued on the 12th at 1118 Zulu, valid from the 12th at 1200 Zulu to the 13th at 1800 Zulu.
• Wind 240 at 8 knots. Visibility greater than 6 statute miles. Few clouds at 5,000.
• From 1500 Zulu: wind 260 at 12, gusting 20 knots. Visibility greater than 6. Scattered at 4,000.
• Temporarily, between 1600 and 1800 Zulu: visibility 2 statute miles in mist. Ceiling broken at 800.
• From 2000 Zulu: wind 310 at 10. Visibility greater than 6. Sky clear.

Read only the top and the bottom, and you’d call this a beautiful VFR day - a fine morning and a clear evening. And you’d be right about both. But sitting in the middle, from 1600 to 1800 Zulu, is a two-hour window where visibility drops to 2 miles and the ceiling sags to 800 feet. That is solid instrument weather. If your arrival lands around 1700 Zulu, right in the fat middle of that window, the forecast you called VFR just warned you that you might be arriving into the goo.

What Does the TEMPO Group Mean in a TAF?

TEMPO stands for temporary. It’s the forecaster saying: I expect these conditions to appear during this window, but I expect each occurrence to last less than an hour and to cover less than half of the time period total.

In the example, the forecaster expects visibility to drop to 2 miles and the ceiling to fall to 800 feet in bursts sometime between 1600 and 1800 Zulu - maybe patchy mist burning off unevenly, maybe a band of light rain drifting through.

Here’s the trap: the word “temporary” makes it sound minor. It is not. A TEMPO condition is one the forecaster is confident enough about to put in writing. When a TEMPO group puts the field below VFR minimums, plan as if that weather will be there when you arrive - not might. The forecaster cannot tell you which 20 minutes inside the window the mist will roll across the numbers, and it is not a VFR pilot’s job to bet on threading the needle between the bad patches.

What Are the TAF Change Groups (FM, BECMG, TEMPO)?

The change groups are a family. Learn them together.

FM (From) signals a rapid and lasting change. At the stated time, everything before it is replaced. In the example, “From 1500 Zulu” means the wind shifts and picks up, gusting to 20, and that becomes the new baseline until the next change group. FM is a clean, permanent handoff.

BECMG (Becoming) is a slower change over a stated window - for example, “becoming between 1400 and 1600 Zulu.” The change happens gradually somewhere in that band and then sticks. When you plan, treat the new conditions as being in effect from the start of the becoming window, since you don’t know whether it shifts at the front edge or the back.

TEMPO (Temporary) comes and goes, doesn’t last, but is expected. Unlike FM and BECMG, it does not replace the baseline - it layers on top of it.

What Does PROB30 or PROB40 Mean in a TAF?

PROB stands for probability - the forecaster putting a number on their uncertainty. PROB30 means a 30% chance of the conditions that follow; PROB40 means 40%.

You’ll never see PROB50 or higher. If a forecaster is more than half sure something will happen, they forecast it outright with an FM, BECMG, or TEMPO. PROB is reserved for low-confidence, high-consequence weather - it’s used most often for thunderstorms and significant drops in visibility. A PROB30 thunderstorm group is the forecaster saying, “I’m not sure this will happen, but if it does, it’s a big deal, and I want it on your radar.”

Use a PROB group with the same discipline as TEMPO. A 30% chance of a thunderstorm at your destination during your arrival window is not a green light with an asterisk - it’s a yellow light with a hard question attached: What’s my plan if it happens? Where’s my alternate? Do I have the fuel to hold or divert? Thirty percent is not small when the thing that’s 30% likely is a thunderstorm sitting on the field you’re trying to land at.

How Do I Actually Use a TAF for a Go/No-Go Decision?

Read every TAF with this workflow:

1. Read the valid period first. Confirm the forecast actually covers your flight. A TAF that expires at 1800 Zulu is no help if your wheels touch down at 1930. Near the end of the valid period, respect that the forecast is at its weakest.
2. Find your arrival time and mark it. Say it out loud: “I’m landing around 1700 Zulu.” That’s the needle you’ll run down through the whole forecast.
3. Walk top to bottom to find the line in effect at arrival. Step through the FM and BECMG groups until you reach the block of time containing your needle. That’s your baseline.
4. Check for an overlapping TEMPO or PROB group. This is the step nobody does, and it’s the whole ballgame. That group doesn’t replace the baseline - it layers on top. In the example, the 1700 Zulu baseline is the 1500 FM line (decent visibility, some wind), but the TEMPO from 1600 to 1800 sits right on your arrival, warning of 2 miles and 800 overcast, on and off. Plan for the worse of the two. The TEMPO is the honest number.
5. Build your out. If the overlapping group puts the field below your personal minimums - and for a VFR pilot, 2 miles and an 800-foot ceiling absolutely does - you need a real alternate: a named airport, a checked TAF, and the fuel to get there with reserves intact.

What Does the Examiner Want on the Checkride?

On both the private and instrument checkrides, weather interpretation lives in the Airman Certification Standards under preflight preparation. The examiner wants more than symbol recognition - anyone can memorize that BKN means broken.

What they’re testing is whether you can take a TAF, find the conditions during your window, and make a go or no-go decision that accounts for the change groups. Point at the TEMPO line and say, “This is why I’m bringing extra fuel,” or “This is why I picked this alternate,” or “This is why we’re not going today.” That last one is a passing answer. Deciding not to go because the TEMPO group scares you is aeronautical decision-making done right.

How Do I Get Better at Reading Aviation Weather?

Reading weather is one of the harder parts of primary training because it isn’t a maneuver you can drill until your hands know it - it’s judgment and ambiguity. Nobody feels confident about this at 40 hours.

The method that works is boring: pull the TAF for your home field every single morning, whether you’re flying or not, decode it in your head, then that afternoon look out the window and grade the forecaster. Do that for a month and the shorthand stops being shorthand and starts being a picture.

What If My Destination Has No TAF?

A TAF forecasts a single airport, and thousands of airports don’t have one. If your destination is a grass strip with no TAF, don’t shrug. Pull the TAFs for the nearest reporting fields that bracket your route, pull the Graphical Forecasts for Aviation for the bigger picture, and build your mental model from the airports that do report. Lean on the TEMPO and PROB groups at those bracketing fields even harder - they’re the closest honest thing you’ve got.

The decoding standards here come straight from the FAA’s Aviation Weather Handbook and the National Weather Service’s TAF documentation, and the Aviation Weather Center publishes all of it if you want to go deeper.

Key Takeaways

• A TAF is a forecast for a specific airport - within about 5 statute miles of the runway complex - while a METAR is a real-time observation.
• TEMPO and PROB groups don’t replace the baseline forecast; they layer additional hazards on top of it, so plan for the worse number.
• TEMPO means the condition is expected in bursts of less than an hour; PROB30/PROB40 flags a 30–40% chance of low-confidence, high-consequence weather like thunderstorms.
• Find your arrival time, identify the change group overlapping it, and if the field drops below your personal minimums, build a real alternate with checked weather and reserve fuel.
• On the checkride, deciding not to fly because a TEMPO group warns of IFR conditions is a correct, passing display of aeronautical decision-making.