The TAF, the Terminal Aerodrome Forecast, and Decoding the FM and TEMPO Lines Before You Launch on a Cross-Country

A Terminal Aerodrome Forecast (TAF) is a National Weather Service prediction of the weather expected within about five statute miles of an airport’s runway complex, typically covering a 24-hour period (30 hours at major airports) and issued four times a day. Unlike a METAR, which reports what the weather is doing right now, the TAF tells you what the weather will do - which is exactly what matters when you won’t land for another three hours. Reading a TAF in full, including its change groups, is the difference between a pilot who gets surprised in the pattern and one who plans on the ground.

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

The name tells you almost everything. Terminal means it’s about a single airport, not your whole route or region. Aerodrome is simply the international term for airport. Forecast is the key word: it’s a prediction made by a trained human forecaster, not an observation.

The METAR and the TAF use nearly the same code but answer two different questions. The METAR answers what is it doing right now. The TAF answers what do we expect it to do. For cross-country planning, the TAF is the one that counts, because you care about the ceiling when you arrive - not the ceiling as you push the throttle up at departure.

Here’s a detail that catches pilots: TAFs are issued roughly every six hours, but the forecast window starts a little after the issue time and runs forward. A TAF issued in the morning is already looking past your planned landing time.

How Do I Read the TAF Header and Valid Period?

Start with the parts that frame everything else.

Airport identifier: A K followed by three letters, the same code as the METAR, telling you which airport the forecast belongs to.

Issuance time: Six digits and a Z, such as 271120Z. Break it apart: the first two digits (27) are the day of the month, the next four (1120) are the time, and Z means Zulu (Coordinated Universal Time) - the single clock all of aviation shares. So this forecast was issued on the 27th at 1120 Zulu.

Valid period: Two groups split by a slash, such as 2712/2812. The first group (2712) is the day and hour the forecast becomes valid - the 27th at 1200 Zulu. The second group (2812) is the day and hour it expires - the 28th at 1200 Zulu. This TAF is good for 24 hours. Read it as valid from, valid to - a start and a finish.

How Do I Decode the Main Forecast Line?

The first forecast line describes the prevailing conditions at the start of the valid window, and it always reads in a fixed order: wind, visibility, weather, sky condition. That order is your friend.

Wind comes first: five or six digits plus “KT,” such as 21012KT - wind from 210 degrees true at 12 knots. Burn this into memory: TAF and METAR winds are referenced to true north, while tower-reported winds and painted runway numbers are magnetic. If gusts are forecast, you’ll see a G: 21018G28KT means from 210 at 18 gusting to 28 knots. That gust number is what you run through your crosswind math against your personal limits and your airplane’s demonstrated crosswind component.

Visibility is next, in statute miles in the U.S.: 6SM is six statute miles. A P in front, as in P6SM, means greater than six statute miles.

Weather only appears when there’s something to report. A minus sign means light, no sign means moderate, a plus means heavy. So -RA is light rain, +SN is heavy snow, BR is mist, and FG is fog. If the group is missing, the air is clean - a good sign.

Sky condition is the ceiling for many of us. The cloud-amount codes are FEW, SCT (scattered), BKN (broken), and OVC (overcast), followed by three digits multiplied by 100 to get feet above ground. BKN010 is broken at 1,000 feet; OVC005 is overcast at 500 feet. Officially, a ceiling is the lowest broken or overcast layer - few and scattered don’t count. When deciding whether it’s a VFR day, your eyes hunt for that first BKN or OVC.

Put the line together and read it as a sentence: Wind from 210 at 12, six miles visibility, no weather, broken at 1,000. Now you’re reading, not decoding.

What Do the FM and TEMPO Change Groups Mean?

A forecast that said one thing for 24 hours straight wouldn’t be worth much. Weather moves, fronts pass, fog forms at dawn and burns off by mid-morning. Change groups are how the forecaster says and then, at this point, expect this instead. Two of them will make or break your planning: FM and TEMPO.

FM (From) is the cleanest, most decisive change group in the TAF. It’s followed by six digits - day, hour, minute - such as FM181500, meaning from the 18th at 1500 Zulu onward. The key word is complete: a From line replaces everything before it. Brand-new wind, visibility, and sky, holding until the next change group or the end of the TAF. A cold front is a classic FM - wind swings from south to northwest, the ceiling tumbles, and from that time the whole scene is different. FM lines are how you build a timeline of your destination: read down the TAF and ask which FM group you’ll be underneath when you land.

TEMPO (temporary) is the one pilots misread most, so slow down. It’s followed by a time range with a slash, such as TEMPO 2718/2722 - between 1800 and 2200 Zulu on the 27th. The crucial part: TEMPO does not replace the prevailing forecast - it rides on top of it. The forecaster is saying the main story still holds, but during this window, for brief periods (each less than an hour, and not adding up to more than half the window), you might see something else pop up - a rain shower, a visibility drop, a gusty burst.

So if the prevailing line says broken at 3,000 and a TEMPO calls for two miles in rain showers, it won’t be two miles the whole time - expect it to come and go. Treat a TEMPO as if it will happen exactly when you arrive. Don’t talk yourself into threading the needle between showers. If the TEMPO covers your arrival window, fly the arrival assuming you get the worst of it. That’s not pessimism; that’s how you avoid getting cornered.

What Does PROB30 Mean on a TAF?

PROB30 (or PROB40) is a probability group. PROB30 2706/2709 means a 30 percent chance of the following conditions between 0600 and 0900 Zulu. The forecaster has a hunch but not enough confidence to commit. You’ll almost always see PROB attached to something that would genuinely wreck your day - thunderstorms, low ceilings, or freezing conditions - so even at 30 percent, you respect it. A three-in-ten chance of a thunderstorm over your destination is a three-in-ten chance of a very bad afternoon.

How Does This Work on a Real Cross-Country?

Picture a three-hour flight landing in the early afternoon. You pull the destination TAF and the prevailing line looks beautiful: light wind, visibility better than six, scattered at 5,000. The app shows a green dot and says go.

But you read down. About an hour before your planned arrival, there’s an FM group: wind swinging northwest and gusting into the twenties, ceiling dropping to broken at 1,200 - a front. Tucked under that new line, a TEMPO calls for two miles in rain showers across the exact hour you’d be entering the pattern.

The green dot lied - not on purpose. The software read the first line and quit. You read the whole story, and now you know your “scattered at 5,000” arrival is actually gusty, low, and rainy, right at the edge of your personal minimums. Maybe you go earlier, plan an alternate, or carry the fuel to hold and let the front clear. Either way, you’re making that decision on the ground with your brain, not in the pattern with your heart rate climbing.

Why Examiners Care About This on the Checkride

On your checkride, the examiner isn’t testing whether you can recite that BKN means broken - anyone can memorize that. They want to watch you take a raw TAF, build a picture of what your destination will be doing when you arrive, and make a real decision with it. The Airman Certification Standards place weather products and risk management at the front of the preflight planning tasks, and this is the skill underneath all of it: reading the sky before you’re in it.

You can pull raw TAFs yourself anytime from the official aviation weather service run by the National Weather Service and FAA. Pick your home field tonight, pull the raw TAF, and read it out loud all the way down.

Key Takeaways

• Read the entire TAF, top to bottom. The trouble is almost never in the first line - it’s in the FM group that lines up with your arrival.
• Match the valid period against your flight. If your landing time is after the TAF expires, you have no forecast for your arrival - that’s a blind spot, not reassurance.
• When a TEMPO or PROB group overlaps your arrival, plan for the worst of it and build your out before you need it.
• TAF and METAR winds are true; tower and runway winds are magnetic. Don’t get burned on crosswind math by forgetting which north you’re using.
• FM completely replaces the prior forecast; TEMPO rides on top of it for brief, come-and-go periods.