The TAF, the TEMPO and PROB Groups That Trip Up Students on Checkrides, and the Go/No-Go Math Hidden in the Terminal Aerodrome Forecast

A Terminal Aerodrome Forecast (TAF) is a point forecast covering a 5-statute-mile radius around a specific airport, issued by the National Weather Service and updated every six hours. Depending on the station, it covers either 24 or 30 hours into the future - major airports typically receive the 30-hour version. The change groups inside that forecast - FM, BECMG, TEMPO, and PROB - each carry a precise technical meaning, and misreading any one of them can turn a confident preflight decision into a dangerous one.

What Does a TAF Cover - and What Doesn’t It Tell You?

The TAF is a point forecast, not a route forecast. It predicts what will happen at one airport, inside that five-mile circle. A clean destination TAF says nothing about conditions along your route of flight, at an airport 50 miles away, or anywhere in between.

Pilots sometimes use a favorable destination TAF to feel confident about an entire flight. That is a misuse of the product. The TAF tells you about the finish line. It says nothing about the race.

How Is a TAF Structured?

Every TAF follows a consistent structure. The first element is the four-letter ICAO identifier - KORD for Chicago O’Hare, KATL for Atlanta Hartsfield. For most U.S. airports, the ICAO code is the familiar three-letter identifier with a K prepended.

Next is the issuance time, formatted as a six-digit group: two digits for day of month, two for the hour, two for the minutes, followed by Z for Zulu (Coordinated Universal Time). A forecast issued on the 15th at 1400 UTC reads 151400Z.

The valid period follows, and it is the first element where many pilots stumble. It is formatted as two groups separated by a slash - start day/hour on the left, end day/hour on the right. A forecast running from noon on the 15th through noon on the 16th reads 1512/1612. Both sides are day-and-hour combinations, not just hours. Reading only the hour digits and ignoring the day prefix is a common oral exam error - know what you are looking at.

How Do I Read Wind, Visibility, and Sky Conditions in a TAF?

After the valid period comes the prevailing conditions expected at the start of the forecast window. The order is always the same: wind, visibility, significant weather, sky condition.

Wind is three digits for direction, two or three digits for speed, and the unit (knots in the U.S.). 27015KT means wind from 270° at 15 knots. A gust is shown with a G between steady speed and gust value: 27015G25KT means from 270° at 15, gusting to 25.

Visibility in U.S. TAFs is in statute miles. P6SM means more than 6 statute miles - the P stands for “plus.” For VFR flight, P6SM is a favorable indicator. For instrument approaches, compare the forecast visibility to the published minimums for the specific procedure you plan to fly.

Weather descriptors include TS (thunderstorm), RA (rain), SN (snow), BR (mist), FG (fog), and DZ (drizzle). A minus sign prefix means light intensity; a plus sign means heavy; no modifier means moderate. -RA is light rain. +RA is heavy rain. The full table of descriptors and intensity modifiers is in the Aeronautical Information Manual (AIM).

Sky condition uses FEW (1–2/8 coverage), SCT (scattered, 3–4/8), BKN (broken, 5–7/8), and OVC (overcast). The number after each abbreviation is height above ground level in hundreds of feet. BKN030 is a broken ceiling at 3,000 feet. SCT020 is scattered clouds at 2,000 feet - and because it is scattered, it is not a ceiling. A broken or overcast layer counts as a ceiling; a scattered layer does not. SKC at the end of a sky condition group means sky clear below 12,000 feet.

What Does FM Mean in a TAF?

FM stands for FROM and indicates a permanent, abrupt change. When FM appears followed by a time, every condition listed before it is completely replaced by what comes after. Nothing carries over.

If a weather element is not listed after an FM group, it is assumed to be at its best - essentially clear skies, good visibility, no significant weather. FM is the cleanest change group in the TAF: conditions change, and they stay changed until the next FM or the end of the forecast.

What Does BECMG Mean in a TAF?

BECMG stands for BECOMING and describes a gradual change over a specified time window. The format BECMG 0608 means the transition begins at 0600 and is complete by 0800. Within that two-hour window, conditions are in transition and the exact timing is uncertain.

BECMG groups only specify what is changing. Any element not listed in a BECMG group remains at the prevailing conditions from before the group. If visibility is not mentioned, assume it is unchanged. Once the window closes, the BECMG conditions become the new prevailing baseline until the next change group or the end of the forecast.

What Does TEMPO Actually Mean - and Why Do Pilots Get It Wrong?

TEMPO stands for temporary, but it carries a specific technical definition in a TAF. It means conditions are expected to last less than one hour at a time and to occur during less than half of the total period over which the TEMPO applies.

The critical misread is treating TEMPO as a maybe. It is not a maybe - it is a when. The forecaster is saying these conditions are expected to happen. The uncertainty is about exactly when within the window each occurrence begins and how long it lasts, not about whether it happens at all.

If a TEMPO group forecasts conditions below VFR minimums, the right question is: what is your plan if those conditions are occurring when you arrive? The forecast is telling you they will occur at some point during that window.

For IFR pilots, a TEMPO ceiling or visibility that falls below approach minimums during the estimated arrival window is a contingency item - not something to scroll past.

When Does a TEMPO Group Trigger the IFR Alternate Requirement?

FAR 91.169 specifies when IFR pilots must file an alternate. The trigger: if the destination TAF forecasts conditions below a 2,000-foot ceiling or 3 statute miles visibility within one hour before or one hour after your estimated time of arrival, you must file an alternate.

Change groups - including TEMPO groups - count in that evaluation. A pilot who looks only at the prevailing forecast conditions and sees them sitting above the threshold may miss a TEMPO group that falls inside the one-hour arrival window and forecasts conditions below it. If that TEMPO is inside your arrival window, the alternate requirement is triggered regardless of what the prevailing forecast shows.

What Do PROB30 and PROB40 Mean?

PROB groups come in two versions. PROB40 indicates a 40% or greater probability of those conditions during the specified period. PROB30 indicates a 30% probability. A higher number means higher probability.

PROB40 is typically used for conditions that are a real possibility but not the forecaster’s most likely outcome. PROB30 is for lower-probability events worth flagging - often thunderstorms, dense fog, or heavy precipitation - where the consequences would be significant if the event occurred.

Neither PROB group means conditions are unlikely. A PROB40 thunderstorm is a four-in-ten chance. A PROB30 is a three-in-ten chance. The pilot in command decides whether those probabilities are acceptable given fuel state, alternate options, and personal experience level - but the starting point is treating them as real, quantified possibilities, not background noise.

How Do I Apply This to a Real TAF Scenario?

Consider a cross-country with a destination TAF valid from noon on the 17th through noon on the 18th.

The prevailing conditions: wind 270° at 15 knots, visibility greater than 6 SM, scattered at 2,000 feet, broken at 4,000 feet. Decent conditions for the first several hours of the forecast.

An FM group at 2000Z (8 PM): wind shifts to 310° at 20 gusting to 30 knots, clouds lift slightly, visibility stays good. From 8 PM onward, this northwest wind is the new reality. Everything before the FM is gone.

A TEMPO group from 0000Z to 0600Z (midnight to 6 AM): wind variable at 5 knots, visibility 1 statute mile in fog, sky OVC002 (overcast at 200 feet).

That TEMPO is not a warning label. The forecaster expects fog to reduce that airport to a 200-foot ceiling and 1-mile visibility at some point between midnight and 6 AM. Not continuously - the less-than-half rule means the full six hours will not be socked in. But it will happen, likely in multiple bursts of under an hour each.

A 7 PM planned arrival stays ahead of the TEMPO window - but now ask what happens if an unexpected headwind slides the arrival to midnight. That arrival falls squarely inside the TEMPO window, and the destination may be well below VFR minimums. A planned 2 AM arrival requires an instrument approach you are current and rated for, fuel for a hold or divert, and an alternate airport with a clean TAF. This is not an edge case. It is exactly what the forecast is describing.

What Does the ACS Require for TAF Interpretation on a Checkride?

The Airman Certification Standards (ACS) for private pilot requires candidates to interpret weather products including TAFs, identify forecast conditions, recognize change groups, and explain how those conditions affect the planned flight. Not a thumbs-up or thumbs-down - a reasoned analysis with specific references to what the forecast says.

If a TEMPO appears during an oral exam, be prepared to state what it forecasts, during what window, and what the contingency plan is. If a PROB40 appears, acknowledge the probability explicitly and explain how it factors into the decision.

A strong answer: “There is a TEMPO forecasting a 200-foot ceiling between midnight and 6 AM. Our planned arrival is 7 PM, which gives us a buffer - but I would monitor our departure time and carry enough fuel to hold or divert to our alternate if we run late.”

A weak answer: “It’s just a TEMPO, so conditions will pass quickly.” The second answer correctly identifies that TEMPO conditions are temporary. It misses the point entirely. The pilot found a word that sounded reassuring and stopped thinking.

Reading the raw TAF alongside a decoded version in ForeFlight or Garmin Pilot is a useful habit to build. Decoded plain-English views are genuinely helpful - but practicing with raw TAFs trains your eye to work with the actual structure. In an oral exam, a designated pilot examiner may hand you a printed TAF to read cold. There is no decoder.

The FAA’s Aviation Weather Handbook and the AIM are both free and are worth having open next to a TAF the first several times you work through one from scratch.

Key Takeaways

• A TAF covers a 5-statute-mile radius around one airport only - it is not a route forecast and says nothing about conditions along the way.
• The valid period format (e.g., 1512/1612) encodes both day and hour on each side of the slash; reading only the hour digits and ignoring the day is a common exam error.
• TEMPO is not a maybe. It forecasts expected conditions. The uncertainty is when and how long each occurrence lasts - not whether it happens.
• FM replaces all prior conditions completely. BECMG only modifies the elements it lists; everything else stays at the prior prevailing baseline. PROB40 is a 40% chance; PROB30 is 30%.
• Under FAR 91.169, TEMPO groups inside the one-hour window around your ETA count toward the alternate requirement - even if prevailing conditions are above the threshold.
• On a checkride, demonstrate reasoning: state what each change group forecasts, when, and what your contingency is.