Reading a TAF
A TAF (Terminal Aerodrome Forecast) uses the same coded shorthand as a METAR, but it forecasts conditions over a future window instead of reporting a single moment. Understanding a TAF is essential for any flight that isn't happening in the next few minutes — mapping surveys, long inspection missions, or any job scheduled hours in advance. This lesson decodes a full TAF and explains the change-group language (FM, TEMPO, BECMG, PROB) that makes TAFs read differently from a METAR.
The example we'll decode
The overall structure mirrors a METAR for the station identifier, issue time, and initial wind/visibility/sky groups. What's new is the valid period at the start of the body, and the change groups that follow, each describing how conditions are expected to shift at a specific point in the forecast window.
Header and valid period: TAF KAPX 151740Z 1518/1624
TAF identifies the report type, KAPX the station, and 151740Z the issue time (15th, 1740Z). 1518/1624 is the valid period: the forecast covers from the 15th at 1800Z through the 16th at 2400Z (written as "24" in this notation). Everything after this valid-period group describes forecast conditions somewhere inside that window — always check that your planned flight time actually falls inside the valid period before relying on any TAF.
The initial (baseline) forecast group
22012G20KT P6SM SCT050 BKN100 is the baseline forecast in effect from the start of the valid period until the first change group takes over. P6SM means "plentiful," or greater than 6 statute miles visibility — TAFs cap reported visibility at 6SM rather than going higher. The wind and sky groups use the exact same format as a METAR.
FM (From) group: FM152200 20008KT P6SM SCT060
FM marks a complete, clean-break change to a new set of conditions starting at the stated time — here, 2200Z on the 15th. Unlike TEMPO or PROB, an FM group is not temporary or probabilistic: from that moment forward, treat the new conditions as the forecast baseline until the next change group. Read FM groups as "everything before this point uses the previous group; everything from this point forward uses this one."
TEMPO group: TEMPO 1600/1604 4SM -RA BR
TEMPO means temporary, fluctuating conditions expected during part (not necessarily all) of the stated period — here, between 0000Z and 0400Z on the 16th. It describes conditions expected to last less than an hour at a time and to affect less than half of the stated period. -RA means light rain; BR means mist. Because TEMPO conditions are intermittent rather than continuous, don't assume they hold for the entire stated window — but do plan for the possibility they occur at any point inside it.
PROB group: PROB30 1604/1608 2SM TSRA BKN008CB
PROB30 gives a probability — here 30% — that the stated conditions will occur during the given period. TSRA means thunderstorms with rain; BKN008CB means broken clouds at 800 feet with cumulonimbus (thunderstorm-producing) clouds present. A 30% probability of thunderstorms is not something to dismiss for a Part 107 flight — even a moderate chance of TSRA in the forecast window is a strong signal to build in a flexible schedule or an earlier flight time.
The example TAF's valid period plotted as a timeline, showing where each change group applies.
View Larger| Group | Meaning | Duration implied | Planning takeaway |
|---|---|---|---|
| FM (From) | Complete change to new baseline conditions | Continues until the next change group | Treat as the new expected baseline from that time forward |
| BECMG (Becoming) | Gradual transition to new conditions over a stated period | Change occurs sometime within the stated 2-hour window | Expect conditions to be "in transition" during the window, settled after |
| TEMPO (Temporary) | Intermittent, temporary fluctuation | Less than 1 hour at a time, affecting less than half the stated period | Plan for the possibility of these conditions occurring, not their certainty or duration |
| PROB30 / PROB40 | Probability (30% or 40%) of the stated conditions | As stated in the accompanying time window | Even 30% probability of hazardous weather (e.g. thunderstorms) warrants a flexible plan |
Reading a TAF for a Specific Flight Window
Follow this sequence to figure out what conditions actually apply to your specific planned flight time.
Confirm your flight time falls inside the valid period
Check the DDHH/DDHH group at the start of the body. If your flight is outside this window, you need a different or newer TAF.
Identify the baseline conditions in effect at your flight time
Start from the beginning of the TAF and move forward through each FM group until you find the one whose start time is at or before your flight time — that's your baseline.
Check whether any TEMPO or PROB window overlaps your flight time
If it does, factor in the possibility of those conditions, even though they aren't guaranteed for the entire period.
Weigh the overall trend, not just a single group
A forecast improving through your flight window is a very different risk picture than one deteriorating, even if the baseline group looks similar at first glance.
| Mistake | Why it happens | Correct understanding | Regulation / source |
|---|---|---|---|
| Assuming TEMPO conditions apply for the entire stated time window | The window looks like a normal duration range, similar to an FM group | TEMPO means intermittent conditions lasting under an hour at a time, affecting less than half the period | NWS TAF format standard |
| Treating a low PROB30 as "probably won't happen, ignore it" | 30% sounds unlikely in everyday language | For hazardous phenomena like thunderstorms, even a 30% probability is a meaningful planning factor for a low-margin sUAS operation | AC 107-2A |
| Reading a TAF issued the previous day without checking whether it's still current | The forecast looks complete and detailed | TAFs are typically reissued about every 6 hours; always pull the most recently issued version | aviationweather.gov |
What's the practical difference between a METAR and a TAF?
A METAR is a report of conditions at a single moment, at a single station. A TAF is a forecast covering a future window of time, typically 24-30 hours, at a specific airport. Use METARs to confirm current conditions right before flight and TAFs to plan ahead for a flight scheduled hours or a day in advance.
Do TAFs exist for every airport?
No. TAFs are only issued for a subset of airports (generally larger or more significant ones), unlike METARs which are available at many more locations. If your operating area has no TAF-issuing airport nearby, use the nearest one available along with graphical forecast products from aviationweather.gov.
What does BECMG mean, and how is it different from TEMPO?
BECMG (becoming) describes a gradual, one-directional transition to new conditions expected to be established and to persist by the end of the stated period — unlike TEMPO, which describes conditions that come and go intermittently without becoming the new steady state.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.