How to Read a TAF

A TAF uses almost the same vocabulary as a METAR, but instead of describing the weather now it forecasts how the weather will evolve over the next 24 to 30 hours. The trick to reading one is understanding the change groups — FM, BECMG, TEMPO, and PROB — and the time windows attached to each. This guide breaks all of that down with two fully worked forecasts.

What a TAF is

TAF stands for Terminal Aerodrome Forecast. It is an official forecast of expected conditions within roughly five statute miles of an airport. TAFs are issued for busier airports, normally four times a day (around 00, 06, 12, and 18 UTC), and are typically valid for 24 or 30 hours. Because a TAF is a forecast, it carries uncertainty — which is exactly why it uses probability and temporary-change groups rather than a single fixed prediction.

The TAF header

TAF KDEN 121130Z 1212/1318 20010KT P6SM SCT050

The valid period 1212/1318 is the single most important thing to read first. Each pair is day-then-hour in UTC: the forecast begins on the 12th at 1200Z and ends on the 13th at 1800Z. Everything that follows describes the prevailing conditions and how they are expected to change inside that window.

The four change groups

FM — From (a clean break)

FM marks a rapid, permanent change. Everything after an FM group completely replaces the previous line until the next change group. FM131500 means "from 15:00 UTC on the 13th, the new conditions are …". FM groups carry a full six-digit day-hour-minute time.

BECMG — Becoming (a gradual transition)

BECMG describes a gradual change over a stated window, usually one to two hours. BECMG 1318/1320 means the change takes place between 18:00 and 20:00 UTC on the 13th; after it completes, the new conditions prevail. Use BECMG for a steady shift, such as a front slowly clearing.

TEMPO — Temporary fluctuations

TEMPO flags temporary conditions expected to last less than an hour at a time and to cover less than half of the stated window. TEMPO 1316/1320 3SM TSRA means brief bouts of 3-mile visibility in thunderstorms and rain are possible between 16:00 and 20:00 UTC on the 13th, but are not the prevailing weather.

PROB — Probability

PROB30 and PROB40 attach a 30% or 40% probability to the conditions that follow. Higher confidence would simply be forecast outright, and lower confidence is not published, so only PROB30 and PROB40 are used. You will often see PROB30 combined with a TEMPO group for low-confidence, short-lived events.

Worked example 1

TAF KDEN 121130Z 1212/1318 20010KT P6SM SCT050
  FM121800 24012G20KT P6SM BKN060
  TEMPO 1220/1224 3SM TSRA BKN035CB
  FM130400 27008KT P6SM SCT080

1. From 1200Z on the 12th: wind 200°/10, better than 6 miles, scattered at 5,000.
2. FM121800 — from 1800Z the wind veers to 240° gusting 20 and clouds become broken at 6,000.
3. TEMPO 1220/1224 — between 2000Z and 0000Z, brief thunderstorms may cut visibility to 3 miles with cumulonimbus at 3,500.
4. FM130400 — from 0400Z on the 13th it settles down: wind 270°/8, scattered at 8,000.

Reading it as a story rather than a code makes the plan obvious: a breezy, showery evening giving way to calmer conditions overnight.

Worked example 2 (deteriorating)

TAF KBOS 061720Z 0618/0724 15012KT P6SM BKN025
  BECMG 0700/0702 12015G25KT 4SM -RA BR OVC012
  TEMPO 0702/0708 1SM RA BR OVC006
  PROB30 0704/0708 1/2SM +RA FG

1. From 1800Z on the 6th: south-southeast wind 12 knots, good visibility, broken at 2,500.
2. BECMG 0700/0702 — between 0000Z and 0200Z on the 7th, wind increases to 15 gusting 25, rain and mist reduce visibility to 4 miles, overcast lowers to 1,200.
3. TEMPO 0702/0708 — temporary drops to 1 mile in rain and mist, overcast 600.
4. PROB30 0704/0708 — a 30% chance of half-mile visibility in heavy rain and fog.

This is a forecast to respect: a marginal-VFR evening becoming solid IFR overnight, with a real chance of near-zero conditions. See Flight Categories for how those numbers map to VFR/IFR.

Reading tips

• Always read the valid period first, then work through the groups in time order.
• FM and BECMG change the baseline; TEMPO and PROB are exceptions layered on top of it.
• A CB (cumulonimbus) note with a TEMPO thunderstorm is a flag for possible turbulence, wind shear, and hail.
• If the TAF period does not cover your entire flight, you are missing part of the picture — check the next issuance or a nearby TAF.

CAVOK, NSW, and visibility shorthand

A few tokens are worth calling out. P6SM means visibility greater than six statute miles — the "P" stands for "plus", and six miles is the highest value normally forecast. NSW means "no significant weather" and is used in a change group to cancel weather that was forecast earlier. CAVOK (ceiling and visibility OK) is a single word meaning visibility is 10 km or more, there is no cloud below 5,000 ft or the highest minimum sector altitude, and no significant weather — you will see it more often on international TAFs than on US ones.

Low-level wind shear (WS)

A TAF can forecast non-convective low-level wind shear with a WS group, for example WS020/24045KT. That reads as wind shear at 2,000 ft AGL, where the wind is from 240° at 45 knots while the surface wind is much lighter. Shear like this is a serious hazard on approach and departure and is one of the most important things a TAF can warn you about.

Amendments and corrections

Because a TAF is a forecast, it is revised when reality diverges from it. TAF AMD is an amended forecast issued when the forecaster expects conditions to differ materially from the current TAF, and TAF COR is a correction of a coding error. When an amended TAF is issued it fully replaces the previous one, so always work from the latest issuance time shown in the header.

Common mistakes

• Ignoring the valid period and reading conditions for a time the TAF does not cover.
• Treating a TEMPO or PROB group as the prevailing forecast — they are exceptions layered on the baseline.
• Forgetting that FM completely resets the forecast while BECMG only changes the elements that are stated.
• Assuming a TAF describes the whole route. It covers only about five miles around the airport.

Limitations

A TAF covers only about five miles around the airport and only the published valid period. It says nothing about the en-route environment, and it can be amended at any time when the forecast busts. Smaller airports have no TAF at all, so pilots interpolate from surrounding stations and area forecasts. Treat a TAF as a well-informed expectation, not a guarantee.

Official sources

• FAA Aviation Weather Handbook (FAA-H-8083-28) and AC 00-45 — TAF format and change-group definitions.
• NWS Aviation Weather Center — aviationweather.gov.
• Leidos Flight Service — 1800wxbrief.com.