Gera Tools

TAF Decoder

Translate a Terminal Aerodrome Forecast into readable plain-English periods

Paste a raw TAF and get a chronological plain-English breakdown of the prevailing forecast plus FM, BECMG, TEMPO, and PROB change groups. Pilots and operations staff use it to assess departure, alternate, and destination conditions. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Last updated Source: Gera Tools

What is the difference between a TAF and a METAR?

A METAR is an observation of current weather, while a TAF is a forecast of expected conditions at an aerodrome, typically valid for 24 or 30 hours. The TAF uses similar codes but adds change groups describing how conditions evolve over time.

A TAF forecasts how the weather at an airport will change over the next day, but its layered change groups can be confusing until they are spread out on a timeline. This decoder breaks a pasted TAF into its prevailing forecast and each change group, translated into plain English.

Reading the structure of a TAF

Every TAF follows the same pattern: a header block, a prevailing forecast, and then zero or more change groups stacked after it. The change groups are where the complexity lives, because each one modifies or replaces the prevailing conditions for a specific period and must be understood relative to the groups around it.

TAF KSFO 121720Z 1218/1324 28010KT P6SM FEW020
     ↑     ↑        ↑
  Station Issued  Valid period  Prevailing forecast

FM122000 30012KT P6SM SKC        ← from 20Z on the 12th: new prevailing conditions
TEMPO 1220/1223 5SM BR BKN008   ← temporary: 20Z–23Z, fluctuations possible
BECMG 1306/1308 VRB03KT         ← gradual change to variable wind in that window

The decoder reads the groups in sequence and attaches each weather element to the period it applies to, so you see the airport’s expected conditions on a timeline rather than as one dense code string.

How each change group type works

FM (From) marks an abrupt, lasting change. Everything in the prevailing forecast is replaced by the FM group’s conditions from the stated time onward, until the next FM or the end of the validity period. When you see an FM, think “the prevailing conditions from this moment forward.”

BECMG (Becoming) describes a gradual change expected to occur within a window and then persist. The conditions outside the window remain as the prevailing forecast; inside the window, conditions are transitioning. After the BECMG window closes, the new conditions become the prevailing forecast.

TEMPO (Temporary) describes fluctuations that are expected to last less than an hour at a time and to occur for less than half the total period. They do not replace the prevailing conditions — they overlay them. For flight planning, TEMPO conditions are worth checking even if they seem brief, especially for low visibility or ceilings.

PROB (Probability) gives the forecaster’s estimated probability (typically 30 or 40 percent) that a specific condition will occur in the stated period. A PROB30 thunderstorm group means the forecaster assessed a 30% chance of thunderstorm activity in that window. Any PROB group is worth reviewing for flight-critical conditions.

Worked example — what to look for

For a TAF that reads:

TAF EGLL 191720Z 1918/2024 27008KT 9999 FEW030
TEMPO 1920/1924 4000 BR OVC005
BECMG 2006/2008 VRB03KT

The decoder would tell you:

  • Prevailing (18Z–20Z): westerly 8 knots, visibility exceeding 10 km, few clouds at 3,000 ft — good VFR conditions.
  • TEMPO 20Z–24Z on the 19th: temporary reductions to 4,000m in mist, overcast at 500 ft — IFR conditions possible, under an hour at a time.
  • BECMG 06Z–08Z on the 20th: gradual change to light variable winds.

The TEMPO period is the operationally significant group here — brief IFR conditions possible through the night.

Notes

Read a TAF alongside the latest METAR for the same station: the METAR shows what is actually happening now, which tells you whether the TAF’s prevailing forecast is verifying. TEMPO and PROB groups represent conditions to plan around even if they are not the forecast most likely outcome.