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.

What do FM, BECMG, and TEMPO mean?

FM marks a rapid and lasting change from a specific time. BECMG describes a gradual change expected to occur within a window and then persist. TEMPO describes temporary fluctuations lasting under an hour at a time that are not expected to dominate the period.

What does a PROB group mean?

PROB followed by a number, such as PROB30, gives the percent probability that the described conditions will occur during the stated window. It is used for significant but uncertain weather like thunderstorms, and the decoder labels the probability for you.

How do I read the validity period?

The validity group such as 1218/1324 means the forecast runs from the 12th at 18:00 UTC to the 13th at 24:00 UTC. Each change group also carries its own period showing when that variation applies within the overall forecast.

Does P6SM mean the same as 10SM?

P6SM means visibility greater than 6 statute miles and is the standard way North American TAFs report unlimited visibility, since 6 miles is the threshold above which exact values are not forecast. The decoder expands it to plain language.

TAF Decoder — Gera Tools

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.

How it works

The decoder first reads the header, then walks the change groups in order, attaching each weather element to the period it applies to:

TAF KSFO 121720Z 1218/1324 28010KT P6SM FEW020
        └issued   └valid    └prevailing forecast
FM122000 ...   = from the 12th at 20:00Z, conditions change
TEMPO 1220/1223 ...  = temporary fluctuations in that window
BECMG 1306/1308 ...  = gradually becoming, during that window

Within every group the same elements as a METAR are decoded: wind, visibility, present weather, and cloud layers.

Example and notes

The sample TAF for San Francisco issued on the 12th at 17:20 UTC, valid to the 13th at 24:00 UTC, forecasts wind from 280 at 10 knots becoming 300 at 12 knots after 20:00, with a temporary period of 5 statute miles in mist and a broken ceiling at 800 ft between 20:00 and 23:00. Read a TAF alongside the latest METAR, and treat TEMPO and PROB groups as conditions to plan around even when they are not the prevailing forecast.