Definition
In a Terminal Aerodrome Forecast (TAF), FM is a change indicator marking a rapid change in forecast conditions, expected to occur in less than one hour. It is followed by a six-digit time group (day, hour, minute in UTC) and a complete new set of forecast conditions that fully replace the previous line until the next change group or the end of the forecast period.
Plain English
FM means 'from this time onward.' It tells you that, starting at the time shown, the weather is expected to change quickly to a new set of conditions, and those new conditions then apply until something else changes.
Context Anchor
Seen in coded TAF reports, especially when forecast wind, visibility, cloud height, or weather is expected to change at a specific time.
Derivation
FM is simply the shortened form of the English word 'from.' Aviation weather codes use two-letter abbreviations to keep TAFs and METARs compact for transmission, so 'from' becomes FM.
Why Pilots Care
Knowing the exact start time of a weather change helps pilots decide departure times, fuel loads, and whether an alternate airport will be needed.
Intuition Check
Do not read FM as a general hint that weather may change sometime later. In a TAF, FM points to a specific start time for the new forecast conditions.
Example Sentence 1
The TAF read 'FM221800 27015KT P6SM SCT040,' so from 1800Z the forecast called for wind 270 at 15 knots, visibility greater than 6 statute miles, and scattered clouds at 4,000 feet.
Example Sentence 2
After FM 0900, the forecast calls for winds from 240 at 12 knots and light rain.