Appears in METARs and TAFs
BECMG
A gradual, lasting change: once it happens, the new conditions hold.
BECMGBECMG marks a permanent change. The conditions that follow it are expected to set in at some point within the stated period and then to persist for the remainder of the forecast, unless a later group changes them again.
In a TAF it is followed by a period in DDHH/DDHH form — BECMG 0312/0314 means the change is expected somewhere between 12:00 and 14:00 UTC on the 3rd. The change is understood to happen at an unspecified time inside that window, normally over less than two hours. In a METAR trend, BECMG can instead be qualified with AT, FM or TL and a time.
The distinction from TEMPO is the one that matters operationally. BECMG moves the baseline: after it, the forecast reads as though the new conditions are the normal state. TEMPO does not — it describes fluctuations around a baseline that is left unchanged. If a forecast says BECMG 3000, plan on 3 000 m for the rest of the period; if it says TEMPO 3000, plan on the better figure with 3 000 m as an intermittent possibility.
In a real report
TAF EDDF 311100Z 3112/0118 21010KT 9999 SCT030 BECMG 3118/3120 24018G30KT
The wind is 210° at 10 kt until some point between 18:00 and 20:00 on the 31st, after which 240° at 18 kt gusting 30 becomes the new baseline for the rest of the forecast.