PROB30 and PROB40

PROB30 means there is a 30 % probability of the conditions that follow occurring during the stated period. PROB40 means 40 %. They quantify forecaster confidence in an alternative outcome that is real but not likely.

Only those two values exist. A probability below 30 % is considered too low to be worth forecasting and is omitted. A probability above 40 % is high enough that the forecaster states the condition directly, with BECMG or TEMPO or as the prevailing forecast, rather than hedging it. So there is no PROB20, PROB50 or PROB60.

A probability group is never combined with BECMG, because a gradual permanent change and a probabilistic alternative are contradictory ideas. It is regularly combined with TEMPO: PROB40 TEMPO 1200 FG reads as a 40 % chance of temporary spells of fog reducing visibility to 1 200 m. Read on its own, PROB40 followed by a plain group describes a probable alternative for the whole period.