METAR and SPECI are observations of actual weather conditions. When a change is expected to occur within the next 2 hours, the trend forecast shall begin with one of the change indicators "BECMG" or "TEMPO":
TEMPO. Temporary change between the specified period. After the end of the period, the weather will revert back to the state of the last definitive change
BECMG. Gradual change from a previous state to a new one between the specified periods. After the end of the period, the weather will be the described one, nullifying previous changes. "BECMG 4000" => visibility will become 4 000 m within the next 2 hours.
ICAO Annex 3 Appendix 5
1.3.4 Recommendation.- The change indicator "BECMG" and the associated time group should be used to describe changes where the meteorological conditions are expected to reach or pass through specified threshold values at a regular or irregular rate and at an unspecified time during the time period. The time period should normally not exceed 2 hours but in any case should not exceed 4 hours.
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