How to interpret BECMG and TEMPO in TAF?
The BECMG group in a TAF describes a gradual change which will take place over a prolonged period of time, but will be the prevailing weather once that change is complete. The time period described in the TAF is the beginning and ending hour during which the gradual change is forecast to be in progress. At the end of the described time the change is expected to be complete and the forecast weather should be the prevailing weather thereafter.

See the description of BECMG from the US Aviation Weather site's TAF decoder page:


  The BECMG group is used when a gradual change in conditions is expected over a longer time period, usually two hours. The time period when the change is expected is a four-digit group with the beginning hour and ending hour of the change period which follows the BECMG indicator. The gradual change will occur at an unspecified time within this time period. Only the conditions are carried over from the previous time group.


Conversely, with the TEMPO group, the described time period is a beginning and ending period after which the weather is forecast revert to the previously prevailing weather.

See the description of TEMPO from the US Aviation Weather site's TAF decoder page:


  The TEMPO group is used for any conditions in wind, visibility, weather, or sky condition which are expected to last for generally less than an hour at a time (occasional), and are expected to occur during less than half the time period. The TEMPO indicator is followed by a four-digit group giving the beginning hour and ending hour of the time period during which the temporary conditions are expected. Only the changing forecast meteorological conditions are included in TEMPO groups. The omitted conditions are carried over from the previous time group.