I'm still confused on the change indicator BECMG. I am sure of the definition of TEMPO being only a temporary fluctuations in the time frame given, but will a TAF with a change of BECMG continue on with the changes in the BECMG or will it revert back to the general forecast in the beginning of the TAF (basically is it permanent change or is the change only applicable in the time frame given just like TEMPO). As it says that once a TEMPO is expected to be have fluctuations of more than 1 hour it will be classified as BECMG.
This is me quoting from WMO document 306, which is also written in Annex 3 (1.3.3):
The change groups TTTTT YYGG/YeYeGeGe in the form of BECMG YYGG/YeYeGeGe shall indicate a change in forecast meteorological conditions expected to occur at either a regular or irregular rate at an unspecified time within the period YYGG to YeYeGeGe. The duration of the period YYGG to YeYeGeGe shall normally not exceed two hours and in any case shall not exceed four hours. The change groups shall be followed by a description of all the elements for which a change is forecast. When an element is not described in data groups which follow the change groups, the description of this element for the period between Y1Y1G1G1 and Y2Y2G2G2 shall be considered to remain valid subject to Regulation 51.1.5.
Note: The conditions described after the groups BECMG YYGG/YeYeGeGe are those expected to prevail from YeYeGeGe until Y2Y2G2G2, unless a further change is expected, in which case a further set of change groups BECMG YYGG/YeYeGeGe or FMYYGGgg must be used.
51.8.4 The change groups TTTTT YYGG/YeYeGeGe in the form of TEMPO YYGG/YeYeGeGe shall indicate frequent or infrequent temporary fluctuations in forecast meteorological conditions which are expected to last less than one hour in each instance and, in the aggregate cover, less than half of the period indicated by YYGG/YeYeGeGe.
If the modified forecast condition is expected to last one hour or more, Regulation 51.8.2 or 51.8.3 applies, i.e. the change groups BECMG YYGG/YeYeGeGe or FMYYGGgg must be used at the beginning and end of the period during which conditions are expected to depart from those forecast prior to YYGG or YYGGgg.
To keep forecasts clear and unambiguous, the use of change indicators should be carefully considered and kept to a minimum. In particular, the overlapping of change periods should be avoided. At any time during the period of validity of the TAF, only one possible variation in the prevailing forecast conditions should normally be indicated. The subdivision of the forecast period by FMYYGGgg should be used to avoid too complex forecasts in cases where many significant changes in weather conditions are expected to occur throughout the forecast period.