How to interpret BECMG and TEMPO in TAF?

Question

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.

Notes:

1. 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.

2. 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.

Answer 1

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.

Answer 2

TEMPO is a temporary fluctuations in the time frame given, the fluctuations should be not more than an hour in each instant and in aggregate it should be not more than half of the forecasting period. TEMPO will revert back when the forecasting period is over.

BECMG is the permanent change, the process of gradual change is not more than two hours and should not exceed four hours.

Answer 3

In plain words- Becoming means that once the change occurs, you can now relate to the changes noted in the Becoming report as prevailing.

More closely- you regard every improvement occurring as it occurred the end of the time of the becoming period, and deteriorating as occurring right at the beginning of the window. Every aspect of weather that was not noted in the report- shall be regarded as the previous prevailing report.

So, if in the Becoming report we had- BECMG 220/15- only the wind changed, but the cloud coverage stays the same as before. The change is permanent. I am attaching a link to the Jeppsen guide here- Jeppsen general link.