The forecast change indicators FM, TEMPO, and PROB are used when a change in any or all of the forecast elements is expected.

The change group FMYYGGgg (voiced as “from”) is used to indicate when prevailing conditions are expected to change significantly over a period of less than one hour. In these instances, the forecast is subdivided into time periods using the contraction FM, followed, without a space, by six digits, the first two of which indicate the day of the month and the final four indicate the time (in hours and minutes Z) the change is expected to occur. While the use of a four-digit time in whole hours (e.g., 2100Z) is acceptable, if a forecaster can predict changes and/or events with higher resolution, then more precise timing of the change to the minute will be indicated. All forecast elements following FMYYGGgg relate to the period of time from the indicated date and time (YYGGgg) to the end of the valid period of the terminal forecast, or to the next FM if the terminal forecast valid period is divided into additional periods.

The FM group will be followed by a complete description of the weather (i.e., self-contained), and all forecast conditions given before the FM group are superseded by those following the group. All elements of the TAF (e.g., surface wind, visibility, significant weather, clouds, obscurations, and when expected, non-convective LLWS) will be included in each FM group, regardless of if they are forecast to change or not. For example, if forecast cloud and visibility changes warrant a new FM group but the wind does not, the new FM group will include a wind forecast, even if it is the same as the most recently forecast wind.

The only exception to this involves the significant weather group. If no significant weather is expected in the FM time period group, then significant weather group is omitted. A TAF may include one or more FM groups, depending on the prevailing weather conditions expected. In the interest of clarity, each FM group starts on a new line of forecast text, indented five spaces.

Examples:

A change in the prevailing weather is expected on the third day of the month at 0230 UTC and the third day of the month at 0500 UTC.

Note that the wind in the FM010500 group is the same as the previous FM group but is repeated since all elements are to be included in a FM group.

The change-indicator group TEMPO YYGG/Y Y G G is used to indicate temporary fluctuations to forecast meteorological conditions that are expected to:

The first two digits (YY) are the day of the month for the start of the TEMPO. The next two digits (GG) are the starting hour (UTC). After the solidus (/), the next two digits (Y Y ) are the ending day of the month, while the last two digits (G G ) are the ending hour (UTC) of the TEMPO period.

Each TEMPO group is placed on a new line in the TAF. The TEMPO identifier is followed by a description of all the elements in which a temporary change is forecast. A previously forecast element that has not changed during the TEMPO period is understood to remain the same and will not be included in the TEMPO group. Only those weather elements forecast to temporarily change are included in the TEMPO group.

TEMPO groups will not include forecasts of either significant weather in the vicinity (VC) or non-convective LLWS.

Examples:

In the example, all forecast elements in the TEMPO group are expected to be different than the prevailing conditions. The TEMPO group is valid on the 22nd day of the month from 1500 UTC to 1700 UTC.