4.2 Introduction
Forecast change indicator groups are used to describe significant changes to the prevailing conditions and/or to indicate temporary changes to the prevailing conditions. They can be used to forecast low probabilities for a thunderstorm or precipitation occurrence and their 
associated significant weather.
NWS authorized change indicator groups are:
- FM (from)
- TEMPO (temporary)
- PROB30 (30% probability)
The FM group is used to indicate a significant change to the prevailing conditions at a specific point in time and therefore provides the user with more definitive information whereas TEMPO and PROB30 indicate a greater degree of uncertainty in the forecast.
Forecast changes should be kept to a minimum number needed to describe operationally significant changes.
Be careful not to over use conditional indicators like TEMPO or PROB30 because the lowest meteorological conditions forecast in a TAF will impact operational decisions.