Change Group
Meaning
RAPID
A change in prevailing conditions that will take place during a
period of time less than 1/2 hour
GRADU
A change in prevailing conditions that will take place during a
period of time lasting more than 1/2 hour but less than 2
hours
Used to indicate temporary changes in a prevailing forecast
TEMPO
condition. Each change should last less than 1 hour, and if
the change is to occur more than once, the total time of all
occurrences should not exceed one half the total time
covered by the forecast.
Used to indicate intermittent changes from a predominate
INTER
forecast condition. Intermittent changes occur more
frequently than temporary changes and last for shorter
periods of time.
Table 1-24 -- Four TAF change groups
Weather Advisories
In-flight advisories serve to notify enroute pilots of the possibility of encountering
hazardous flying conditions that may not have been forecast at the time of the preflight
weather briefing.
NWS Flight Advisories
The NWS issues in-flight weather advisories designated Convective SIGMETs (WST),
SIGMETs (WS), and AIRMETs (WA) (see Table 1-25). These advisories are often
transmitted to air traffic control facilities via the flight data input/output (FDIO) system.
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