Aviation Verify Program: Multiple Forecasts At One Time

During any single 5-minute interval, one or two forecasts may be in effect, depending upon whether or not a TEMPO or PROB group is in effect. If neither a TEMPO nor PROB group is in effect, verification is done by simply comparing the prevailing forecast (FM or BECMG group) to the observation. If a TEMPO or PROB group is in effect, then two forecasts (one from the prevailing group and one from the TEMPO or PROB group) are valid for that 5-minute interval. For every 5-minute interval that a PROB group is in effect, the lowest (worst) forecast condition is used for verification. For every 5-minute interval that a TEMPO group is in effect, a two step process is used to determine which forecast is used for verification. The first step checks the observational database for variability since TEMPO groups are only justified when atmospheric conditions are varying. Once justification based upon variability is established, the second step determines which of the two forecasts are most operationally significant, and that forecast is used for verification. Step 1. For each 5-minute interval each element is checked separately for variability to see if a TEMPO forecast is justified. A TEMPO group is justified if conditions change twice or more during a period of three hours or less. For ceiling or visibility, change is defined as movement between categories as listed in Section 4.1.1 (CIG) and section 4.1.2 (VIS). For weather, each element (i.e., thunderstorm, rain, fog, snow, and freezing precipitation) is evaluated separately and a change is defined as the starting or stopping of that element. For wind speed, change is defined as an increase or decrease in the sustained speed of greater than 7 knots. For wind gusts, change is defined as an increase or decrease in the gust speed of greater than 9 knots. Change is also defined as whenever subsequent observations shift from the occurrence of gusts to the occurrence of no gusts or vice versa. For wind direction, change is defined as a shift in direction of greater than 30 degrees. Step 2. Follow the appropriate instruction, below: If the TEMPO group is justified, each 5-minute interval is evaluated with the forecast from either the prevailing or TEMPO conditions, whichever produces the most favorable verification score. If the TEMPO group is not justified, each 5-minute interval is evaluated with the forecast representing the lowest (worst) conditions, the conditions for which the pilot must plan.

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