Notice of Intent to Change AVN MOS Guidance

Effective with the 1200 UTC forecast cycle on or about July 24, 2001, guidance for categorical forecasts of precipitation amount (QPF) will be added to the new AVN MOS messages. These messages are identified with WMO headers FOPA20, FOUS21-FOUS26, and FOAK37-FOAK39, and are stored as MAV products in the AWIPS text data base. In addition, new forecast equations to predict the probability of precipitation (PoP) will be implemented. The PoP guidance provides forecasts of the probability of precipitation equaling or exceeding 0.01 inches of liquid-equivalent amount in a 6- or 12-h period. The 6-h probabilities for all sites are labeled as P06 and are valid 6-12, 12-18, 18-24, ..., and 66-72 hours after 0000 or 1200 UTC. Except for the stations in the FOPA20 message, the 12-h probabilities labeled as P12 are valid for periods of 12-24, 24-36, 36-48, 48-60, and 60-72 hours after either 0000 or 1200 UTC. For the FOPA20 sites, the 12-h probabilities are valid for periods of 6-18, 18-30, 30-42, 42-54, and 54-66 hours after 0000 or 1200 UTC. The QPF guidance labeled as Q06 and Q12, respectively, for the 6- and 12-h periods, provides categorical forecasts of precipitation amount for the same projections as the PoP. The categorical definitions are as follows:

0 = no precipitation;
1 = 0.01 to 0.09 inches;
2 = 0.10 to 0.24 inches;
3 = 0.25 to 0.49 inches;
4 = 0.50 to 0.99 inches;
5 = > 1.00 inches (Q06); 1.00 - 1.99 inches (Q12)
6 = > 2.00 inches (Q12 only)
More details on the new guidance may be found at the following sites:

In addition to the new QPF guidance, we are updating the equations used to produce the visibility and obstruction to vision guidance available in the MAV message. This update eliminates spurious forecasts of low visibility conditions that had been noted by several forecasters.

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Page created July 20, 2001