Effective with the 1200 UTC forecast cycle on September 17, 2002, guidance for the probability of thunderstorms and severe thunderstorms will be added to the Eta MOS messages. The probability of thunderstorm guidance provides forecasts of the probability of a thunderstorm in a 6-, 12- or 24-h period. Because the thunderstorm guidance was developed from lightning strike data extracted from the National Lightning Detection Network (NLDN), guidance is only provided for sites in the contiguous United States (CONUS). Both the thunderstorm and conditional severe storm probabilities are available year-round for stations in the CONUS. Forecast equations were developed for three seasons: Spring (March 16 - June 30), Summer (July 1 - October 15), and Cool (October 16 - March 15).

Sample Message

KSAT   ETA MOS GUIDANCE    8/09/2002  0000 UTC
DT /AUG   9            /AUG  10                /AUG  11
HR   06 09 12 15 18 21 00 03 06 09 12 15 18 21 00 03 06 09 12
.
.
.
T06      8/ 0  7/ 0 47/10 28/ 5  6/ 1  6/ 0 57/12 32/ 6 11/ 2
T12           15/ 0       57/12       14/ 2       61/13

The T06 line represents forecasts for the probability of thunderstorms (to the left of the diagonal) and the conditional probability of severe thunderstorms (to the right of the diagonal) during a 6-h period. The 6-h probability forecasts are valid for intervals of 6-12, 12-18, 18-24, 24-30, 30-36, 36-42, 42-48, 48-54, and 54-60 hours after the initial data times (0000 and 1200 UTC). In the message, the pair of forecast values is displayed under the ending time of the 6-h period. The thunderstorm probability is given to the nearest whole percent. Values range from 0 to 100%. A missing forecast value is indicated by 999. The conditional severe thunderstorm probability is given to the nearest whole percent. Values range from 0 to 98%. A missing forecast value is given by 99.

The T12 line represents forecasts for the probability of thunderstorms (to the left of the diagonal) and the conditional probability of severe thunderstorms (to the right of the diagonal) occurring during a 12-h period. The 12-h probability forecasts are valid for intervals of 6-18, 18-30, 30-42, and 42-54 hours after the initial data times (0000 and 1200 UTC). In the message, the pair of forecast values is displayed under the ending time of the 12-h period. The thunderstorm probability is given to the nearest whole percent. Values range from 0 to 100%. A missing forecast value is indicated by 999. The conditional severe thunderstorm probability is given to the nearest whole percent. Values range from 0 to 98%. A missing forecast value is given by 99.

During the development, the lightning data were placed on a 48 km grid, and all of the strikes that occurred anywhere in the 48 km grid box were assigned to the center point of the grid box. This is an areal forecast covering the grid box, and the forecast verifies when a thunderstorm occurs anywhere in the grid box.

For the text messages, a nearest neighbor approach was used to match each CONUS MOS site to the closest thunderstorm grid point. For the GRIB and graphics products, the forecasts have been analyzed to a 40 km Lambert Conformal grid covering the CONUS and about 150 km beyond.