FORECASTING MOVEMENT OF THUNDERSTORMS

Radar can be an invaluable aid in determining the speed and the direction of movement of the thunderstorm. Sometimes it is desirable and necessary to estimate the movement from winds aloft. There is no completely reliable relationship between speed or direction of the winds aloft in forecasting thunderstorm movement. However, one study reveals that there is a marked tendency for large convective rainstorms to move to the right of the wind direction in the mean cloud layer from 850 to 500 hPa (or the 700-hPa wind) with a deviation of about 25 degrees. There appears to be little correlation between wind speed and speed of movement at any level, although the same study mentioned above revealed that 82 percent of the storms move within plus or minus 10 knots of a mean speed of 32 knots.

Figure 5-7.-Computation of the Showalter Stability Index (SSI).

FORECASTING MAXIMUM GUSTS WITH NONFRONTAL THUNDERSTORMS

Maximum gusts associated with thunderstorms occur over a very small portion of the area in which the thunderstorm exists, and usually occur immediately before the storms passage. Nevertheless, the possibility of damage to aircraft and installations on the surface is so great that every available means should be used to make the best and most accurate forecast possible to forewarn the agencies concerned.

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