Estimation of Gusts From Climatology and Storm Intensity

The forecaster is aware that the season of the year and the station location have a great bearing upon the maximum winds to be expected. Certain areas of the United States, and the world, have a history of severe thunderstorm occurrence with associated strong winds during the most favorable seasons of the year. For this reason you should have the thunderstorm climatology for your station, as well as for the general area, available as to time of occurrence, season of occurrence, and the associated conditions.

The storms intensity and reports from neighboring stations can give you a good indication of conditions to expect at your station.

Forecasting Peak Wind Gusts Using the USAF Method

Refer to the AG2 TRAMAN, volume 2, unit 6, for an explanation of two methods for forecasting maximum wind gusts of convective origin. One involves the use of the Dry Stability Index (T1) and the other the downrush temperature subtracted from the dry-bulb temperature (T2).

Hail, like the maximum wind gusts in thunderstorms, usually takes place in a narrow shaft that is seldom wider than a mile or 2 and usually less than a mile wide. The occurrence of hail in thunderstorms was discussed earlier in this chapter. However, a few additional facts concerning the occurrence and frequency of hail in flight should be discussed at this point.

. Since hail is normally associated with thunderstorms, the season of the maximum occurrence of hail is coincident with the season of maximum occurrence of thunderstorms.

. When the storm is large and well developed, an assumption should be made that it contains hail.

. Encounters of hail below 10,000 feet show the hail distribution to the equally divided between the clear air alongside the thunderstorm, in the rain area beneath the storm, and within the thunderstorm itself.

. From 10,000 to 20,000 feet, the percentages range from 40 percent in the clear air alongside the storm to 60 percent in the storm, with 82 percent of the encounters outside the storm beneath the overhanging cloud.

. Above 20,000 feet, the percentages reflect 80 percent of the hail is encountered in the storm with 20 percent in the clear air beneath the anvil or other cloud extending from the storm.

Climatology is of vital importance in predicting hail occurrence, as well as hail size. Good estimations of the size of hail can be gleaned from reports of the storm passage over nearby upstream stations. Here too, modifying influences must be taken into account.

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