There are two methods for calculating convective gusts. These are the T₁ and T₂ methods. The T₁ method is used to predict average maximum gusts and the maximum wind gust in air mass thunderstorms. The T₂ method should be used to compute the maximum wind gust in frontal thunderstorms or in a prefrontal squall line, when numerous air mass thunderstorms are expected, or when an air mass meso-scale convective thunderstorm complex threatens your station. In brief, use the T₁ method for thunderstorm gusts at your station when thunderstorms are expected in the vicinity, and use the T₂ method if a thunderstorm is expected to pass directly over your station with heavy rain. The maximum downrush wind or microburst will occur as the transition between the mature stage and the dissipating stage of a strong thunderstorm coincident with the beginning of the heaviest precipitation.

The most severe weather appears to occur in areas where the height of the Wet-Bulb-Zero (WBZ) is less than 10,500 feet. Wet-Bulb-Zero heights between 7,000 and 9,000 feet above the ground are most closely associated with destructive surface winds caused by microbursts from thunderstorms. Microbursts are extremely rare with a WBZ height lower than 5,000 feet or higher than 10,500 feet. It seems the WBZ height is a reliable preliminary indicator of microbursts and the associated strong wind gusts from thunderstorms.

If the WBZ height on your Skew T falls between 5,000 and 11,000 feet, then the analysis should be continued with either the T₁ method or the T₂ method, as applicable.

The T₁ method uses the calculated difference between the temperature of a parcel of moist surface air raised moist adiabatically to 600 millibars and the 600-millibar temperature as a guide to determine the surface wind gusts. The computation procedure is as follows:

1. Have the forecaster determine what time the thunderstorms are expected to occur, and the maximum temperature expected during that time period.

2. If an inversion is present within 200 millibars of the surface and the inversion will not be removed by diurnal changes, from the warmest point in the inversion ascend moist adiabatically to 600 millibars, and record the temperature. If no strong inversion is present, ascend moist adiabatically from the predicted maximum temperature to 600 millibars, and record this temperature.

3. Subtract the actual 600-millibar tempera-ture from the temperature you just found and recorded. The difference between these two temperatures is the T₁ temperature.

4. Enter table 6-2-2 with T₁ to find the average maximum gust speed (V') The table is

based on the formula with some empirical adjustments.

5. Calculate the mean wind speed in the surface-to-5,000-foot layer.

6. Add one-third of the mean surface-to- 5,000-foot wind to V' to find your maximum probable gust speed, This is the T₁ predicted gust speed.

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