Forecast Exercise: T2 Method

Hypothetical sounding for wind gust forecast exercise Compute maximum wind gust using the T2 method.

To answer the next question, you may want to open one or both of the following:

Step 1: Determine T2.
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then click Done.

T2 = °C

The correct answer is T2 = 10°C.
From the wet-bulb zero level (750 hPa), proceed down a moist adiabat to the surface where the corresponding temperature is 13°C.
T2 = TMAX - 13 = 23 - 13 = 10°C.

Step 2: Determine V.
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then click Done.

V = kt

The correct answer for V is 8.33 knots.
If we take the wind speed from the sounding, the average wind speed in the lowest 5,000 feet (surface to 850 hPa level in this case) is 25 knots.
Average wind speed = (10 + 20 + 30 + 40)/4 = 25.
V = 25/3 = 8.33 knots

Step 3: Determine V2.
V2 = V + 13*sqrt[(T1+T2)/2]
Enter your answer in the text box below,
then click Done.

V2 = kt

The correct answer for V2 is 52 knots.
With T1=12, T2=10, and V=8.33:
V2 = 8.33 + (13 * sqrt[(12+10)/2]) = 52 knots

Note that the T2 method which is more robust, predicted a maximum wind gust 7 knots higher than the T1 method, in part because it more realistically takes into account the negative buoyancy of the evaporatively-cooled downdraft air that drives convective wind gusts. The T2 method is preferred when time allows or an automated computer routine to calculate it is available. However, the T1 method can be a useful first guess for when you are confronted with an unexpected, rapidly evolving convective situation.
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