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Gfs  (Showalter-Index)                                                   source:http://weather.unisys.com
White lines with values in wet bulb potential temperatures (deg C).
Critical values for Showalter Index:
(not to be used with frontal inversion between 850-500 hPa)

< +3: showers, possible thundery.
<  -2: severe thundersorms.
<  -5: risk for tornados.

Showalter Index calculator
Enter the 500 hPa parcel temperature (*) °C
Enter the 500 hPa temperature °C
The Showalter Index is

SSI   > 2:   no thunderstorms.
SSI =< 2:   thunderstorms.

(*) To find 500 hPa parcel temperature: construct LCL for parcel at 850 hPa. From this point ascend saturated adiabatically to the 500 hPa level and read off.

This Index doesn't work well if a frontal surface or inversion is present between 850 hPa and 500 hPa.

Formula used: SSI = T - Tparcel  (both T at 500 hPa)  



a. The SI only assesses instability in one level of the troposphere. Unlike SI, CAPE is better at assessing instability in the troposphere as a whole.

b. Take note of the elevation of the sounding station. If the 850 mb level is near the surface, then the LI and SI will basically be using the same method.

c. Always look at the sounding to see what is causing the index to be stable or unstable. Sounding will show the depth of the air masses and how relevant using the SI is. By http://www.theweatherprediction.com