2. Parameters

Stability Assessment » Showalter Stability Index (SSI) Definition

Definition: Showalter Stability Index

The Showalter stability index (SSI) is a popular severe weather index. It is similar to the lifted index (LI), but while the LI starts with the mean of the lowest 100-hPa AGL (above ground level) layer, the SSI uses a parcel lifted from 850 hPa to 500 hPa. At 500 hPa the parcel temperature is subtracted from the sounding temperature. More negative SSI values indicate greater instability.

SSI values have been empirically linked to convective events as follows:

SSI Value Event
+3 to +1 Rain showers,
some thundershowers
+1 to -2 Thundershowers
-3 to -6 Severe thunderstorms
less than -6 Severe thunderstorms,
possible tornadoes

These threshold values are valid for lower elevation localities in the eastern 2/3 of the United States. As with LI or CAPE, you should never rely solely on SSI to evaluate the convective potential.