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Stability Index Values


Listed below are the values of several stability indices and their typical interpretation. It should be noted that these index values provide a relative potential for thunderstorm occurrence based on the stability index alone. These relationships were developed when it was believed that stability indices alone could tell you the potential for thunderstorms. Recent research in thunderstorm forecasting has shown that instability is a necessary but not sufficient condition for the occurrence of thunderstorms. As a result, index values, by themselves, are only a part of the total evaluation needed to forecast thunderstorms. Use these tables with care.

Lifted Index

Index Value Thunderstorm Indication
0 to -2 thunderstorms possible
good trigger action needed
-3 to -5 unstable
thunderstorms probable
less than -5 very unstable
heavy to strong thunderstorms possible

source: reference 1

Showalter Index

Index Value Thunderstorm Consideration
3 to 1 thunderstorms possible
strong trigger needed
0 to -3 unstable
thunderstorms probable
-4 to -6 very unstable
good heavy thunderstorm potential
less than -6 extremely unstable
good strong thunderstorms potential

source: reference 1

K Index

West of Rockies East of Rockies Thunderstorm Probability
less than 15 less than 20 none
15 to 20 20 to 25 isolated thunderstorms
21 to 25 26 to 30 widely scattered thunderstorms
26 to 30 31 to 35 scattered thunderstorms
above 30 above 35 numerous thunderstorms

K values may not be representative of an airmass if the 850 mb level is near the earth's surface. Western United States values are forced to approximate areal coverage used by George [2].

source (west of Rockies): reference 1
source (east of Rockies): reference 2

Vertical Totals, Cross Totals, Total Totals

Vertical Totals (VT)
VT Values West of Rockies
with adequate moisture
less than 28 no thunderstorms
29 to 32 few thunderstorms
more than 32 scattered thunderstorms

source: reference 1

Total Totals (TT)
VT Values West of Rockies
48 isolated or few thunderstorms
52 scattered thunderstorms
few of moderate intensity
55 scattered thunderstorms
a few moderate, isolated severe
58 scattered moderate thunderstorms
few severe, isolated tornadoes
61 scattered to numerous moderate thunderstorms
few to scattered severe, few tornadoes
64 numerous moderate thunderstorms
scattered severe thunderstorms
scattered tornadoes

source: reference 1

East of the Rockies
CT Values VT Values TT Values Forecast
18-19* 26 or more* 44 isolated or few thunderstorms
20-21 26 or more 46 scattered thunderstorms
22-23 26 or more 48 scattered thunderstorms
isolated severe
24-25 26 or more 50 scattered thunderstorms
few severe, isolated tornadoes
26-29 26 or more 52 scattered to numerous thunderstorms
few to scattered severe, few tornadoes
30 26 or more 56 numerous thunderstorms
scattered severe, scattered tornadoes

* along the immediate Gulf Coast and over the Gulf Stream, CT of 16 or more and VT or 23 or more
TT is the most important.

source: reference 3

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[1] Western Region Technical Attachment, No. 84-14, May 1984, National Weather Service, Salt Lake City, UT.

[2] George, J.J., 1960: Weather Forecast for Aeronautics. Academic Press, New York.

[3] Miller, R.C., 1972: Notes on Analysis and Severe Storm Forecasting, Procedures of the Air Force Global Weather Central, AFGWC Technical Report 200 (rev).

last updated on 3/09/10