CAPE- Convective Available Potential Energy. A measure of the amount of energy available for convection. CAPE is directly related to the maximum potential vertical speed within an updraft; thus, higher values indicate greater potential for severe weather. Observed values in thunderstorm environments often may exceed 1,000 joules per kilogram (j/kg), and in extreme cases may exceed 5,000 j/kg. However, as with other indices or indicators, there are no threshold values above which severe weather becomes imminent.
Value/ Event
1 - 1,500 Positive for thunderstorms
1,500 - 2,500 Large
2,500+ Extreme
Lifted Index (or LI)- A common measure of atmospheric instability. Its value is obtained by computing the temperature that air near the ground would have if it were lifted to some higher level (around 18,000 feet, usually) and comparing that temperature to the actual temperature at that level. Negative values indicate instability - the more negative, the more unstable the air is, and if thunderstorms develop they are more likely to be stronger.
Value /Event
+3 to +1 Rain showers, some thundershowers
+1 to -2 Thundershowers
-3 to -6 Severe thunderstorms
< -6 Severe thunderstorms, possible tornadoes
K Index. The K Index is primarily applicable in the prediction of air mass thunderstorms. Low values of the K Index in the presence of other strong thunderstorms indicators (sharp trough, high level jet, etc.) may suggest a severe thunderstorm potential. For example, a low K value might result from a 700 mb dry tongue. Limitations: Favors non severe convection. This index is a measure of thunderstorm potential but has nothing to do with severity of storms. Can not be used in mountain areas. The K value may not be representative of the airmass if the 850mb level is near the surface.
K-index value (in °C) / Thunderstorm Probability
< 20 None
20 to 25 Isolated thunderstorms
26 to 30 Widely scattered thunderstorms
31 to 35 Scattered thunderstorms
Above 35 Numerous thunderstorms
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