Lifted Index — The map displays contours of the Lifted Index, at 4-unit intervals. The Lifted Index (LI) is a measure of the stability of the atmosphere. It is simply the difference between the observed 500 milllibar (mb) temperature and the temperature that a parcel of air would have if it were lifted from the boundary layer to the 500 mb level. If the LI is positive, the atmosphere is stable. If the LI is negative, the atmosphere is unstable. In terms of thunderstorms, the chance for thunderstorms increases as the LI decreases.
For practical purposes, this is a general relationship between LI and thunderstorm development:
LI Value |
Thunderstorm Probability |
> 4 |
None |
1 to 4 |
Slight Chance of Thunderstorms |
-2 to 0 |
Thunderstorms Possible, slight risk of Severe Thunderstorms |
-3 to -5 |
Thunderstorms Likely, moderate risk of Severe Thunderstorms |
< -5 |
Strong risk of Severe Thunderstorms |
Maps are updated 4 times per day and are valid at 0000, 0600, 1200 and 1800 UTC. Maps are available 1½ hours past the valid time. Maps are created for the analysis, 12- and 24-hour forecast period each time they are generated. The LI maps are computed from Radiosonde and numerical model data.