Guide to Convective Parameters

INDEX VALUE INTERPRETATION
CAPE

(convective available potential energy)

<1000 weak convection
1000 to 2500 moderate convection
>2500 strong convection
 
CIN*

(convective inhibition)

<50 weak cap that can be easily broken by surface heating
50 to 200 moderate cap that can be broken by strong heating/synoptic scale forcing
>200 strong cap that impedes thunderstorm development
 
LI

(lifted index)

>0 atmosphere is stable to convection
0 to -2 thunderstorms possible, but good trigger needed
-3 to -5 thunderstorms probable
<-5 heavy to strong thunderstorms probable
 
KI*

(k index)

W of Rockies E of Rockies  
<15 <20 thunderstorms unlikely
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
>30 >35 numerous thunderstorms

*Some useful notes:

CIN: A measure of the unlikelihood of thunderstorm development. The presence of CIN doesn't preclude thunderstorm development as long as it isn't too high. In fact, a moderate amount of CIN in the morning hours is actually favorable to the development of heavy afternoon thunderstorms in the presence of a large amount of CAPE. High CAPE and low CIN morning environments tend to cloud up very quickly after a small amount of surface heating is introduced. Watch for the CIN to go to zero in the afternoon on high CAPE days. This is warning flag that afternoon convection is likely (at least from the point of view of the model).

K Index: This index is used as a guide to forecast air mass thunderstorms, i.e. thunderstorms that develop in the heat and humidity on a typical summer day in the SE. Note that there are two separate scales for the W part of the US and the E part of the US. The K Index is a linear combination of temperature and dewpoint at various levels (KI = T850 - T500 + Td850 - (T700 - Td700)).

A final word of caution:

There can be times where all the indices seem to be pointing toward a very convectively active day and nothing happens. So, I wouldn't blindly use these indices as my primary source to forecast precipitation amounts. Rather, if you think convection will occur via the models, NWS discussions, SPC, etc, use these tools as supplements to fine tune your forecast.

Acknowledgements: This page was prepared from the class notes of Roger Wakimoto from course AS C110 at UCLA.