KI <20:         thunderstorms unlikely.
KI 20 to 25: isolated thunderstorms.
KI 26 to 30: scattered thunderstorms.
KI 31 to 35: few thunderstorms.
KI 36 to 40: few to numerous thunderstorms .
KI > 40:        almost 100% risk for thunderstorms.
 

The K Index is primarily used to forecast heavy rain and thunderstorm potential. It ain't a good predictor for severe vs non severe weather. The K Index was developed for pulse (air mass) thunderstorm forecasting. It works best used in summer period for air mass thunderstorms (no dynamical triggering mechanism).

Tests on the field showed there is a good correlation between no occurrence of thunderstorms and KI values <26.

Formula: K = (T850 -T500) + Td850 - (T700-Td700).