The first 20 minutes of lightning in this storm was composed entirely of relatively infrequent (less than 5 per minute) intra-cloud (IC) flashes between a main negative charge layer at 7 km MSL and a lower positive charge near 5 km. During this first 20 minutes, maximum updraft did not exceed 10 m/s and volume of updraft exceeding 10 m/s did not exceed 100 km^3.
At 21:50 UTC, a presumed upper positive charge layer began participating in IC flashes, and flash rates surged to near 50 flashes/min. Maximum updrafts surged to 20 m/s, but volume of updraft exceeding 10 m/s remained below 100 km^3. Following this first surge, the storm experienced several more surges in total lightning rate. For the most part, each lightning surge was accompanied by a surge in updraft volume.
Up until 23:30 UTC, this storm produced only a couple CG flashes and its volume of updraft exceeding 10 m/s grew steadily to near 700 km^3. At 23:30, positive CG flashes began in earnest and total flash rates surged to over 300 per minute. Coincident with this lightning surge, the volume of updraft exceeding 10 m/s rapidly increased from 700 km^3 to over 1500 km^3, and the storm produced an F1 tornado.
Supplementary URL: http://radarmet.atmos.colostate.edu/~kwiens/rm3/kwiens/SLS_2002.html