This loop of GOES-8 Sounder-derived Convective Available Potential Energy (CAPE) shows the instability of the air mass that was being advected northeastward into Arkansas and the southern Mississippi valley region on 21 January 1999. CAPE values exceeded 2000 J kg-1 (yellow enhancement) after about 20:00 UTC. ETA model 850 hPa wind streamlines are plotted at 6-hour intervals on the imagery.