Supercell SW of Rapid City

Thunderstorm development began shortly after we began. Cells north of Rapid City became severe, with hail as the main threat.

Shortly after 2000Z, a supercell thunderstorm developed rapidly along the Wyoming/South Dakota state line. Visible satellite imagery showed a nice flanking line of towering cumulus feeding into the southern part of the supercell, and the storm was showing signs of rotation in the lower levels, although the radar beam was at about 6500 feet AGL, and about 10,000 ASL.

In monitoring the 2-minute (1-minute update) GLM event density lightning, there was a noticable increase in flash events in the 10 minutes or so leading to where low-level rotation was observed, with a quick drop off in lightning from that point on for the next 20 minutes as the storm probably began producing large hail.

In checking the ProbSevere products, the hail showed a large increase into the 85% range (the MESH hail size was around 1.25") and a smaller increase into the 30% range of wind. Interestingly, even with the low leve rotation and some evidence of a hook echo, the ProbTor probabilities stayed less than 2%.

Gary Cannalte