During the afternoon of June 23, 1998, a slow moving supercell thunderstorm produced a series of three tornadoes just north of Rapid City, SD. Luckily, little damage was caused by these impressive tornadoes as they slowly moved to the north of I-90 that afternoon. This page briefly outlines the meteorological conditions and radar imagery associated with this event. Please note...the photographs linked in the text below (except the one above, courtesy of Molly Zeitler) are from the excellent web page of Tom Warner.
The atmosphere was very unstable throughout much of the early afternoon of June 23rd, with weak (20 kt) southwest flow aloft. Several strong, but transient storms formed in this unstable (CAPE = ~ 2000 J/Kg, LI = -6) environment in a convergent area downstream (to the northeast) of the Black Hills (approximately 20 miles north-northwest of Rapid City). Some large hail (up to 1.75" in diameter) was reported with these short-lived early afternoon storms. From 1 PM to 4 PM, the atmospheric wind shear increased significantly (as the surface winds strengthened out of the southeast), and the atmosphere further destabilized (CAPE = ~4000 J/Kg, LI = -13), allowing for the explosive formation of more organized (supercellular) tornadic storms. The atmospheric conditions within which the storm developed are well represented by the skewt/log P and hodograph from the June 24 00Z Rapid City sounding.
The tornado-producing storm was initiated downstream (downwind) of the Black Hills, about 20 miles northwest of Rapid City, at approximately 4:30 PM. The storm quickly became severe, and soon developed a vivid hook echo, with strong convergent rotation by 5:40 PM.. The first tornado developed within this initial mesocyclone at 6:04 PM, about 11 miles to the north of Rapid City. It is interesting to note that the reflectivity features were not as striking at the time of the tornadic development as they were about 20 minutes earlier. Click here for a velocity image at the time of the first tornado. This initial tornado lasted approximately 10 minutes, becoming quite visually impressive, even from the Rapid City NWS office. At about this time, the radar imagery showed a secondary appendage/hook echo quickly forming on the southwest flank of the initial storm. By 6:32 PM, a thin, rope-like tornado formed with this new mesocyclone / appendage which lasted about 5 minutes. At this time, the storm exhibited two strong mesocyclones and appendages (hooks). As the storm slowly progressed to the east, the primary (first) mesocyclone strengthened considerably, and produced the third tornado from this storm at 6:49 PM. (reflectivity image, velocity image). This was the last confirmed tornado with this storm, though it continued to exhibit strong rotation for over another hour or two. This series of tornadoes was perhaps one of the best observed tornadic events from Rapid City in recent memory, as all three were easily visible from many locations.
Other interesting notes about the storm:
There was considerable crop damage associated with this supercell as it moved through Meade county. One observer reported hen's egg-sized hail (2") falling for over an hour, as the storm continuously re-developed over the same area. For those who are interested, here's a listing of the WSR-88D TDA output from this event (compiled by Matthew Bunkers).
|22:57 Z 23 June||ETVS|