NOAA/NWS 1982 Williamson County / Marion, IL F4 Tornado Web Site
|Then / Now
A review of the 12 UTC (7 a.m. CDT) 500 mb and surface analyses from the morning of May 29, 1982 reveals a great deal about the potential for severe thunderstorms later that day. The 12 UTC analysis had a 500 mb low over Montana, with a positively tilted trough extending south from the low into northern California. A high pressure ridge was situated from James Bay in Canada south into New York state. With a trough to our west and ridge in the east, this placed the immediate region in a west-southwesterly flow pattern. Looking at the finer scale, a shortwave disturbance was analyzed near the Missouri/Illinois border with west-northwest winds immediately behind it over western Missouri. In addition, a band of 20 to 30 knot southwest winds, perhaps some form of a weak jet max, was punching into central Oklahoma from New Mexico and Arizona.
On the 12 UTC surface chart, a cold front was draped from Minnesota southwest across Iowa, Kansas, Oklahoma, and into Texas. A triple point was found just south of Des Moines, Iowa, with a warm front jutting east across northern Illinois and Indiana. Morning dew points ranged from 65°F at St. Louis, Missouri and Louisville, Kentucky, to 72°F at Memphis, Tennessee. 70 or 71°F dew points were found in Fort Smith, Arkansas, Springfield and Cape Girardeau, Missouri, and Fort Campbell, Kentucky. An outflow boundary was draped over Central Illinois and Missouri from overnight convection that was decaying as it tracked east across the two states.
By early afternoon, archived surface observations from both Carbondale and Marion, Illinois (KMDH & KMWA) showed temperatures had reached the mid to upper 80s with dew points just above 70°F. Judging by the conditions at both the surface and several thousand feet up, we can safely assume that all the necessary ingredients were present for the development of thunderstorms later that day, including moisture, lift, and instability.
(1.) Moisture - Southwest flow and morning dew points in the upper 60s to lower 70s indicated there was a steady stream of warm, moist Gulf of Mexico air surging north across the Mississippi Delta Region into the Lower Ohio River Valley. By early afternoon, surface dew points likely ranged from 70 to 75°F over southern Illinois.
(2.) Lift - Afternoon satellite imagery showed a synoptic-scale circulation (the surface low) entering northwest Illinois. While the surface cold front likely had advanced into Missouri and the western fringe of Illinois by early afternoon, this was probably not the lifting mechanism that contributed directly to the thunderstorm development. The convection firing over southwest Illinois and southeast Missouri appeared to be along another boundary--more than likely the outflow boundary left over from overnight convection. The weak 500 mb jet max noted over the Southern Plains earlier in the day may have also contributed as a lifting mechanism as it headed northeast across the Ozarks by afternoon.
(3.) Instability - Intervals of midday sunshine boosted temperatures into the middle and upper 80s by early afternoon. This combined with 70's dew points certainly made for a very unstable air mass.
(4.) Wind Shear - The presence of moisture, lift, and instability certainly made the atmosphere ripe for thunderstorm development. While no significant speed or directional shear was detected in the surface or upper-air analyses, the boundary along which the Williamson County storm tracked apparently provided enough localized shear for the storm to spin up a tornado.