1990 Plainfield Tornado - Atmospheric Conditions

Atmospheric Conditions

For late August standards, August 28 was a very humid, very warm day. Temperatures reached into the low 90s ºF (about 11 °F warmer than the normal of 79 °F), but dew points soared into the upper 70s °F. The presence of such a high dew point did not necessarily predict a severe thunderstorm outbreak; the prior day, similar conditions existed in northern Illinois with the exception of a warmer mid level troposphere. A warmer atmosphere inhibits the rising of surface air through the atmosphere; a requirement for convective precipitation, that is, precipitation resultant from humid surface air ascending to condense in a cooler atmosphere above, to occur. This warmer air aloft can describe either, or both, weak lapse rates thus weak instability or a capping inversion.

The atmosphere on the 28th was significantly more unstable as the approach of a low-pressure system from the northwest cooled the mid levels (and also caused dynamic lifting) as instability continued to build in the capped, muggy environment, although the wind fields (strong, but out of the west-northwest and unidirectional) were not suitable for significant tornadic development. As a result of the very high low-level temperature and dew point, convective available potential energy (CAPE) values were in excess of 8,000 J/kg; generally, values of 1,500 J/kg are considered to be moderately unstable, whereas values of more than 4,000 J/kg are considered "extreme". The lifted index (LI), the dominant estimate of instability used at the time, was also extreme. Generally, an LI value of -6 °C or below readily supports severe thunderstorm development, but during the day the LI value ranged from -12 °C to -14 °C. Such extreme instability can lead to explosive thunderstorm development, very strong updrafts, and modulates the updraft to better enable tornadogenesis.

Conditions were ripe for severe thunderstorm development, and with both low level and high level steering winds from the west-northwest, the National Severe Storms Forecast Center issued a Severe Thunderstorm Watch for northern Illinois valid 1:30 p.m. through 8:00 p.m. CDT that night. The NSSFC predicted a derecho-type event for later that night as rapidly developing thunderstorms along a cold front in Wisconsin would be carried by the steering flow into the Chicago area. One of these storms developed supercellular characteristics south of Rockford, steering 30 to 40 degrees to the right of prevailing steering winds. By now, mid-level steering winds had begun to veer, causing a wind profile slightly more favorable for tornadic development. This helped develop a high precipitation supercell.

The "HP" supercell produced multiple funnel sightings, hail damage, and damaging winds south of Rockford before heading southeast toward Will County and dropped a rain wrapped tornado. This is why many in the path did not see the actual tornado but more of a wall of water and dark clouds coming toward them. As the storm continued south it produced a 16-mile swath of damage ranging from F1 to F5. The National Weather Service in Rosemont, IL (serving all of Central and Northern Illinois at the time) did not issue a Tornado Warning until 10 minutes after the tornado had hit Plainfield. Using the traditional radar technology at the time (until the installation of NEXRAD Doppler Radar) the storm had a well defined "hook echo" meaning the radar was picking up the wrapping of the rain around the circulation of the tornado.