Plainfield Tornado
| F5 tornado | |
|---|---|
| Formed | August 28, 1990 3:15 p.m. to 3:45 p.m. CDT |
| Max rating1 | F5 tornado |
| Damage | $165 million |
| Areas affected | Plainfield, Crest Hill, and Joliet, Illinois |
| 1Most severe tornado damage; see Fujita scale | |
The 1990 Plainfield tornado was a devastating tornado that occurred on the afternoon of Tuesday, August 28, 1990. The violent tornado killed 29 people and injured 353. It is the only F5 tornado ever recorded in August and the only F5 tornado to strike the Chicago area. There are no known videos or photographs of this heavily rain-wrapped tornado. The Plainfield tornado was part of a small outbreak that produced several tornadoes in the northern US and in Canada.
For late August standards, August 28 was a very humid, very warm day. Temperatures reached into the low 90s °F (32–34 °C; about 11 °F or 6 °C warmer than the normal of 79 °F – 26 °C), but dew points soared into the upper 70s °F (25–27 °C). 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.
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