From Independence Day through July 9th, many areas in the Upper Mississippi Valley experienced severe weather on a day to day basis. The Chicago Metropolitan Area and northeast Illinois were no exception, with hail, damaging winds, and flooding being a continual threat throughout that period.

During the afternoon of July 6, the northeast Illinois area experienced several hail-producing thunderstorms. The large hail was the primary threat with over two dozen locations in northern and northeast Illinois receiving at least severe weather criteria hail (0.75 in. or greater). A spatial representation of the hail reports reveals a littering of dime, quarter, half-dollar, and golf ball size hail. The strongest storms affected the circled areas with hail > 1.75” in diameter

The potential for large hail producing thunderstorms was manifest early in the afternoon by looking at the 18UTC ILX (Lincoln IL) sounding. It would be reasonably representative of northeast Illinois given the southwesterly flow present. A nearly adiabatic lapse rate in the lowest two kilometers with convective available potential energy (CAPE) value well over 5,000 J/kg. No capping inversion was present. Wet-bulb zero elevation was a little over 11,000 ft.

A key component to the day’s severe weather was boundaries which separated air with different meteorological characteristics. The lake breeze from Lake Michigan was one such boundary that had initiated in the late morning. The lake breeze is easily seen in the 1836 UTC reflectivity from the KLOT Doppler radar. Initiation of thunderstorms had already taken place on the lake breeze in Lake County, and the slight reflectivity shown over northern Cook County was the beginning of a new thunderstorm. Note also the developing storm over McHenry County.

By 1905 UTC, the thunderstorm over northern Cook County had moved northeast and was located along the border of Lake and Cook counties. Such high reflectivity values (60-70 dBz) as seen in the lowest elevation scan, were seen at all elevation angles of the radar beam between 2600 ft and 13,000 ft (0.5° to 3.4°). The vertical integrated liquid (VIL) at 1859 UTC at approximately the same time denoted a concentrated group of greater than 70 kg/m² in the Buffalo Grove and Wheeling vicinity. VIL is a quantitative measure of the water (hail etc) mass density within a sample column. High values (depending upon airmass characteristics) can denote the presence of hail.

Values in excess of 70 Kg/m2 correlate well to ground reports of golf ball size hail (1.75 in.) from Arlington Heights to Wheeling from 1859 – 1906 UTC and Buffalo Grove from 1902 – 1909 UTC. The hardest area hit was along the Lake-Cook county border. Large hail was falling from this storm twenty-five minutes after the initial reflectivity was detected from the storm.

This storm dissipated and moved offshore as seen by the 1940 UTC reflectivity image. Note though that a second boundary-induced thunderstorm had moved into western Lake County and had intensified. The hook-like appendage on the south side of this supercell remained intact as the storm moved east across Lake County. Wall clouds and a funnel cloud were observed near Interstate 94 in eastern Lake County. Also throughout its lifetime, the storm exhibited a right curving bias which reflects the rotating nature of the cell. The storm ‘peaked’ around 2003 UTC. The northern portion of this cell would move over Lake Bluff at 2009 UTC and produce golf ball size hail in that community at 2015 UTC.

This supercell created an outflow that moved southward during the mid-afternoon. As seen on the 2147 UTC reflectivity, this boundary was draped across northern Kane and Cook Counties. At the same time a synoptic boundary was nearing the DeKalb-Kane county line. The momentum behind this boundary was apparent with a 15 knot wind out of the WNW at the DeKalb observing site. Note the beginning of a thunderstorm in central Kane County near Elburn. This storm would be the most rapid to intensify, and would do so as it intersected the southward moving outflow boundary to its east.

A 0.5 deg reflectivity image from 2217 UTC revealed just that. Calculated VIL at 2212UTC exceeded 60 Kg/m2 large hail was reported in Addison, Bloomingdale, Glendale Heights, Lombard, West Chicago, and Wheaton between 2217-2230 UTC. The largest hail reported from that storm was two inches in diameter and reported in Carol Stream in northern DuPage County.

By 2237 UTC storm had weakened rapidly as it moved into western Cook Co. and ingested more stable air which had originated from earlier convection and Lake Michigan.

In summary, storms which developed during the afternoon of July 6th in northeast Illinois were a product of very unstable air and mesoscale boundaries of various origins. Large damaging hail occurred with the stronger storms and fell over densely populated areas.

MTF /KML
August 2003