Supercell of July 13, 2004

A deepening synoptic trough over the western Great Lakes combined with strong low level instability, supported development of a long-duration supercell over northern Illinois. The storm featured very large hail (4"+), wind damage and a brief period of F0 tornado production (NE Bureau County) during the 5 hours it affected our County Warning Area (CWA).

Environmental Aspects...

The environmental sounding from Davenport (DVN) at 1200 UTC on July 13, 2004 revealed that above 950 mb a deep layer of steep lapse rates and dry air existed. Winds veered gradually with height reaching 40 knots by 15000 ft MSL. While the middle and some lower levels of the atmosphere were 'dry', surface dew points over eastern Iowa into central Illinois were in the middle to upper 70s. The existence of a shallow surface inversion in the sounding data suggested the initial convection would most likely be slightly elevated (off the surface).

As the morning progressed, northwest winds increased aloft and a surface cold front approached the area from southeast Minnesota. The Storm Prediction Center (SPC) forecasted a 'moderate risk' of severe convection across northern Illinois, with the primary threat being large hail and possible tornadoes. The steep lapse rates noted at DVN supported rapid parcel acceleration once storms initiated, and the dry air allowed for a low wet-bulb zero height (evaporative cooling at or below 32 F). Thus, tall organized convective towers had an extended zone below the freezing level inducing large hail growth.

The SPC regional analysis at 1700 UTC (in the midst of the event) testified to the extreme instabilities present as well as some of the dynamic involved in subsequent movement of the system.

One of the significant contributing factors to the events of this day were the unusually high surface dew points (thus high theta-e values) feeding into the system from the southwest. A 1700 UTC surface map overlaid with objectively analyzed theta-e and the Chicago (KLOT) Doppler radar, reflectivity image depicts warm surface dew points of 77-81°F across all of west central Illinois. Evapotranspiration was likely a dominant contributor to the warm dewpoints due to the extensive corn fields in the area. The corresponding CAPE values as calculated at the SPC revealed a maxima of over 7500 J/kg coincident with the highest dew point air. The track of the main cell is noted on the image.

As noted on the DVN 1200 UTC sounding winds above 4 km exceeded 40 kt. This is re-enforced by the SPC 0-6 km shear diagnosis at 1700 UTC which showed an extensive area of 40 kt winds spreading into western Illinois. Thus the ambient shear profile of the atmosphere was sufficient to support supercell characteristics.


Shortly before1500 UTC (10 AM CDT), a lone thunderstorm developed south of Dubuque Iowa. The storm then moved slowly into Carroll County Illinois. The 4-panel reflectivity image at 1610 UTC indicates the cell contained a large reflectivity gradient across it's rear flank and developed a strong central precipitation core as it worked into western Ogle County. At this early stage the storm's lifespan horizontal rotation was resolved by the KLOT radar through much of the storm's depth.

The storm continued southeast during the next hour reaching southern Lee County by 1730 UTC. Hail to one inch in diameter was reported periodically during that period. By 1727 UTC the four panel reflectivity image depicted a well organized storm with an appendage along the western (rear) flank and strong central core. Representative VIL values at 1219 UTC reached 80 g/kg. Unknown at the time was the occurrence of near 4" diameter hail between Sublette and Amboy in Lee County. The photo of the stone (shown here) was taken over an hour after the event. The approaching storm may have appeared like this.

While the large hail was falling over Lee County, the appendage along the west flank of the storm exhibited concentrated rotation as the rotation descended into lower levels. The storm-relative motion (SRM) return at 1227 UTC has the strongest rotation signature at the 1.8 deg scan (lower left, beam about 11500 ft agl), although the circulation was a relatively broad and disorganized. At the 0.5 degree scan (upper left, beam about 4500ft agl) the signature was similar but weaker.

13 minutes later, at 1740 UTC, the action shifted into northwest LaSalle and northeast Bureau Counties. Golf ball size hail was reported from Mendota. The reflectivity signature over northeast Bureau County had wrapped around the cyclonic circulation and when combined with a strong updraft, created a Bounded Weak Echo Region (BWER), resolved in the 1.8 deg scan elevation (lower left image).

The character of the SRM image had also changed with a much tighter (gate-to-gate) return through the depth of the sampling. Ground surveys reported multiple F0 scale tornado touch downs in northeast Bureau County around Van Orin at this time. 4" hail was also reported from La Moille. Tornado generation had occurred close to 3 hours after the storm initiated.

The well defined circulation associated with the Bureau County tornadoes dissipated as the storm continued slowly SSE. The storm motion recurved slightly more to the SE as the tornadoes ceased but continued to produce large hail. One hour later at 1847 UTC the cell approached Streator in southeast LaSalle County. The concave reflectivity notch just south of Streator suggested a circulation likely still existed with the storm.

By 1942 UTC, nearly 5 hours into this singular event, the parent storm moved southward through Ford County. The reflectivity notch was still present. However (and fortunately) only hail and isolated wind damage reports were received at this time. Further to the west, however, the situation was more dangerous given a F4 tornado was occurring near Roanoke in Woodford County (area circled in image). The parent updraft of that storm developed quickly along the western flank of our main cell, feeding off the extremely buoyant high dew point air over western and central Illinois.

The SRM return from KLOT certainly does not do the Roanoke storm justice but one can still detect a gate-to-gate circulation at around 7000 ft agl. The original supercell we were dealing with now appears have bowed out at this point. Bad data on this image makes interpretation difficult but subsequent images depicted bowing had indeed begun.

The 1959 UTC image was taken about the time widespread damage was occurring just west of Paxton in Ford County. A ground survey revealed tree and building damage over a wide area. The damage appeared to be the result of straight line winds. While the corresponding SRM image was still plagued by questionable data, a post event ground damage survey indicated that the cold pool from this cell was rapidly bowing out.

From start to finish this one storm lasted over 5 hours within our County Warning Area. It evolved from a large hail and occasional tornado producer into a wind (and some hail) event further down state. For an animated view of the entire event click here (there is a 33 min gap toward the end of the loop). is the U.S. government's official web portal to all federal, state and local government web resources and services.