July 27 2003



Keywords: july 27 2003
Description: Mesoscale features and interactions often control where and when the most noteworthy weather occurs. The month of July 2003 has been marked by a continuing parade of severe weather events across

Mesoscale features and interactions often control where and when the most noteworthy weather occurs. The month of July 2003 has been marked by a continuing parade of severe weather events across northern Illinois. The episode which took place on July 27 th featured a band of damaging winds across the far southwest and southern Chicago suburbs as well as 4.90” of rain which fell during the midday hours at the NWS office in Romeoville.

A visible satellite image at 1601UTC provides an overview of the elements in play, A stalled frontal zone just south of Chicago was already supporting an east-west band of thunderstorms in a very unstable airmass. Further to the west, just crossing the Mississippi River, is a fast moving band of thunderstorms which originated over central Iowa. These storms were moving straight eastward and exhibiting a bowing structure characteristic of strong straight line winds.

To the north a fine line of arced clouds was moving westward though Lake and McHenry counties of northeast Illinois. These clouds marked the leading edge of a surge of cooler drier air working south and west from Lake Michigan. Ahead of the Iowa convection a 1200 UTC sounding from KDVN adds some insight to the atmospheric structure associated with this rapidly moving feature. While there are pockets of steep lapse rates above 5000 ft, two notable items are apparent. The first is the 35kt ambient wind at 850mb. The second is a dry pocket about the same elevation. The combination of momentum (wind) and evaporation potential relatively close to the surface suggest a mechanism to sustain a bowing event in an active convection zone. Note also the positive buoyancy (CAPE) of 3789 j/kg.

About 45 minutes later convection south of Chicago has blossomed while the thunderstorms from Iowa continue to race east and the backdoor cool push remains evident. A reality check from the KLOT WSR-88D radar better defines the situation. Surface observations (not necessarily matching the time of the radar reflectivity) show wind gusts in excess of 50mph associated with the eastward moving Iowa bow. The convective band across KLOT was nearly stationary while cells were training slowly to the east. The encroaching backdoor front has some push as winds were gusting to 22 kt at O’Hare Intn’l (ORD) and 18 kt at Waukegan (UGN). By 1701UTC it is clear that there would likely be a direct interaction between the eastward rushing Iowa bow and westward moving synoptic front. It most likely will occur in an area already convectively active around KLOT. Doppler interrogation of the 1704UTC base velocity field (right image) shows a peak in bound value of about 80 kt at about 4500ft agl. The leading edge of the wind surge is entering northwest LaSalle County, coincident with the leading edge of the reflectivity (left image).

2500ft agl) rushes across northern LaSalle County. Ongoing convection lies in its path along an east-west line and the back door cool front continues to sag toward KLOT.

A half hour later, 1758UTC all elements have come together in the vicinity of KLOT. Wind damage was reported with the bow to the west over Kendall and DeKalb counties. The inbound velocity pool is just west of KLOT with the sampling zone below 1000ft agl. A closer view of the reflectivity and surface data shows the back door front extrapolates to KLOT and highlights the airmass difference present across northeast Illinois. Under the influence of cooler water and drier source region the temperatures over Cook County northward into southeast Wisconsin are less than the dew points further south in the undisturbed air at Kankakee. While the Iowa bow continued on toward northwest Indiana…the interaction between it, the preexisting east-west band of thunderstorms over KLOT and the westward moving back door boundary was sufficient to initiate an intensification of the convection directly over KLOT. This can be seen in the 1808UTC reflectivity image (left frame) and is the heart of the heavy rain that fell at the station.

During the following hour the Iowa bow kept on pushing into Indiana while the back door front kept moving westward as a defined feature. Images at 1827UTC and 1902UTC document the parting of their ways. Convection over KLOT subsequently decreased as the forcing dissipated.

The above sequence is a fairly dramatic example of synoptic and mesoscale interactions which maintain their identity both before and after they collide. The resulting rainfall pattern analyzed using 24hr rainfall totals ending 1200UTC of July 28 th vividly illustrates the localized nature of the event.

While this case involved a radar defined westward moving lake enhanced back door frontal interface, the process of enhancing a baroclinic zone by cooler air emanating from Lake Michigan (not lake breeze) is not an unknown complicating factor in northeast Illinois weather. Notable instances of this include slow moving spring warm fronts retarded by a cool component off the water which stall southwest of Chicago and the July 1996 flood. In the latter case a sustained wind off the water was instrumental in maintaining a strong thermal ribbon from Rockford to the SW Chicago suburbs resulting in nearly 17” of rain in Aurora.






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