A very narrow band of "training" thunderstorms moved over the city of Chicago into northwest Indiana Wednesday evening producing flash flooding, killing at least one person. The graphic shows the Composite Reflectivity product from the Chicago (LOT) NWS radar at 7:21 p.m. CDT Wednesday. In addition to the flooding rain, large hail was reported over parts of the city, the hail size estimate from the radar showing in the graphic. Notice the slow storm motion, a critical factor needed to produce such high rainfall amounts.
There earlier was a band of showers and storms that moved from west to east over the metro area, and then out over the lake. A small area of storms followed the first batch of weather, moving southeast out of McHenry county. Nothing unusual was occurring with these new storms until they reached over the city. The storms then suddenly intensified and changed direction to move south-southeast parallel to the lake.
Based on the storm motion I saw on the TV, it looks like the storms intensified and suddenly produced the torrential rainfall when they most likely hit a little lake breeze that the earlier rain activity moved westward from the lake. This is a mesoevent that typically occurs over the metro area in the first two weeks of August, not the middle of September. However, in this case the atmosphere was typical of one in early August: very moist, slow storm motion, and something to bring a lake breeze to the shoreline.
Rainfall totals over the area can be found here. The highest amounts of 4+ inches fell along the lake in Chicago, and then downstream over northwest Indiana. The exact location of the rainfall axis will be determined by where a mesoscale lake breeze exists when the storms come along to feed off of the enhanced convergence. In this case the front or boundary must have been very close to the lakefront for the max rainfall axis to lie from the lakeshore southeast into Indiana.
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