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You can’t prepare for what isn’t forecast: excessive rains July 23-24

July 26, 2011

Just 12 miles separated Carol Stream’s 2.92″ of rain on the morning of July 23, from widespread 6-8″ totals in Arlington Heights / Des Plaines / O’Hare Airport. The spectacular rain totals come on the heels of a humid, but precipitation-free, pattern that resulted in the beginning stages of drought in the area.

What’s worse, the high-impact event rainfall event, accompanied by nearly constant lightning over almost a one-hour period, was completely missed by both computational models and forecasters. The HPC only estimated that less than two inches of rain would affect the area during the time period. No flash flood watches were issued prior to the event. There was no indication of a slight risk (or better) of flash flooding until the event had passed.

The entire rainfall episode can be stepped through on NEXRAD radar at this link. Precipitation totals for the event can be viewed here.

At 11:30p, strong thunderstorms begin forming along a subtle surface boundary along the Cook County-Lake County line. Where this boundary originated is somewhat of a mystery; it could have been from a storm that passed through Wisconsin earlier in the day.

By 1:00a, strong convection begins to fire to the east of Carol Stream, over the city of Chicago proper. Most likely, the mid-or-upper level system approaching from the west was beginning to draw tropical air over a lake breeze boundary. Rainfall rates within these storms exceeds 2″ per hour, and at the airport occasionally exceeds 8.0″ / hour (for one minute)! These types of rainfall rates are almost never found in the mid-latitudes where Chicago is located, and are certainly not a rate of rainfall to which the local terrain has evolved.

The first line of rain sweeps through Carol Stream at 2:00a, when other areas to the north and east have already received 4-5″ of rain. This line brings an astounding 2.36″ of rain in 40 minutes, for an hourly rate of 3.5″ / hr. A total of 2.92″ falls in Carol Stream through 9:00a, ultimately causing some nuisance flooding.

Later systems die out as they approach the area, something that generally occurs because air sinks in the area after a thunderstorm passes. (Thunderstorms form when air rises, condensing water vapor.) Only larger scale weather can induce further precipitation at that point (an approaching cold front, or a tropical system moving in from the south).

A late night (11:30p-1:00a) light show caused 3-4″ of rain to fall on Aurora and Naperville, but these storms were moving too quickly to generate the 7″ tallies of 7/23.

For summer storms in moist tropical air masses, the only way to be in touch with a rapidly-changing weather situation is to download an app like RadarScope for the iPhone, or by maintaining and reading your own rain gauge. Had the lake-effect breeze boundary been further west, across the Fox River, this could have been a major flood event for Carol Stream – essentially unforecasted, and largely taking place from midnight to 4:00a.

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