Flash flood emergency in Chicago; dangerous severe weather outbreak possible today

Torrential downpours from endless thunderstorms fed by an atmospheric river of moisture have pounded the Chicago area since noon Wednesday, forcing declaration of a flash flood emergency in the city this morning. Four to six inches of rain has fallen in the past 24 hours over most of Northern Illinois, with more than half of that total falling since midnight in many areas. Chicago's Ohare Airport received 5.85" as of 10 am CDT, and the 3.84" of rain that has fallen so far today is the heaviest rain ever recorded in a calendar day in the Windy City so early in the year. Radar-estimated rainfall amounts in excess of 8" have occurred in areas of Northwest Illinois about 150 miles east of Chicago. Chicago's all-time 24-hour rainfall record is 9.35", set August 13 - 14, 1987. Another 1 - 2" of rain is likely today in the city. Flooding has been reported in at least 500 basements and homes, and countless roadways are reported under water and impassible. Portions of the Eisenhower, Edens, and Kennedy expressways have been flooded and closed. Several communities have declared a state of emergency, but no injuries have been reported.


Figure 1. Heavy rains in Chicago this morning caused a sinkhole to open up near 96th and Houston streets. The sinkhole was due to a water main break from a pipe constructed in 1915. Image credit: WGN Morning News.


Figure 2. Radar-estimated rainfall amounts in excess of 8" have occurred in areas of Northwest Illinois about 150 miles east of Chicago, and 4 - 6" over most of Northern Illinois.

Chicago flooding a threat to wash Asian Carp into Lake Michigan
The rains have brought the Des Plaines River on the east side of Chicago to major flood stage this morning, and a record flood crest is expected on Friday. The Asian Carp, a dangerous invasive species that would cost billions of dollars were it to get loose in the Great Lakes, is present in the Des Plaines River. Today's flood event is capable of washing significant numbers of Asian Carp from the Des Plaines River into a canal that feeds directly into Lake Michigan, where they might be able to set up a breeding population capable of devastating the Great Lakes' fishing industry. However, in October 2010, the U.S. Army Corps of Engineers completed the Des Plaines River barricade, a 13-mile-long system of concrete barriers and a specially fabricated wire mesh that allows water to flow through the fence but prevents the passage of fish. Hopefully, this barricade will withstand the flood and prevent Asian Carp from washing into Lake Michigan.


Figure 3. The Des Plaines River on the east side of Chicago is at major flood stage, and is expected to crest at record flood levels on Saturday. Image credit: USGS.


Figure 4. The Des Plaines River Asian Carp barrier. Image credit: USACE.

Dangerous severe weather situation for Thursday
NOAA's Storm Prediction Center has placed their "Moderate Risk" forecast of severe weather over an usually large swath of the U.S., from Michigan to Mississippi. A tornado watch issued this morning for most of Indiana and small portions of Eastern Illinois, Southern Lower Michigan, and Northwest Ohio is advising residents that this is "A Particularly Dangerous Situation." NOAA only uses this wording for unusually potent severe weather situations. The risk today is that isolated rotating supercell thunderstorms may develop early this afternoon in advance of a cold front sweeping through the Midwest, and these thunderstorms will bring a moderate risk of EF-2 or stronger tornadoes, hail bigger than golf balls, and wind gusts in excess of 74 mph.


Figure 5. Severe weather outlook for Thursday, April 18, calls for a "Moderate Risk" of severe weather over an usually large swath of the U.S. Large hail, damaging winds, torrential flooding rains, and a few strong EF-2 and EF-3 tornadoes are likely in this region.

Jeff Masters