Heavy rains with more on the way may cause significant flooding problems

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Heavy rains in excess of 5-inches already with another 2-3-inches possible may cause significant flooding problems across the Chicago area this evening/overnight

Flooding is likely across the Chicago area this evening and overnight, especially along and south of the Interstate-88 and Interstate-290 corridors on into Chicago. DuPage County has apparently seen the heaviest rainfall as of 6PM CDT with 5.19-inches reported at Lisle, 4.85-inches at Downers Grove and 4.50-inches in Darien with the rain still coming down hard. Midway had received 2.88-inches and O’Hare 2.48-inches as of late this afternoon.

Below is a Mesoscale Heavy Rainfall Discussion issued by the National Weather Prediction Center. It has rather technical verbage, but is deemed worth displaying here.

MESOSCALE PRECIPITATION DISCUSSION 0887
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
623 PM EDT SAT OCT 14 2017

AREAS AFFECTED…CENTRAL/NORTHERN IL…NORTHWEST IN…FAR
SOUTHWEST LOWER MI

CONCERNING…HEAVY RAINFALL…FLASH FLOODING LIKELY

VALID 142220Z – 150420Z

SUMMARY…REPEATING ROUNDS OF HEAVY SHOWERS AND THUNDERSTORMS THIS
EVENING WILL FOSTER AREAL CONCERNS FOR RUNOFF PROBLEMS AND FLASH
FLOODING.

DISCUSSION…A WAVE OF LOW PRESSURE ACROSS CENTRAL IA LATE THIS
AFTERNOON IS INTENSIFYING AS A LARGER SCALE TROUGH EJECTS OUT OF
THE ROCKIES AND ACROSS THE NORTHERN/CENTRAL PLAINS REGION.
MULTIPLE LEAD SHORTWAVE IMPULSES ARE SEEN IN THE SUITE OF
NON-OPERATIONAL GOES-16 WV CHANNELS…WITH ONE IMPULSE SHEARING
DOWNSTREAM ACROSS SOUTHERN LOWER MI…AND A SECONDARY IMPULSE
QUICKLY EJECTING THROUGH CENTRAL/EASTERN IA AND INTO WESTERN IL.
THE INITIAL IMPULSE ALREADY PRODUCED HEAVY RAIN TODAY ACROSS A
LARGE AREA OF EASTERN IA…NORTHERN IL AND SOUTHERN WI WITH SOME
AREAS RECEIVING 3 TO 5 INCHES…AND THIS HAS LED TO A SATURATION
OF THE AREA SOILS.

THE LATEST RADAR AND GOES-VIS/IR SATELLITE IMAGERY IS SHOWING
RENEWED ORGANIZATION AND EXPANSION OF CONVECTION ACROSS FAR
EASTERN IA AND INTO WESTERN IL AS THE SECOND SHORTWAVE INTERACTS
WITH THE NORTHWARD TRANSPORT OF A VERY MOIST AND UNSTABLE AIR
AIRMASS ACROSS THE LOWER MO AND MID MS VALLEYS. THE LATEST RAP
ANALYSIS SHOWS A NOSE OF 1500 J/KG OF MUCAPE NOSING UP ACROSS
NORTHERN MO AND INTO WESTERN IL WHERE IT IS INTERSECTING AND
OVERRUNNING A WELL-DEFINED AND QUASI-STATIONARY FRONTAL ZONE.
MEANWHILE…PWATS ARE VERY HIGH FOR THIS TIME OF THE YEAR AT 1.7
TO 1.9 INCHES…WHICH IS OVER 3 SIGMAS ABOVE NORMAL.

SMALLER SCALE SHORTWAVE DYNAMICS INTERACTING WITH THE FRONT AND
THE AFOREMENTIONED AIRMASS COUPLED WITH LARGER SCALE 250 MB
RIGHT-ENTRANCE REGION JET FORCING AHEAD OF THE LARGER SCALE TROUGH
SHOULD FOSTER A RIPE ENVIRONMENT FOR ORGANIZED CONVECTION WITH
HEAVY RAINFALL THIS EVENING ACROSS THE MIDWEST WITH A FOCUS ON
ESPECIALLY CENTRAL AND NORTHERN IL AND INCLUDING THE CHICAGO
METROPOLITAN AREA. ACCOUNTING FOR THE LATEST RADAR TRENDS…THE
HRRR IS LIKELY TOO FAR NORTH INTO SOUTHERN WI WITH ITS HEAVIER QPF
AXIS…WHEREAS OBSERVATIONAL TRENDS SUPPORT MUCH GREATER IMPACTS
FARTHER SOUTH INTO CENTRAL/NORTHERN IL. THESE AREAS ARE LIKELY TO
SEE AS MUCH AS AN ADDITIONAL 2 TO 3 INCHES OF RAIN WITH ISOLATED
HEAVIER AMOUNTS THROUGH ABOUT 04Z. ADJACENT AREAS OF NORTHWEST IN
AND SOUTHWEST LOWER MI WILL ALSO SEE THE POTENTIAL FOR THESE
HEAVIER RAINS LATER INTO THE EVENING AS THE UPSTREAM SURFACE LOW
LIFTS TOWARD SOUTHWEST WI…AND THE UPSTREAM CONVECTION PUSHES
FARTHER EAST ALONG WITH THE COLD FRONT.

THESE ADDITIONAL RAINS ON TOP OF THE SATURATED SOIL WILL LIKELY
PROMOTE RUNOFF PROBLEMS AND SOME FLASH FLOODING…WITH AN EMPHASIS
ON THE URBANIZED CORRIDORS AND INCLUDING THE CHICAGO METROPOLITAN
AREA.