Meso Saurday

Strong thunderstorms moving through Chicago area – primary threat heavy downpours/local flooding

by Paul Dailey on 08|23|14 at 2PM

&nbsCentral Great Lakes sector loopp;

Update 4:30PM CDT

At 4:30PM CDT the leading edge of the storms has moved east and is approaching a South Bend-Logansport, Indiana line (check radar mosaic. Remnant showers and isolated thunderstorms continue over northwest Indiana with scattered showers back to west over Chicago and south. All activity is moving east.

The Flash Flood Warning remains in effect over northern Lake County, Indiana where 3+ inch rains have triggered local flooding.

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Update 3:30PM CDT

At 3:30PM CDT the leading edge of the band of thunderstorms was out over Lake Michigan with the heaviest rains having left Chicago behind. The front edge of the storms extended south out of lake Michigan to just east of a Gary-Rensselaer Line. The Strongest storm/heaviest rains appeared to be in the northwest corner of Indiana, just west of Gary.

Heavy/locally flooding rainfall was still the primary concern with this line of storms, however wind gusts to 49 miles per hour were recorded on a buoy on Lake Michigan 4 and a half miles northeast of Chicago.

Some additional rainfall reports during the past hour…

Norridge                       1.02 inch with minor flooding in the area

Lansing                          1.40 inch in 30 minutes – minor flooding

Lincolnwood               1.06

Park Ridge                    0.95

Oswego                          0.48

St. Charles                    0.34

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Update 2:30PM CDT

At 2:30 PM CDT the line of thunderstorms was over the city of Chicago moving into Lake Michigan – extending south along the Indiana-Illinois state line – pushing into northwest Indiana. The individual thunderstorm cells remain strong and capable of gusty winds with the primary threat locally heavy/flooding rainfall of over an inch/hour.

During the past hour Crystal Lake reported 1.48 inches while 1.57 inches was recorded in the Palatine area. Minor flooding was noted in Bull Valley and Palatine.

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Update 1:30PM CDT

Heavy downpours have resulted in 2.56 Inches of rain in Winslow, Sephenson County and Rockford in Winnebago County between 11:30 and 1PM CDT.

As of 1:30PM the leading edge of the eastward-moving storms were approaching an Arlington Heights, Naperville, Joliet, Kankakee line.

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A north-south-oriented band of thunderstorms (see radar mosaic above) is moving east and will be approaching/moving through the Chicago area during the next couple hours. At 12:45PM,  the strongest portion of the band appeared  to be along and south of Interstate-80. However the storms could intensify/decrease as the band moves east. Wind gusts in excess of 50 miles per hour, hail and heavy downpours could accompany some of the strongest storms.

Following is a discussion by the National Storm Prediction center on the map above, describing meteorologically the synoptic situation form their perspective.

MESOSCALE DISCUSSION 1602
   NWS STORM PREDICTION CENTER NORMAN OK
   1118 AM CDT SAT AUG 23 2014

   AREAS AFFECTED...NRN AND EASTERN ILLINOIS INTO WRN INDIANA

   CONCERNING...SEVERE POTENTIAL...WATCH UNLIKELY 

   VALID 231618Z - 231815Z

   PROBABILITY OF WATCH ISSUANCE...20 PERCENT

   SUMMARY...IN ADDITION TO THE RISK FOR VERY HEAVY RAINFALL AND
   FREQUENT CLOUD TO GROUND LIGHTNING...THE POTENTIAL FOR STRONG WIND
   GUSTS MAY BEGIN TO INCREASE WITH STORMS OVERSPREADING THE REGION
   THIS AFTERNOON.  THE PROBABILITY OF A SEVERE WEATHER WATCH ISSUANCE
   STILL SEEMS LOW...BUT TRENDS IN STORM COVERAGE AND ORGANIZATION WILL
   CONTINUE TO BE MONITORED.

   DISCUSSION...VIGOROUS CONVECTION PERSISTS WITH A BAND OF FORCING FOR
   UPWARD VERTICAL MOTION ASSOCIATED WITH A WEAK SHORT WAVE IMPULSE NOW
   PROGRESSING AROUND/THROUGH THE CREST OF MID-LEVEL RIDGING ACROSS
   NORTHERN ILLINOIS AND SOUTHERN WISCONSIN.  A LINE OF THUNDERSTORMS
   APPEARS LIKELY TO CONTINUE SPREADING EASTWARD INTO THE CHICAGO
   METROPOLITAN AREA THROUGH THE 18-19Z TIME FRAME...BUT THE STRONGEST
   ACTIVITY IS BECOMING FOCUSED ON THE SOUTHERN FLANK OF THE LINE
   ADVANCING NEAR/NORTH THROUGH EAST OF PEORIA...AIDED BY INFLOW OF
   SOMEWHAT WARMER AND MORE MOIST AIR.

   AS INHIBITION FOR VERY MOIST BOUNDARY LAYER PARCELS /WITH SURFACE
   DEW POINTS IN THE MID 70SF/ CONTINUES TO WEAKEN WITH INSOLATION
   ...FURTHER INTENSIFICATION OF CONVECTION OR SUBSTANTIVE NEW
   DEVELOPMENT APPEARS PROBABLE DURING THE NEXT FEW HOURS ACROSS OR TO
   THE EAST SOUTHEAST OF THE PEORIA/BLOOMINGTON/DECATUR AREA.  DESPITE
   THE PRESENCE OF WEAK TO MODEST MID-LEVEL LAPSE RATES... SEASONABLY
   HIGH MOISTURE CONTENT...WITH PRECIPITABLE WATER WELL IN EXCESS OF
   TWO INCHES...IS SUPPORTING SIZABLE CAPE AND THE POTENTIAL FOR HEAVY
   PRECIPITATION LOADING.  THIS COULD BEGIN TO GRADUALLY CONTRIBUTE TO
   AN INCREASING RISK FOR STRONG DOWNBURSTS...AND PERHAPS AN
   EXPANDING/STRENGTHENING SURFACE COLD POOL.

   WITH DEEP LAYER MEAN FLOW FIELDS TRANSITIONING FROM LIGHT WESTERLY
   TO LIGHT NORTHWESTERLY ACROSS ILLINOIS INTO INDIANA...THE TENDENCY
   MAY BE FOR STRONGEST STORMS TO EVENTUALLY TAKE ON A MORE
   SOUTHEASTERLY PROPAGATION THROUGH THE AFTERNOON HORUS.

   ..KERR/MEAD.. 08/23/2014

Paul Dailey

Paul earned his BS in Math-Physics at Ball State University. As a weather Officer in the Air Force he had a year of study in meteorology at the University of Wisconsin Madison. He has a Masters in Meteorology from Purdue University with additional studies at Rutgers University. He entered the National Weather Service in 1962 and held various positions over the ensuing years – Pennsylvania State Climatologist, Lead Forecaster in Indianapolis and Des Moines, Central Region Chief of Operations in Kansas City, Meteorologist-In-Charge(MIC) at Cincinnati and MIC at Chicago (1990-2002). WGN-TV/Chicago Tribune Meteorologist since 2002.

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