Strong thunderstorms moving through Chicago area – primary threat heavy downpours/local flooding
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.
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
Park Ridge 0.95
St. Charles 0.34
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.
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.
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