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Mesoscale potential heavy rainfall discussion covers Chicago area currently under a Flash Flood Watch

The National Weather Prediction Center has outlooked  an area covering portions
of Iowa, southern Wisconsin and northern Illinois (see area depicted on the
highlighted map) for the possibility of heavy flood-producing rains over the next several
hours. Over two-inches per hour may occur with the strongest storms with totals over
an extended period possibly reaching 6-inches. This includes all of the Chicago area
currently under a Flash Flood Watch from 4PM this afternoon through Saturday morning,
and agrees well with an Excessive Rainfall Outlook issued earlier today.

See detailed discussion below…

 

MESOSCALE PRECIPITATION DISCUSSION 0525
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
239 PM EDT FRI JUL 21 2017

AREAS AFFECTED…PORTIONS OF IA, IL, & WI

CONCERNING…HEAVY RAINFALL…FLASH FLOODING LIKELY

VALID 211839Z – 220039Z

SUMMARY…THUNDERSTORMS WITH HEAVY RAIN ARE EXPECTED NEAR AN
EAST-WEST BOUNDARY OVER THE NEXT SEVERAL HOURS. HOURLY RAIN RATES
TO 2.5″ WITH LOCAL AMOUNTS TO 6″ ARE POSSIBLE.

DISCUSSION…SATELLITE AND RADAR IMAGERY SHOW TWO BATCHES OF
THUNDERSTORMS NEAR THE 42.5 PARALLEL BETWEEN NORTHEAST NE AND THE
WESTERN WI/IL BORDER, WITH THE NEW WESTERN BATCH FORMING NEAR THE
INTERSECTION OF A WARM FRONT AND WEAK DEW POINT GRADIENT NEAR THE
NE/IA BORDER. AN APPROACHING SHORTWAVE IS SEEN ON WATER VAPOR
IMAGERY ACROSS EASTERN SD. INFLOW AT 850 HPA IS SOUTHWEST AT
20-30 KTS PER VAD WIND PROFILES, NEAR THE MAGNITUDE OF THE MEAN
850-400 HPA WIND. PRECIPITABLE WATER VALUES ARE ~2″. ML CAPE
VALUES SOUTH OF THE EFFECTIVE FRONT ARE 2000-4000 J/KG AND RISING.
EFFECTIVE BULK SHEAR OF 40-45 KTS LIES ACROSS THE REGION.

THE WARM FRONT/OUTFLOW BOUNDARY IS EXPECTED TO RETURN NORTHWARD TO
ROUGHLY 42.5N LATITUDE, THE SOUTHERN EDGE OF THE EXTENSIVE
CLOUDINESS LEFT BEHIND BY THE EASTERN COMPLEX. HOWEVER,
CONVECTIVE PROGRESSION IS EXPECTED TO BE EAST TO EAST-SOUTHEAST AT
25 KTS PER FORWARD PROPAGATING VECTORS. THE RAP GUIDANCE SHOWS
INFLOW INCREASING TO 40-45 KTS AND BECOMING INCREASINGLY
CONVERGENT, NEARLY DOUBLE THE MEAN WIND WHICH WOULD LEAD TO
CONVECTIVE BACKBUILDING AND MAKE FOR EXCELLENT RAINFALL
EFFICIENCY. THE COMBINATION OF LOW-LEVEL INFLOW AND INSTABILITY
COULD ALLOW FOR CONVECTIVE BOWS TO DEVELOP BY 22Z, WHICH WOULD
NORMALLY LIMIT RAINFALL TOTALS. HOWEVER, THE DEGREE OF BULK SHEAR
INDICATES THE PRESENCE OF MESOCYCLONES IN THE UPCOMING CONVECTIVE
ACTIVITY, WHICH COULD ALSO HOLD UP CONVECTIVE BOWS AND ENHANCE
RAIN RATES. HEAVY RAINFALL IS ALSO POSSIBLE ALONG THE WESTERN AND
SOUTHWESTERN-MOST FLANKS OF ANY BOWING CONVECTIVE SEGMENTS,
WHETHER OR NOT MESOCYCLONES ARE PRESENT.

HOURLY RAIN RATES TO 2.5″ ARE EXPECTED. THE MESOSCALE GUIDANCE
HAS A STRONG SIGNAL FOR HEAVY RAINFALL, SHOWING LOCAL AMOUNTS NEAR
6″. DESPITE THE SOUTHWARD SHIFT SEEN SINCE THEIR 12Z RUNS, THEY
COULD STILL END UP BEING TOO FAR NORTH WITH THEIR AXIS — THE 14Z
HRRR, 12Z WRF NSSL, AND 12Z NMMB LOOKS FAIRLY REASONABLE, ALTHOUGH
THEIR CONVECTIVE PROGRESSION/DEVELOPMENT TO THE EAST MIGHT NOT BE
QUICK ENOUGH. SOILS ARE SATURATED FROM THE PAST FEW DAYS OF RAIN
ACROSS PORTIONS OF NORTHEAST IA, NORTHERN IL, AND SOUTHERN WI.
FLASH FLOODING IS CONSIDERED LIKELY.