MESOSCALE PRECIPITATION DISCUSSION 0163
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
454 PM EDT SAT APR 29 2017

AREAS AFFECTED...ILLINOIS

CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE

VALID 292100Z - 300130Z

SUMMARY...MCV AND BROAD MODERATE RAIN SHIELD LIFTING NORTHEAST
WITH EMBEDDED CONVECTIVE ELEMENTS MAY POSE LOCALIZED FLASH
FLOODING CONCERNS.

DISCUSSION...RADAR MOSAIC DENOTES WELL EVOLVED MESOSCALE BETA TO
ALPHA SCALE CONVECTIVELY INDUCED VORTEX LIFTING NORTHEAST OUT OF
MO INTO CENTRAL IL. HAVING SPANNED SCALES WITHIN A FAVORABLE UPPER
LEVEL DIVERGENT ENVIRONMENT (AT THE APEX OF ANTICYCLONIC
CURVING/ACCELERATING 3H JET) SUPPORTS STRENGTHENING AND BACKING
850MB FLOW ACROSS SOUTHERN AND EASTERN IL IN ADVANCE OF THE
SYSTEM.  THIS FLOW IS SUPPORTING ISENTROPIC ADVECTION OF MODEST
INSTABILITY TOWARD DEEPLY SATURATED ENVIRONMENT WITHIN THE
CONVECTIVE SHIELD WITH MUCAPES NOW REACHING 500-1000 J/KG (NEARING
1500 J/KG NEAR MCV CENTER).   AS SUCH THERE IS SOME INCREASE IN
EMBEDDED CONVECTION ALONG THE DEFORMATION ZONE ACROSS WESTERN IL
TO NORTHEAST IL.  THIS DEFORMATION ZONE SHOULD REMAIN FAIRLY
STATIONARY ALLOWING FOR A PROLONGED MODERATE RAINFALL WITH
.5-1.0"/HR RAINFALL BUT GIVEN DURATION...EXPECTED TOTALS OF 2-3"
ARE POSSIBLE TO EXCEED 3HR FFG VALUES  MAINLY FOCUSED ALONG THE
I-55 CORRIDOR FROM STL TO CHICAGO.

FURTHER SOUTH...DEEPER CONVECTION ALONG AND JUST SOUTH OF THE MCV
WILL BE CLOSE ENOUGH TO THE WARM SECTOR TO BE MORE SURFACE BASED
AND EFFECTIVELY MORE EFFICIENT THOUGH SHOULD ALSO BE MORE EASTWARD
PROPAGATING LIMITING THE OVERALL TOTALS.  THOUGH WITH SBCAPES TO
500-1000 J/KG AND TPWS UP TO 1.5"...RATES OF 1.5" ARE
POSSIBLE...INITIALLY THESE RATES WILL TRAVERSE HIGHER FFG BUT WILL
PUSH TOWARD AREAS AFFECTED BY HIGHER RAINFALL YESTERDAY/EARLY THIS
MORNING NORTH OF I-70 AND SOUTH OF I-74 IN E IL.  GIVEN 6HR FFG
BELOW 1.0" IN SOME COUNTIES...IT IS MORE LIKELY THAT FF WOULD
OCCUR HERE TOWARD 00-01Z AS THE LINE MARCHES THROUGH.

GALLINA

ATTN...WFO...DVN...ILX...LOT...LSX...

ATTN...RFC...NCRFC...OHRFC...

LAT...LON   41878848 41718760 40778760 39658767 38668784
            38978942 39128974 39569094 40419060 41188987