MESOSCALE PRECIPITATION DISCUSSION 0292
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
853 PM EDT MON JUN 11 2018

AREAS AFFECTED...SOUTH-CENTRAL ILLINOIS...SOUTHWEST INDIANA

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

VALID 120052Z - 120500Z

SUMMARY...THUNDERSTORMS WERE BECOMING MORE NUMEROUS BY 7 PM CDT
ACROSS SOUTH-CENTRAL ILLINOIS ALONG A STATIONARY FRONT. THESE
STORMS SHOULD CONTINUE TO COALESCE INTO LARGER CLUSTERS AS THEY
SLOWLY MOVE EAST-SOUTHEAST...AND MAY POSE A FLASH FLOOD THREAT.
RAIN RATES UP TO 2 IN/HR WILL BE POSSIBLE...AND THE STORMS MAY
EVENTUALLY SPREAD INTO SOUTHWEST INDIANA.

DISCUSSION...SURFACE OBSERVATIONS DEPICTED A WELL-DEFINED FRONT
ACROSS SOUTH-CENTRAL ILLINOIS OVER THE PAST COUPLE HOURS. WHEN
CONVECTION WAS BEGINNING TO INITIATE AROUND 22Z...THE REGION SOUTH
OF THE BOUNDARY WAS CHARACTERIZED BY SURFACE TEMPERATURES
GENERALLY 89-93 DEGREES WITH DEWPOINTS IN THE LOW 70S AND
SOUTHWEST WINDS. THE INFLOW REGION ALSO HAD ANALYZED SBCAPE IN
EXCESS OF 3000 J/KG INDICATING A STRONGLY UNSTABLE AIR MASS. TO
THE NORTH...GOES-16 VISIBLE SATELLITE IMAGERY SHOWED MORE STABLE
STRATUS CLOUDS...EASTERLY FLOW...AND SURFACE TEMPERATURES
GENERALLY BELOW 80 DEGREES. IN THE FEW HOURS SINCE...CONVECTION
HAS BECOME MORE NUMEROUS ALONG AND JUST NORTH OF THE BOUNDARY
WHERE THE RAP ANALYZED A POOL OF HIGHER 925-850MB DEWPOINTS (+16C
TO +18C AT 850MB) AND A COINCIDENT RIBBON OF HIGHER PRECIPITABLE
WATER VALUES. THE SHARP LOW-LEVEL FOCUS FOR CONVECTION COMBINED
WITH THE UNSTABLE AND MOIST ENVIRONMENT SHOULD CONTINUE TO SUPPORT
FAIRLY WIDESPREAD THUNDERSTORMS AND A LIKELY CORRIDOR WHERE THEY
WILL TRACK.

THE STORMS WERE EXHIBITING VERY SLOW MOTIONS AROUND 00Z...WITH THE
RAP ANALYZED FORWARD PROPAGATING MOTION VECTORS ONLY AROUND 10-15
KNOTS. AS THE CONVECTION CONTINUES TO COALESCE INTO LARGER
CLUSTERS...THEY MAY BEGIN TO ACCELERATE TO THE SOUTHEAST BUT THE
FORWARD SPEED IS STILL LIKELY TO BE SOMEWHAT LIMITED. THIS MAY
PROLONG HEAVY RAINFALL IN A GIVEN LOCATION AND INCREASE THE FLASH
FLOODING THREAT. ADDITIONALLY...NEW UPDRAFT GROWTH CONTINUES AHEAD
OF THE DOMINANT CLUSTERS (CURRENTLY IN THE I-70 CORRIDOR) AND THIS
MAY FAVOR FURTHER CELL MERGERS AND HEAVY RAINFALL THREAT. GIVEN
THIS...RAIN RATES TO AROUND 2 IN/HR SEEM ACHIEVABLE. WITH REDUCED
FLASH FLOOD GUIDANCE AND ELEVATED SOIL MOISTURE ACROSS PORTIONS OF
THE AREA THAT HAVE RECEIVED HEAVY RAINFALL IN THE PREVIOUS 24
HOURS...THESE RAIN RATES COULD BE SUFFICIENT TO PRODUCE SOME
INSTANCES OF FLASH FLOODING.

LAMERS

ATTN...WFO...ILX...IND...LMK...LSX...PAH...

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

LAT...LON   39358907 39358849 39098732 38818637 38248621
            37818694 37978818 38268907 38618976 39058976