MESOSCALE PRECIPITATION DISCUSSION 0083
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
205 AM EDT TUE APR 03 2018

AREAS AFFECTED...SC ILLINOIS...SRN/CTRL INDIANA...SW OHIO...FAR
NRN KY

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

VALID 030604Z - 031200Z

SUMMARY...ELEVATED THUNDERSTORMS WILL CONTINUE TO BECOME MORE
NUMEROUS EARLY THIS MORNING AND MAY POSE A FLASH FLOOD THREAT.
HOURLY RAINFALL RATES MAY REACH AROUND 1 IN/HR...WITH RAINFALL
TOTALS REACHING 2-3 INCHES IN THE HEAVIEST RAIN BANDS.

DISCUSSION...ELEVATED THUNDERSTORMS CONTINUE TO BLOSSOM FROM FAR
EASTERN MISSOURI INTO SOUTH-CENTRAL ILLINOIS AS OF 06Z PER
SATELLITE AND RADAR OBSERVATIONS. GOES EAST IR SATELLITE SHOWED
RELATIVELY LIMITED COVERAGE OF COLD CLOUD TOPS AS RECENTLY AS
03Z...BUT OVER THE PAST THREE HOURS CLOUD TOPS COLDER THAN -40C
HAVE MARKEDLY INCREASED IN AREAL EXTENT WITH CONTINUED COOLING
(MIN VALUES NOW AROUND -70C) AND SIGNIFICANT AMOUNTS OF LIGHTNING
OVER ILLINOIS. THESE ARE ALL SIGNALS THAT THE CONVECTION IS
BEGINNING TO FULLY REALIZE THE APPROX 1000 J/KG OF MUCAPE
AVAILABLE. STORMS ARE UNDOUBTEDLY ELEVATED WITH SURFACE
TEMPERATURES AND DEWPOINTS MERELY IN THE 40S...AND PARTS OF THE
REGION HAVING JUST RECEIVED SNOWFALL IN THE PAST 24-36 HOURS. THE
EFFECTIVE INFLOW LAYER LOOKS TO BE AROUND 900-850MB PER A VARIETY
OF MODEL FORECAST SOUNDINGS...AND OBJECTIVE RAP ANALYSIS SHOWED
INCREASING FRONTOGENESIS IN THAT SAME LAYER FROM CTRL MISSOURI
INTO SC ILLINOIS ON THE NOSE OF THE LOW-LEVEL JET. THE COMBINATION
OF IMPROVING LOW-LEVEL FORCING AND SUBSTANTIAL ELEVATED
INSTABILITY SHOULD ALLOW THUNDERSTORMS TO CONTINUE TO DEVELOP AND
ORGANIZE ACROSS THE REGION.

THE INFLOW REGION OVER SRN MISSOURI AND NRN ARKANSAS IS
CHARACTERIZED BY PRECIPITABLE WATER VALUES AROUND 1.1 TO 1.2
INCHES (PER RAP ANALYSIS AND GPS-PW OBS). WHILE THESE VALUES ARE
NOT EXTREMELY HIGH...THEY ARE ABOVE NORMAL FOR THIS TIME OF
YEAR...AND WHERE THEY EXTEND INTO THE IMMEDIATE VICINITY OF THE
OHIO RIVER ARE ROUGHLY IN THE 80-90TH PERCENTILE. THE LACK OF
SUBSTANTIALLY HIGHER PWS MAY LIMIT THE MAXIMUM HOURLY RAIN RATES
SOMEWHAT CLOSER TO 1 IN/HR. HOWEVER...GIVEN THE ABUNDANCE OF
DEVELOPING STORMS IN THE PAST FEW HOURS...TRAINING CONVECTION
SEEMS LIKELY AND MAY LEAD TO SEVERAL INCHES OF RAIN IN JUST 2-4
HOURS IN THE HEAVIEST RAIN BANDS. 3-HR FLASH FLOOD GUIDANCE ACROSS
MUCH OF THE OUTLINED DISCUSSION AREA IS AROUND OR JUST BELOW 2
INCHES...WHICH SEEMS ACHIEVABLE GIVEN HI-RES MODEL FORECASTS AND
RADAR TRENDS. FURTHERMORE...SNOW MELT FROM YESTERDAY SHOULD HAVE
ADDED TO THE MOISTURE LEVELS IN THE TOP LAYERS OF THE SOIL...AND
THE NATIONAL WATER MODEL SHOWS ELEVATED SOIL MOISTURE LEVELS AND
STREAMFLOW VALUES ACROSS MUCH OF THE SAME AREA. THEREFORE...FLASH
FLOODING WILL BE POSSIBLE WHERE HEAVIER RAIN CAN PERSIST FOR AT
LEAST A COUPLE HOURS.

LAMERS

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

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

LAT...LON   40568646 40488509 40118439 39398345 38928356
            38618429 38558514 38508641 38338725 38348871
            38468954 38869041 39399025 39828961 40188823