MESOSCALE PRECIPITATION DISCUSSION 0120
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
836 PM EDT WED MAY 02 2018

AREAS AFFECTED...SRN IOWA...NRN ILLINOIS...FAR NRN MISSOURI

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

VALID 030035Z - 030635Z

SUMMARY...DEVELOPING THUNDERSTORMS ALONG A STALLING FRONT FROM
SOUTHERN IOWA INTO NORTHERN ILLINOIS MAY BEGIN TO TRAIN AND
PRODUCE LOCALLY VERY HEAVY RAINFALL. HOURLY RAIN RATES OF 1-2
IN/HR MAY PRODUCE SOME FLASH FLOODING THIS EVENING ACROSS THE
REGION.

DISCUSSION...GOES-16 SATELLITE IMAGERY AND REGIONAL RADARS SHOWED
A BLOSSOMING OF CONVECTION BETWEEN 22Z AND 2330Z ALONG A STALLED
FRONT OVER SOUTHERN IOWA. RAPIDLY COOLING CLOUD TOPS WERE
NOTED...AND THIS DEVELOPING CONVECTION HAS GENERALLY SINCE
MAINTAINED ITS INTENSITY WITH AN UNINHIBITED INFLOW OF A
MOIST...UNSTABLE AIR MASS IMMEDIATELY TO THE SOUTH OVER NORTHERN
MISSOURI. OBJECTIVE RAP ANALYSIS AT 00Z SHOWS MLCAPE VALUES AROUND
1000-2000 J/KG WITH AROUND 50 KNOTS OF DEEP LAYER EFFECTIVE SHEAR.
THIS ENVIRONMENT SHOULD SUPPORT THE MAINTENANCE OF THIS CONVECTION
INTO THE EVENING HOURS.

OVER THE PAST COUPLE HOURS...RADAR AND LIGHTNING TRENDS HAVE SHOWN
VERY LITTLE MOTION OFF THE EXISTING FRONT IN SOUTHERN IOWA...AND
850-300MB MEAN FLOW AND CORFIDI VECTORS ARE ORIENTED NEARLY
PARALLEL TO THIS BOUNDARY. INCREASING LOW-LEVEL INFLOW WITH THE
NOCTURNAL INCREASE OF THE LLJ AND A CONFLUENCE TO THIS FLOW NEAR
THE NOSE OF THE LLJ...WILL PROVIDE CONTINUED LOW-LEVEL FORCING IN
A CORRIDOR THAT SHOULD BE FAVORED FOR MOVEMENT OF EXISTING STORMS.
THE COMBINATION OF POSSIBLE NEW CONVECTIVE DEVELOPMENT IN CONCERT
WITH A CHANNELING OF EXISTING CONVECTION INTO A SIMILAR PATH
SHOULD LEAD TO THE THREAT OF FLASH FLOODING THIS
EVENING...PARTICULARLY FROM SOUTHERN IOWA INTO NORTHERN ILLINOIS.
THE DEVELOPMENT OF ADDITIONAL CONVECTION WILL ALSO BE AIDED BY THE
OVERLAP OF THE RIGHT ENTRANCE REGION OF A 150 KNOT UPPER LEVEL JET
(WITH STRONG DIVERGENCE SIGNATURE NOTED ON THE RAP FORECASTS).

HI-RES MODELS ARE NOT HANDLING THE ONGOING DISTRIBUTION OR MOTION
OF CONVECTION VERY WELL. EVEN THE MODELS THAT APPEAR TO BE CLOSER
TO THE 00Z OBSERVED NATIONAL RADAR MOSAIC (18Z NAM CONEST, HRRR,
HRRR-E) DO NOT APPEAR TO MATCH THE INTENSITY OF ONGOING RAINFALL
OR THE OBSERVED CELL MOTIONS. HOWEVER...THE MESOSCALE PATTERN DOES
FAVOR CONTINUED INSTANCES OF CELL TRAINING. PRECIPITABLE WATER
VALUES AROUND 1.2 TO 1.3 INCHES COULD SUPPORT SOME 1 INCH PER HOUR
RAIN RATES...BUT THE PRESENCE OF A DRY LAYER AROUND 700-500MB
COULD REDUCE PRECIPITATION EFFICIENCY. THEREFORE...FLASH FLOODING
WOULD BE MOST LIKELY WHERE THE DURATION OF RAINFALL WOULD
COMPENSATE FOR THE LIMITING FACTORS OF A LACK OF MID-LEVEL
MOISTURE AND MORE SUBSTANTIAL PWATS.

LAMERS

ATTN...WFO...DMX...DVN...EAX...ILX...LOT...LSX...OAX...

ATTN...RFC...MBRFC...NCRFC...

LAT...LON   42278874 42198762 41058766 40439012 39859313
            39869476 40279535 41049483 41729278 42069077