MESOSCALE PRECIPITATION DISCUSSION 0135...CORRECTED
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
242 AM EDT SAT MAY 12 2018

CORRECTED FOR TYPOS

AREAS AFFECTED...SOUTH-CENTRAL IA & NORTHWEST IL

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

VALID 120636Z - 121036Z

SUMMARY...THUNDERSTORMS ARE EXPANDING IN COVERAGE AND INTENSITY
ACROSS SOUTH-CENTRAL IA.  HOURLY TOTALS TO 1.5" WITH LOCAL AMOUNTS
TO 3" ARE EXPECTED.

DISCUSSION...A SHORTWAVE ENTERING SOUTHWEST IA HAS LED TO
INCREASING THUNDERSTORMS ACROSS SOUTH-CENTRAL IA NEAR AN 850 HPA
SLICE OF THE FRONTAL BOUNDARY, WITH THE BEST CONCENTRATION OF
CONVECTION CURRENTLY WEST AND SOUTH OF DES MOINES.  IR CLOUD TOPS
CONTINUE TO COOL AND EXPAND WITH THE STORMS PER GOES-16 IMAGERY. 
PRECIPITABLE WATER VALUES OF 1.3-1.4" LIE HERE PER GPS DATA. 
INFLOW AT 850 HPA IS ~25 KTS FROM THE SOUTHWEST, WHICH IS
IMPORTING MU CAPE VALUES OF 2000 J/KG INTO THE BAND.  EFFECTIVE
BULK SHEAR OF 40-50 KTS EXISTS HERE PER SPC MESOANALYSES.  OF
CONCERN IS A FAIRLY CONSTANT REDEVELOPMENT OF CONVECTION NEAR THE
IA/NE BORDER WHICH IS LENGTHENING THE BAND.

THE 00Z HREF PROBABILITIES OF REACHING 0.5" AND 1" IN AN HOUR HAVE
UNDERSOLD THE POTENTIAL/BEEN TOO LOW THUS FAR.  WHILE THE
MESOSCALE GUIDANCE HAS A SIGNAL FOR 2" AMOUNTS LOCALLY, 3" AMOUNTS
APPEAR MORE LIKELY PER RECENT TRENDS AND EXPECTATIONS.  THE PEAK
TIME FOR RAINFALL/CELL COVERAGE APPEARS TO BE 08Z.  WITH TIME, THE
LOW-LEVEL INFLOW IS EXPECTED TO VEER AND SLIGHTLY WEAKEN, WHICH
SHOULD STOP THE BACKBUILDING/REDEVELOPMENT AND FREE THE CONVECTION
TO SHIFT EASTWARD.  THE RAIN THREAT SHOULD BE WANING AFTER 10Z. 
UNTIL THEN, HOURLY RAIN TOTALS TO 1.5" WHERE CELLS TRAIN AND MERGE
ARE EXPECTED.

ROTH

ATTN...WFO...DMX...DVN...LOT...OAX...

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

LAT...LON   42229115 42109008 41748924 41199029 40979130
            40979389 41319613 41749614 41909498