MESOSCALE PRECIPITATION DISCUSSION 0086
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
919 PM EDT TUE APR 03 2018

AREAS AFFECTED...SE IN...S OH...SW PA...NC/NE KY...WV...PNHDLE MD

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

VALID 040118Z - 040715Z

SUMMARY...FLASH FLOODING WILL REMAIN POSSIBLE ACROSS PORTIONS OF
THE MIDDLE AND UPPER OHIO RIVER VALLEY THIS EVENING AS SEVERAL
WAVES OF THUNDERSTORMS MOVE THROUGH THE REGION. DESPITE SOME
GRADUAL DECREASE IN THE RAIN RATES...LOCALIZED 6-HOUR RAIN TOTALS
MAY REACH 2-3 INCHES IN ISOLATED HEAVIER BANDS...WHICH COULD LEAD
TO FLOODING.

DISCUSSION...A WELL-DEFINED SURFACE LOW WAS SITUATED IN NORTHEAST
INDIANA AS OF 01Z...WITH A SYNOPTIC COLD FRONT STRETCHING TO THE
SSW/SW AND BEGINNING TO PROGRESS MORE QUICKLY TO THE EAST.
HOWEVER...THERE WAS ALSO A SIGNIFICANT AMOUNT OF CONVECTION
OCCURRING WELL OUT AHEAD OF THE FRONT. ONE ARCING CONVECTIVE LINE
HAS ACCELERATED AS FAR EAST AS SWRN PENNSYLVANIA AND THE OHIO-WEST
VIRGINIA BORDER. THIS HAD ESTABLISHED AN OUTFLOW BOUNDARY ALONG
THE SOUTHERN BORDER OF OHIO WITH AN ELONGATED MESO-HIGH JUST TO
THE NORTHWEST... INDICATIVE OF A BOUNDARY LAYER COLD POOL THAT WAS
ALTERING THE SURFACE WIND FIELD. BEHIND THAT...A PRE-FRONTAL BAND
OF CONVECTION STRETCHED FROM NEAR CLEVELAND OH... TO COLUMBUS
OH... TO LEXINGTON KY... TO BOWLING GREEN KY. THIS WAS SITUATED
JUST INSIDE THE EASTERN EDGE OF THE PLUME OF HIGHER LOW-MID LEVEL
MOISTURE AND INSTABILITY.

FLOODING IS LIKELY ONGOING ACROSS PORTIONS OF INDIANA AND OHIO
WHERE HEAVY RAIN HAS ALREADY OCCURRED EARLIER IN THE DAY. THE
FLASH FLOOD GUIDANCE IN THESE AREAS IS CONSIDERABLY LOWER...AND IT
WOULD NOT TAKE MUCH ADDITIONAL RAINFALL TO LEAD TO RENEWED
FLOODING OR EXACERBATE EXISTING FLOODING. HOWEVER...MODELS
GENERALLY SHOW DECREASING RAIN RATES IN THESE AREAS OVER THE NEXT
SIX HOURS...LIKELY DUE TO ITS INCREASINGLY LIMITED ACCESS TO THE
MORE UNSTABLE INFLOW NORTH OF THE OUTFLOW BOUNDARY.
ADDITIONALLY...THE CONVECTIVE LINES MAY BEGIN TO MOVE MORE QUICKLY
TO THE EAST AS THE COLD FRONT BEGINS TO ACCELERATE. THESE FACTORS
WOULD TEND TO LIMIT FLASH FLOOD POTENTIAL NORTH OF THE OHIO
RIVER...BUT GIVEN SATURATED SOIL CONDITIONS ANY HEAVIER RAINFALL
COULD LEAD TO ISOLATED FLASH FLOODING.

FURTHER TO THE SOUTH...RAIN RATES MAY BE HIGHER AND HI-RES MODELS
ARE STRUGGLING TO ADEQUATELY MODEL THE SITUATION. IN
GENERAL...THEY LIMIT THE PRE-FRONTAL CONVECTION FAR MORE THAN WHAT
IS CURRENTLY BEING OBSERVED ON RADAR. THE HIGHER MUCAPE VALUES
(500-1000 J/KG) WITH PRECIPITABLE WATER VALUES AROUND 1 INCH
SHOULD PROVIDE AN ENVIRONMENT CONDUCIVE FOR 1 IN/HR RAIN
RATES...AND THE EXISTING OUTFLOW BOUNDARY COULD PROVIDE MORE OF A
FOCUS FOR TRAINING RAIN BANDS THAN IS CURRENTLY SUGGESTED BY
HI-RES MODELS. THE LIMITING FACTORS IN THIS REGION (NC/NE KY INTO
S/C WV) WOULD BE HIGHER FLASH FLOOD GUIDANCE...AND THE CURRENT
POSITION OF PRE-FRONTAL CONVECTION RELATIVE TO THE INSTABILITY AND
MOISTURE AXIS. AS IT IS GENERALLY ON THE EASTERN EDGE...FASTER
FORWARD PROPAGATION COULD PUSH THE CONVECTION INTO A MORE
UNFAVORABLE ENVIRONMENT THAT COULD ACT TO REDUCE RAIN RATES. THE
PLUME OF HIGHER PWATS WILL EVENTUALLY SWEEP ACROSS THE ENTIRE
REGION WITH THE COLD FRONT...BUT IT REMAINS UNCERTAIN WHETHER OR
NOT CONVECTION WILL BE ONGOING WITH ACCESS TO MORE ROBUST
INSTABILITY WHEN THAT OCCURS. ALTOGETHER THIS SUGGESTS A THREAT
FOR ISOLATED FLASH FLOODING ACROSS THE ENTIRE OUTLINED REGION.

LAMERS

ATTN...WFO...CLE...CTP...ILN...IND...JKL...LMK...LWX...PBZ...
RLX...RNK...

ATTN...RFC...MARFC...OHRFC...

LAT...LON   41087949 40887874 40487832 39867859 39427903
            38927953 38318001 37948076 37508138 37428292
            37408390 37778515 38128589 38388662 39218637
            39918572 40348464 40568320 40568231 40738165
            40828099 40948034