MESOSCALE PRECIPITATION DISCUSSION 0166
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1234 PM EDT FRI MAY 18 2018

AREAS AFFECTED...WRN VA/NC BORDER INTO CNTRL VAH

CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY

VALID 181633Z - 182230Z

SUMMARY...THUNDERSTORM COVERAGE WILL INCREASE OVER THE NEXT FEW
HOURS...POSING A FLASH FLOOD THREAT FROM WESTERN NORTH
CAROLINA/VIRGINIA TO THE CHESAPEAKE BAY. PERIODS OF TRAINING AND
REPEATING CONVECTION ARE EXPECTED TO PRODUCE 2-4+ INCHES THROUGH
22Z.

DISCUSSION...THINNER CLOUD COVER OVER PORTIONS OF SOUTHEAST
VIRGINIA INTO NORTH CAROLINA...LOCATED SOUTH OF A STATIONARY
FRONT...HAS ALLOWED FOR MODEST WARMING AND INCREASING
CAPE...RELATIVE TO LOCATIONS NORTH OF THE FRONT. AT 16Z...REGIONAL
RADAR IMAGERY SHOWED THE GREATEST COVERAGE OF CONVECTION NEAR THE
FOOTHILLS OF THE APPALACHIANS OVER WESTERN VIRGINIA/NORTH CAROLINA
WHERE AN UPSLOPE COMPONENT WAS NOTED TO THE 850 MB FLOW. KFCX
RAINFALL ESTIMATES WERE ABOUT 0.5 TO 1.0 IN/HR...NEAR SOME OF THE
VERY LOW FLASH FLOOD GUIDANCE VALUES IN THE AREA. THE SPC
MESOANALYSIS PAGE INDICATED MLCAPE WAS WEAK AS OF 15Z...BETWEEN
250 AND 750 J/KG...BUT 12Z AREA RAOBS SHOWED PRECIPITABLE WATER
VALUES NEAR THE 95TH PERCENTILE AND FLOW WAS UNIDIRECTIONAL FROM
THE SOUTH ABOVE THE BOUNDARY LAYER.

AS CONTINUED HEATING OCCURS...WEAK INCREASES IN CAPE ARE EXPECTED
TO COMBINE WITH FORCING FROM SEVERAL WEAK MESOSCALE VORTICITY
MAXIMA IN THE WARM SECTOR MOVING NORTHWARD TO INCREASE COVERAGE OF
CONVECTION TOWARD THE EAST IN THE VICINITY OF THE STATIONARY FRONT
WHICH WILL ACT AS A SOURCE OF LOW LEVEL CONVERGENCE. ALOFT...FLOW
IS BROADLY DIFFLUENT WHICH WILL HELP TO SUPPORT EVACUATION ALOFT
AND VERTICAL MOTIONS. LOW LEVEL UPSLOPE FLOW TO THE WEST AND
UNIDIRECTIONAL FLOW TO THE EAST SHOULD HELP TO SUPPORT PERIODS OF
TRAINING AND REPEATING CONVECTION INTO THE LATE AFTERNOON. GIVEN
ONLY SMALL INCREASES TO CAPE ARE EXPECTED GIVEN CLOUD COVER AND NO
CHANGES TO MID-LEVEL TEMPERATURES...RAIN RATES ARE EXPECTED TO
PEAK MOSTLY IN THE 1-2 IN/HR RANGE...WITH 6 HOUR TOTALS IN THE 2-4
INCH RANGE...WITH LOCALLY HIGHER TOTALS. GIVEN AREA-WIDE LOW FLASH
FLOOD GUIDANCE...FLOODING AND FLASH FLOODING ARE CONSIDERED LIKELY.

OTTO

ATTN...WFO...AKQ...GSP...LWX...MRX...RAH...RNK...

ATTN...RFC...LMRFC...MARFC...OHRFC...SERFC...

LAT...LON   38237641 37937614 37637615 37267664 36317796
            36027887 35937925 35648058 35628137 35838172
            36348193 36888169 37218101 37467974 37757905
            38097713