MESOSCALE PRECIPITATION DISCUSSION 0498
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
349 AM EDT SAT JUL 21 2018

AREAS AFFECTED...EASTERN NC & FAR SOUTHEAST VA

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

VALID 210749Z - 211349Z

SUMMARY...THUNDERSTORMS WITH HEAVY RAIN ARE BEGINNING TO STREAM
NORTHWARD INTO EASTERNMOST NC.  HOURLY RAIN TOTALS TO 2.5" WITH
LOCAL AMOUNTS TO 5" ARE POSSIBLE THROUGH 14Z.

DISCUSSION...SURFACE OBSERVATIONS INDICATE A PAIR OF LOW PRESSURE
AREAS: ONE JUST WEST OF WILMINGTON WITH A SECOND ONE IMPLIED JUST
EAST OF NEW RIVER NC.  CENTRAL PRESSURES ARE VERY SLOWLY FALLING
IN THIS REGION AHEAD OF A SHORTWAVE MOVING INTO CENTRAL VA AND
CENTRAL NC.  SATELLITE IMAGERY REVEALS ENHANCED DIVERGENCE IN THE
CIRRUS PATTERN AND THE EXISTENCE OF A CONVECTIVE BAROCLINIC LEAF
CENTERED CLOSER TO THE EASTERN OF THE TWO LOWS.  THUNDERSTORMS
WITH HEAVY RAINFALL AND COOLING CLOUD TOPS ARE MOVING INTO EASTERN
NC FROM THE ATLANTIC WITH A COUPLE MESOSCALE CIRCULATIONS NOTED
JUST EAST AND WELL SOUTH OF MOREHEAD CITY NC.  PRECIPITABLE WATER
VALUES ARE 2.25-2.5" PER GPS DATA.  INFLOW AT 850 HPA IS 20-25 KTS
FROM THE SOUTH PER VAD WIND PROFILES AND RAP FORECAST DATA. 
EFFECTIVE BULK SHEAR IN THE REGION REMAINS UNDER 25 KTS PER SPC
MESOANALYSES.  ML CAPE VALUES ARE 1000-2000 J/KG ACROSS THE OUTER
BANKS OF NC, AND ARE TRENDING UPWARD ACROSS NORTHEAST NC AND
SOUTHEAST VA.  CIN REMAINS MINIMAL.

INFLOW AT 850 HPA IS EXPECTED TO RAMP UP IN THIS REGION, WHICH
SHOULD CONTINUE TO SHIFT INSTABILITY NORTHWARD UP THE COAST NEAR
AND AHEAD OF THE SURFACE LOW.  THE MESOSCALE GUIDANCE HAS A
UNIFIED SIGNAL FOR LOCAL AMOUNTS OF 5" THROUGH 14Z ACROSS EASTERN
NC, BUT DISAGREE AS TO HOW MUCH RAIN FALLS ACROSS SOUTHEAST VA. 
AS LONG AS CIN REMAINS MINIMAL, BOUNDARY LAYER MOISTURE
CONVERGENCE WOULD DETERMINE THE AXIS FOR HEAVY RAINFALL.  RAP
FORECASTS INDICATE THAT THIS AXIS/WARM FRONT PIVOTS INTO NORFOLK
VA AND GETS VERY CLOSE TO THE VA PORTION OF THE EASTERN SHORE. 
THE 850 HPA SLICE OF THE FRONT GETS INTO CHESAPEAKE BAY BY THE END
OF THE PERIOD, WHICH WOULD BECOME A MORE IMPORTANT CONVECTIVE
FOCUS SHOULD CIN DEVELOP.  THE AMOUNT OF MOISTURE AND INSTABILITY
ALLOWS FOR HOURLY RAIN TOTALS UP TO 2.5" ACROSS EASTERN NC AND
NEAR NORFOLK VA.  THIS DEGREE OF RAIN WOULD BE PROBLEMATIC IN
URBAN AREAS.  UNCERTAINTY IN THE FORECAST SCENARIO IS HIGHER THAN
NORMAL AS HEAVY RAINFALL IN SOUTHEAST VA COULD CLIP NORFOLK AND
SAIL EAST OF THE EASTERN SHORE.

ROTH

ATTN...WFO...AKQ...MHX...

ATTN...RFC...MARFC...SERFC...

LAT...LON   38007555 37997527 37367559 36787589 36337562
            35887541 35207543 35137578 34877616 34647638
            34527654 34567668 34617676 34587705 34577717
            34647723 34787720 35267719 35907699 37257634
            37877587