MESOSCALE PRECIPITATION DISCUSSION 0165
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
215 AM EDT FRI MAY 18 2018

AREAS AFFECTED...NORTH CENTRAL NC...CENTRAL/SOUTHERN VA...SOUTHERN
MD

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

VALID 180615Z - 181215Z

SUMMARY...HEAVY RAINFALL THREAT INCREASING THROUGH THE EARLY
MORNING HOURS. GIVEN THE WET ANTECEDENT CONDITIONS...FLASH
FLOODING IS POSSIBLE.

DISCUSSION...CONVECTIVE CLOUDS TOPS ARE COOLING ACROSS A FAIRLY
LARGE AREA OF NORTH-CENTRAL NC AND UP ACROSS SOUTH-CENTRAL VA AS
HEAVY SHOWERS AND A FEW EMBEDDED THUNDERSTORMS ARE SEEN ORGANIZING
ACROSS THIS REGION. THERE IS EVIDENCE OF A WEAK SHORTWAVE IMPULSE
ACTING AS AT LEAST A PARTIAL FACILITATOR WHICH IS SEEN TRAVERSING
THE CENTRAL APPALACHIANS AT THIS TIME. THE FLOW TO THE EAST OF
THIS IS BECOMING AT LEAST MODESTLY DIVERGENT AT THE UPPER LEVELS
OF THE COLUMN WHICH IS PROVIDING SOME UPTICK IN LARGER SCALE
ASCENT. MEANWHILE...A QUASI-STATIONARY FRONT OVER THE REGION IS
FOCUSING LOW LEVEL CONVERGENCE WITH A POOL OF WEAK INSTABILITY
ALONG IT.

THESE INGREDIENTS APPEAR TO BE COMING TOGETHER EARLY THIS MORNING
IN A WAY THAT WILL FACILITATE AN AXIS OF SUSTAINABLE CONVECTION
AND RESULTING HEAVY RAINFALL. GIVEN THE DEPTH OF TROPICAL MOISTURE
OVER THE REGION...WITH ENHANCED RAINFALL EFFICIENCY AND THE VERY
COLD CONVECTIVE TOPS...IT IS POSSIBLE THAT RAINFALL RATES MAY
REACH UP TO NEAR 2 INCHES/HR. THE CONVECTION IS EXPECTED TO LIFT
UP TO THE NORTHEAST AND BECOME ORIENTED IN A SW/NE FASHION THAT
WILL GRADUALLY INCLUDE THE NORTHERN NECK OF VA AND ALSO ADJACENT
AREAS OF SOUTHERN MD AND THE MIDDLE CHESAPEAKE BAY REGION.

THE LATEST HRRR/HRRRX GUIDANCE SUPPORTS AN ADDITIONAL 3 TO 4
INCHES OF RAIN THROUGH 12Z...WITH MUCH OF THE REGION ALREADY
SEEING THEIR SOIL CONDITIONS VERY MOIST IF NOT SATURATED FROM
EARLIER HEAVY RAINFALL. THEREFORE...A CONCERN FOR ENHANCED RUNOFF
AND FLASH FLOODING EXISTS.

ORRISON

ATTN...WFO...AKQ...LWX...PHI...RAH...RNK...

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

LAT...LON   38767656 38687631 38427601 37857612 37317643
            36267736 35667829 35407902 35657970 36207982
            37107903 37677846 38227768 38637707