MESOSCALE PRECIPITATION DISCUSSION 0500
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
252 PM EDT SAT JUL 21 2018

AREAS AFFECTED...EASTERN VA...CENTRAL MD...SOUTHEAST PA...SOUTHERN
NJ...DELMARVA PENINSULA...

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

VALID 211852Z - 220052Z

SUMMARY...PROLONGED MODERATE RAINS WITH OCCASIONAL EMBEDDED
CONVECTIVE ELEMENTS OVER 2"/HR POSE LIKELY INUNDATION FLOODING
WITH POSSIBLE LOCALIZED FLASH FLOODING ESPECIALLY NEAR SURFACE LOW
TRACK...

DISCUSSION...VERY DYNAMIC SYNOPTIC SETUP EVOLVING ACROSS THE
MID-ATLANTIC THIS AFTERNOON INTO EVENING HOURS.  A SHORTWAVE WITH
TROPICAL ORIGINS OUT OF THE EASTERN GULF OF MEXICO IS DRAWING A
STRONG WARM CONVEYOR BELT ALONG THE COASTAL CAROLINAS AND OFF
SHORE WITH TPWS OVER 2.25" NEARING 2.5" NEAR THE MOUTH OF THE
CHESAPEAKE BAY.  THIS SHORTWAVE IS NICELY ORIENTED IN THE SYNOPTIC
TROF THAT IS GOING NEGATIVE TILT AS THE 80KT 3H JET ROUNDS THE
BASE OF THE TROF ACROSS N GA WITH BROAD DIFFLUENCE ALOFT ACROSS
THE REGION.  AS SUCH 18Z SFC ANALYSIS DEPICTS A 1004MB LOW OVER NE
NC WHICH HAS DROPPED 3MB IN THE LAST 3 HOURS AND IS LIKELY TO
CONTINUE.  GIVEN THE CONFIGURATION A NARROW WEDGE BAROCLINIC
SHIELD EXISTS ACROSS SE PA TO THE NECKS OF VA WITH STRONG
LLJ/WESTERN BRANCH OF THE WARM CONVEYOR PROVIDING STRONG
ISENTROPIC ASCENT WITH 25KTS OF SELY 7H/25KTS EASTERLY 85H AND
30KTS OF NNELY 92H OF DEEP MOIST FLOW/FLUX PER VWP AT TADW AND
TBWI.  STILL INSTABILITY HAS BEEN LIMITED DUE TO 5C/KM LAPSE RATES
AND LIMITED INSOLATION AND LIMITED TO THE WARM CONVEYOR
OFFSHORE...STILL THE SLANTWISE FLOW IS STRONG ENOUGH TO DRAW
MUCAPES AS GREAT AS 1000 J/KG TO THE WESTERN SHORE OF THE
CHESAPEAKE BAY IN MD AND VA ATTM.  THIS IS SUPPORTING HOURLY RATES
UP TO .3-.5"/HR WITH SOME ISOLATED EMBEDDED CONVECTIVE ELEMENTS UP
TO .75"/HR WITHIN THE STRONGEST CONVERGENCE AT THE NOSE OF THE
LLJ/PSEUDO WARM FRONT/COOL CONVEYOR BELT INTERFACE CURRENTLY OVER
THE S DELMARVA TOWARD S NJ.

STRONG WESTERN GRADIENT ALONG THE DEFORMATION ZONE FROM KRIC TO
LOUDOUN COUNTY VA ALONG THE RIGHT ENTRANCE TO THE 50KT JET
ENTRANCE IN THE BEST ALSO SUPPORTS ENHANCED/FOCUSED MOISTURE
CONVERGENCE AND RATES UP TO .75"/HR WHICH WILL LIFT NORTHEAST IN A
TRAINING TYPE MANNER LEADING TO A SECONDARY MAXIMA THROUGH THE
EVENT INTO WEST CENTRAL MD AND ALONG FRONT RIDGE LINES OF S
CENTRAL PA TOO. WITH 2-4" TOTALS POSSIBLE THROUGH 00Z. THE MAJOR
LIMITING FACTOR IS THE VERY HIGH FFG VALUES AND THE DURATION OF
THE RAINFALL...WHICH MAY NOT BE EXCEEDED THROUGH 1/3/6HR TIME
FRAMES BUT PROLONGED INUNDATION IS LIKELY THROUGH MUCH OF THE AREA
OF CONCERN.

THE FLASHY STYLE ELEMENTS ARE LIKELY TO BE LIMITED ALONG AND JUST
WEST OF THE NORTHWARD TRACK OF THE SURFACE LOW.  RECENT 1HR
RAINFALL RATE OF 2.11" AT FNK IN SOUTHEAST VA IS INDICATIVE OF
THIS THREAT.  STRONG ISALLOBARIC FLOW/MST CONVERGENCE AND
FAVORABLE SHEAR VECTORS SHOULD SUPPORT MORE VERTICAL DEVELOPMENT
BUT ALSO A BACKBUILDING NATURE TOWARD THE CENTER OF LOW PRESSURE
ON THE DOWNWIND SIDE OF THE INNER MOST BAND OF CONVERGENCE. 
CURRENT TRENDS/TRACK WOULD SUGGEST THE LOW TO SLIP THROUGH THE
NORFOLK METRO THEN HEAD JUST WEST OF DUE NORTH ALONG OR CENTERED
IN THE CHESAPEAKE BAY... THIS WILL LEAVE THE GREATEST THREAT OF
2"/HR RAIN RATES OVER THE ALREADY 1-3" TOTALS THAT WOULD HAVE
FALLEN PRECURSORY TO THIS SMALL CONVECTIVE COMPLEX.  IF ADDED BY
FRICTIONAL CONVERGENCE ALONG THE INLETS/NECKS OF THE RIVERS INTO
ST. MARY'S/CALVERT COUNTIES IN MD...FURTHER .25-.5"/HR RATES WOULD
BE POSSIBLE (PER RECENT HRRR/NSSL-WRF UP TO 3+"/HR IN THE 22-01Z
TIME FRAME)...BUT THIS SCENARIO REMAINS A BIT TOO UNCERTAIN AT
THIS TIME.

GALLINA

ATTN...WFO...AKQ...CTP...LWX...PHI...

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

LAT...LON   40377477 39787413 39317440 38857476 37857530
            37367558 36617590 36537658 36737707 38357774
            39557802 40317770 40347585