MESOSCALE PRECIPITATION DISCUSSION 0209
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
737 PM EDT SUN MAY 27 2018

AREAS AFFECTED...CENTRAL VA...SOUTHERN AND EASTERN MD...SOUTHERN
DE...EXT SOUTHEAST WV

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

VALID 272335Z - 280435Z

SUMMARY...NARROW CORRIDOR OF MOIST CONFLUENCE/CONVERGENCE FOR
EFFICIENT RAINFALL RATES WITH SLOW TRAINING AND POTENTIAL
STATIONARY CELLS. 

DISCUSSION...RAP ANALYSIS FIELDS DEPICT A NARROW MID-LEVEL
CONFLUENCE ZONE FROM NEAR APPOMATTOX COUNTY TO SOUTHERN DE, WHERE
20KT 7H FLOW ROUNDS THE SE VA RIDGE INTO THIS ZONE WHILE WEAK
5-15KT FLOW NORTH OF THE ZONE CONVERGES AS WELL LEADING TO MASS
PILING AND TPWS UP TO 2.0".  ADDITIONALLY SUFFICIENT DAYTIME
HEATING ALSO ALLOWED FOR A RESERVOIR OF INSTABILITY TO BUILD UP
SOUTH OF THE ZONE WITH SBCAPES UP TO 2500 J/KG STILL GENERALLY
UNCAPPED.  AS SUCH ACTIVE CONVECTION HAS BEEN DEVELOPING ALONG THE
LINE WITH NUMEROUS OVERSHOOTING TOPS FROM LYNCHBURG TO THE
NORTHERN NECK OF VA SEEN IN GOES-16 IMAGERY.  WITH NEAR NEGLIGIBLE
LOSS TO MOISTEN THE COLUMN, CELLS WILL BE HIGHLY EFFICIENT WITH
RATES UP TO 2.0"/HR.  CELLS DUE TO THE WEAK CONFLUENCE AND
STEERING IN THE MID-LEVELS CELLS WILL SLOWLY TRACK ENE WITH
TRAINING POSSIBLE.  THE GOOD NEWS IS THAT THERE IS STRONG GUIDANCE
TO SUPPORT THAT THE RESERVOIR OF INSTABILITY WILL BE EXHAUSTED
WITH LOSS OF DAY TIME HEATING AND CELLS WILL WEAKEN.  ANOTHER
LIMITING FACTOR COULD BE OVERLOADING OF DOWNDRAFTS WHICH COULD
DISRUPT UPDRAFTS GIVEN VERY WEAK INFLOW. RATES AND SLOW CELL
MOTIONS EVEN IF DISRUPTED SHOULD BE SUFFICIENT TO EXCEED THE LOW
1HR FFG VALUES BELOW 1.5" OR LOWER...THOUGH FF WILL BE MORE
LOCALIZED IN THE PIEDMONT AND SW VA AND SE WV. 

AREA OF GREATEST CONCERN EXISTS NEAR THE SLOW SOUTHWESTWARD
SAGGING 'BACKDOOR' FRONTAL ZONE THAT BISECTS THE AREA OF CONCERN
NOW ACROSS NOVA TO SOUTHERN MD AND MORE EASTWARD FROM CGE TO
FENWICK ISLAND; WITH THIS LAST SEGMENT FAIRLY PARALLEL TO THE
CONFLUENCE ZONE.  AS SUCH INCREASED SFC CONVERGENCE AND ASCENT
THAT SHOULD BE INCREASED TO ALLOW FOR STRONGER INFLOW WILL SUPPORT
STRONGER FLUX AS WELL AS LONGER AREA OF ASCENT...AS SUCH 3-5" IS
POSSIBLE ACROSS THE CENTRAL DELMARVA AND 2-4" ACROSS S MD. THIS IS
IN LINE WITH THE GEM-REGIONAL AND NAM CONEST AND SUPPORTED BY
RECENT HRRR RUNS.  

GALLINA

ATTN...WFO...AKQ...LWX...PHI...RLX...RNK...

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

LAT...LON   39107548 38837469 38097540 37467724 37037883
            36938023 37218142 37778149 37818055 37737961
            38347787 39037639