MESOSCALE PRECIPITATION DISCUSSION 0643
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
858 AM EDT MON AUG 07 2017

AREAS AFFECTED...N VA...DC...MD...DE...S NJ...EXT SE PA

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

VALID 071300Z - 071900Z

SUMMARY...TRAINING EMBEDDED CONVECTIVE ELEMENTS WITHIN DEFORMATION
ZONE POSES LONGER DURATION FLASH FLOOD/FLOODING RISK.

DISCUSSION...GOES-E WV AND RADAR MOSAIC DEPICT MULTIPLE SHORTWAVES
ACROSS THE MID-ATLANTIC THAT ARE FAVORABLY ORIENTED TO ALLOW FOR
NARROW CORRIDOR OF TRAINING SHOWERS WITH POTENTIAL FOR EMBEDDED
SHALLOW CONVECTIVE ELEMENTS. A SHORTWAVE LIFTING OUT OF EASTERN PA
TOWARD N NJ ANGLES A TRAILING BOUNDARY/DEFORMATION ZONE ACROSS N
DE/CENTRAL MD TOWARD THE APEX OF THE DEEPER SHORTWAVE CURRENTLY
OVER E CENTRAL WV.  THIS ORIENTS AN IDEAL CORRIDOR FOR PERSISTENT
MOISTURE CONVERGENCE AND CONFLUENCE/MERGERS FOR CELLS TO GRAVITATE
TO BEFORE TRAINING ALONG THE CORRIDOR. STRONG DPVA WILL MAINTAIN
SUFFICIENT LARGE SCALE ASCENT ACROSS THE MID-ATLANTIC.

LIMITING FACTOR FOR CONVECTIVE GROWTH IS IN AVAILABLE INSTABILITY.
RAP ANALYSIS SUGGESTS A NOSE OF CAPE OVER SE VA POINTED TOWARD MD
FROM THE CHESAPEAKE BAY...WITH 12Z RAOBS FROM IAD/RNK/WAL SUGGEST
VERY SKINNY SATURATED PROFILES WITH CAPES OF 250-750 J/KG
AVAILABLE.  AS SUCH RECENT DIX/DOX/AKQ/LWX RADAR TREND SHOWS
STRONG CONVERGENCE NEAR/ALONG THE DEFORMATION ZONE FROM JUST SOUTH
OF DC METRO ACROSS ANNAPOLIS TOWARD N DE WHERE SLANTWISE/SHALLOW
CONVECTIVE ELEMENTS HAVE FORMED.  GIVEN SATURATED PROFILES TO
1.75" TPWS PER RAOBS...RAINFALL RATES UP TO 1-1.5"/HR ARE POSSIBLE
THOUGH HIGHLY TRANSIENT...NECESSITATING THE UPSTREAM
REDEVELOPMENT/TRAINING FOR COMPOUNDED TOTAL. 

ANY STRONGER/VERTICALLY ORIENTED CONVECTION THAT DEVELOPS FURTHER
SOUTH WITHIN THE WEDGE OF INSTABILITY OVER CENTRAL VA/SE
VA...SHOULD TRACK TOWARD THE DEFORMATION ZONE AND INCREASE THE
FLASH FLOODING RISK GIVEN RATES OVER 2" (ESPECIALLY ALONG THE
EASTERN SHORE OF THE CHESAPEAKE GIVEN INCREASED SFC CONVERGENCE ON
ACCELERATED LL FLOW...QUEEN ANNE/TALBOT COUNTIES) AS SUGGESTED BY
MODELS WITH THE GREATEST  INSTABILITY SUCH AS THE 09Z HRRRX/06Z
NAM-CONEST.  TRENDS WOULD SUGGEST THESE MODELS ARE TOO AGGRESSIVE
BUT THE RECENT HRRR (WHILE IDEALLY ORIENTED TO CURRENT
OBSERVATIONAL TRENDS) APPEARS TOO LIGHT.

THIS WOULD SUGGEST 1-2" ACROSS N VA/DC... WITH 2-4" ACROSS SE
CENTRAL MD INTO THE CENTRAL EASTERN SHORE OF MD/DE THROUGH
19/20Z...WITH MORE TO COME UPSTREAM.  AS SUCH FLASH FLOODING IS
POSSIBLE BUT WITH TIME LONGER DURATION FLOODING IS BECOMING MORE
LIKELY ESPECIALLY FURTHER EAST (GIVEN LONGER DURATION).     

GALLINA

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

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

LAT...LON   40287511 39607399 38317482 37957580 37477825
            38007907 38657927 39107835 39507703 39877622