MESOSCALE PRECIPITATION DISCUSSION 0532
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
741 PM EDT TUE SEP 29 2015
 
AREAS AFFECTED...S PENNSYLVANIA...MARYLAND...DELAWARE...E
VIRGINIA...SW NEW JERSEY...DISTRICT OF COLUMBIA 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 292340Z - 300440Z
 
SUMMARY...INCREASED DEEP CONVECTION AT APEX OF DEEPLY CONFLUENT
FLOW...TO POTENTIALLY LEAD TO POSSIBLE FLASH FLOODING.

DISCUSSION...WV LOOP SHOWS THE ADVANCEMENT OF THE S/W ACROSS E WV
INTO W MD WITH A SE ORIENTED TROF ACROSS W VA.  STRONG PVA AHEAD
OF THIS AXIS WITH S/W RIDGING ALOFT ALLOWING FOR DOWNSTREAM
DIVERGENCE CONTINUES TO ENHANCE LARGE SCALE ASCENT AND EXPANSION
OF BAROCLINIC SHIELD PRECIPITATION NORTH ACROSS MUCH OF CENTRAL
PA. IN THE LOW LEVELS...NEARLY 90 DEGREES OF CONFLUENT FLOW IS
BEING FOCUSED ACROSS A NARROW CORRIDOR/CONVERGING OVER N VA ...
FROM EASTERLY FLOW OFF THE DELMARVA TO SOUTHERLY FLOW OUT OF
EASTERN NC.  THIS HAS ALLOWED FOR AN UPTICK SEEN IN IR WITH
COOLING IR TOPS SOME REACHING AS COLD AS -70C AT TIMES. GIVEN TPWS
OF 2.0"+ THROUGH SUCH A HIGH FLUX (30-35KT FLOW) AHEAD OF THE TROF
AXIS...RAIN RATES TO 2.0" PER HOUR CAN BE EXPECTED IN THESE DEEPER
CORES...SURROUNDED BY .75-1.0"/HR IN THE SHALLOWER ASCENT.  THIS
HAS BEEN ALLOWING A BROADER AREA OF 2-3" TOTALS SEEN ACROSS N
VA...WHICH SHOULD CONTINUE TO TRACK SLOWLY EASTWARD THOUGH
ISOLATED POCKETS OF HIGHER TOTALS IN THE 4-5" RANGE COULD NOT BE
RULED OUT GIVEN CELL EFFICIENCY. WITH STRONG 7H THETA-E ADVECTION
AND MOISTURE CONVERGENCE COLLOCATION...HVY STRATIFORM WITH SHOULD
SHIFT NORTH ACROSS N MD INTO SE PA AND POSSIBLY SW NJ BY 06Z.

ACROSS SOUTH CENTRAL VA...CELLS ARE BIT MORE SCATTERED AS THE
CONVERGENCE ALONG THE BOUNDARY IS DRIVEN MAINLY BY THE SLOW
EASTWARD ADVANCEMENT OF THE TROF.  THESE CELLS HAVE SOME INCREASED
CAPES TO 1000 J/KG HELPING FOR INCREASED RATES...THOUGH SCATTE   
IN BOTH ZONES...FFG VALUES ARE GENERALLY HIGH AND COULD SUPPORT
THESE TOTALS...BUT URBAN AND HARD SOIL SURFACE DUE TO LONGER TERM
DRIER CONDITIONS ACROSS THE AREA COULD LEAD TO RAPID RUN-OFFS FOR
SOME ISOLATED FLOODING CONCERNS.  THIS IS GENERALLY SUPPORTED BY
MUCH OF THE HI-RES CAMS... PARTICULARLY THE ESRL EXPERIMENTAL HRRR
AND CANADIAN GEM-REGIONAL AS WELL AS THE 12Z ARW AND NMMB...THOUGH
APPEAR TO BE A BIT SLOWER WITH EASTWARD PROGRESS.  

GALLINA

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

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

LAT...LON   40707653 40577520 39887509 39037525 37427604 
            37067661 37257762 38627778 39117792 39987827 
            40427776