MESOSCALE PRECIPITATION DISCUSSION 0921
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
759 PM EDT SUN OCT 29 2017

AREAS AFFECTED...ERN PA...NJ...NY...CT...RI...MA...SRN VT...SRN NH

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

VALID 292359Z - 300559Z

SUMMARY...STEADY MODERATE TO LOCALLY HEAVY WILL INCREASE ACROSS
THE NRN MID-ATLANTIC INTO THE NORTHEAST. FLOODING AND FLASH
FLOODING WILL BE A GROWING CONCERN IN THE 00-06Z TIME FRAME WITH
ADDITIONAL RAINFALL TOTALS OF 2-4 INCHES.

DISCUSSION...23Z GOES WATER VAPOR IMAGERY SHOWED A WELL DEFINED
NEGATIVELY TILTED SHORTWAVE TROUGH OVER VA/NC WITH A DEVELOPING
COMMA-HEAD FEATURE OVER THE NRN MID-ATLANTIC INTO UPSTATE NY. GOES
HD WINDS AGREED REASONABLY WELL WITH RAP ANALYSES OF THE JET
STREAM AT 200 MB...WHICH SHOWED A 130-150 KT JET CORE EXITING INTO
SRN QUEBEC. FARTHER SOUTH...WHILE NO GOES HD DATA WAS AVAILABLE TO
CONFIRM...RAP ANALYSES SHOWED A 90-110 KT UPPER LEVEL JET OVER
COASTAL NC AT THE BASE OF NEGATIVELY TILTED TROUGH LEAVING A
CLASSIC COUPLED JET SIGNATURE CONTAINING STRONG UPPER LEVEL
DIVERGENCE OVER NJ. AT THE SURFACE...A DOMINANT LOW WAS BEGINNING
TO TAKE OVER 60 MILES TO THE EAST OF THE SRN DELMARVA PENINSULA
WITH AN ESTIMATED PRESSURE OF 989 AT 23Z. THIS LOW IS LIKELY TO
TRACK NNW OVER THE NEXT 6 HOURS ACROSS CNTRL NJ...FOLLOWING
MAXIMUM 3-HOUR PRESSURE FALLS INDICATED BY A BUOY 20 MILES EAST OF
DE SHOWING AN 8 MB DROP IN THE PAST 3 HOURS.

AS THE SURFACE LOW TRACKS NORTH...STRONG LOW LEVEL FRONTOGENESIS
ORIENTED FROM SOUTH TO NORTH SHOULD SET UP BENEATH A STRONG
DEFORMATION AXIS OVER ERN PA INTO CNTRL NY. 850 MB WINDS NEAR 95
KT SOUTH OF ERN LONG ISLAND ARE FORECAST BY RECENT RAP CYCLES TO
ALIGN WITH PRECIPITABLE WATER VALUES BETWEEN 1.8 AND 2.0
INCHES...CAUSING MOISTURE FLUX ANOMALIES AT THIS LEVEL TO BE OFF
THE CHARTS. WHILE INLAND 850 MB WIND SPEEDS WILL BE LESS...50-60
KT VALUES ARE STILL EXPECTED TO ENHANCE UPSLOPE FLOW INTO THE
POCONO AND CATSKILL MOUNTAINS WITH HIGHER VALUES OVER FAR SRN NEW
ENGLAND. POWERFUL MOISTURE FLUX CONVERGENCE AT 850 MB IS EXPECTED
TO WRAP AROUND FROM NORTHEAST TO NORTHWEST OF THE LOW CENTER
IMPACTING THE OROGRAPHICALLY FAVORED REGIONS OF THE
POCONOS/CATSKILLS BETWEEN 02-05Z WITH 1-3 INCHES OF RAIN JUST IN
THE 3-HOUR PERIOD. SOME OF THESE RAINS WILL FALL ON TOP OF 2-4
INCH OBSERVED RAINFALL IN THE CATSKILLS OVER THE PAST 24 HOURS.

FARTHER EAST...LONG ISLAND IS EXPECTED TO SEE A QUICK HIT OF
RAINFALL RATES OF 0.5 TO 1.0+ IN/HR TO THE EAST OF A DEVELOPING
DRY SLOT...WITH THE STRONG MOISTURE FLUX AXIS IMPACTING THE
BERKSHIRES/TACONIC MOUNTAINS FOR A LONGER DURATION WHERE HI-RES
GUIDANCE IS SHOWING 2-4 INCHES OF RAIN ENDING 06Z. SIX HOUR TOTALS
OF 1-2 INCHES WILL BE POSSIBLE OVER LOWER ELEVATIONS OF SRN
VT/MA/CT/RI WHERE RAINFALL OVER THE PAST WEEK HAS LOWERED FLASH
FLOOD GUIDANCE VALUES TO 2 INCHES OR LESS IN 3 HOURS.

OTTO

ATTN...WFO...ALY...BGM...BOX...BTV...BUF...CTP...GYX...OKX...
PHI...

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

LAT...LON   44357376 43977270 43137168 41997147 41377177
            40787240 40337323 39967384 39367419 39217462
            39347497 39797546 40417596 41257621 42587622
            43237599 43897533 44247454