MESOSCALE PRECIPITATION DISCUSSION 0920
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
157 PM EDT SUN OCT 29 2017

AREAS AFFECTED...NJ...E PA...S NY...LONG ISL...W CT...W MA

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

VALID 291754Z - 292350Z

SUMMARY...THE THREAT OF FLASH FLOODING WILL GRADUALLY INCREASE
OVER THE COURSE OF THE AFTERNOON WITH A MORE SIGNIFICANT RAMP UP
IN RAIN RATES EXPECTED WITH A SURGE OF TROPICAL MOISTURE AROUND
6-8 PM EDT (22-00Z). THE FLASH FLOOD THREAT WILL BE HIGHEST IN
URBAN AREAS...PARTICULARLY NEW YORK CITY METRO AREA...AND IN AREAS
OF TERRAIN IN SRN NEW YORK...WRN CONNECTICUT...AND WRN
MASSACHUSETTS.

DISCUSSION...RAIN WAS BEGINNING TO INCREASE ACROSS THE REGION AS
STRONG LOW-MID LEVEL MOISTURE TRANSPORT CONTINUES DOWNSTREAM OF AN
AMPLIFIED TROUGH. THUS FAR RAIN BANDS HAVE BEEN SOMEWHAT
DISORGANIZED...WITH GREATER ORGANIZATION EXPECTED TO ARRIVE WITH
STRONGER FORCING THIS EVENING AS THE NEGATIVE TILT SHORTWAVE AND
RAPIDLY DEEPENING SURFACE LOW APPROACH. HOWEVER...BETWEEN 17Z AND
18Z...SHALLOW RAIN BANDS WERE BECOMING MORE FOCUSED NEAR AND JUST
OFFSHORE OF THE NEW JERSEY COASTLINE. THIS WAS LIKELY OCCURRING TO
THE EAST OF A WEAK LOW-LEVEL FRONTAL WAVE...WITH BOUNDARY LAYER
CONVERGENCE AND LOW-LEVEL MOISTURE TRANSPORT BRIEFLY MAXIMIZED TO
THE EAST...IN THE VICINITY OF NJ. THESE FOCUSING MECHANISMS FOR
SHALLOW CONVECTIVE RAIN BANDS SHOULD GRADUALLY WANE IN INTENSITY
AS THE DOMINANT FORCING FEATURE BECOMES THE EVOLVING CYCLONE TO
THE SOUTH. HOWEVER OVER THE NEXT FEW HOURS (18-22Z)...FLASH
FLOODING MAY BEGIN TO DEVELOP IN AREAS WHERE CONVECTIVE RAIN BANDS
ARE A LITTLE MORE PERSISTENT. RAIN RATES UP TO AROUND 1.0 TO 1.25
IN/HR SEEM POSSIBLE PRIOR TO THE MAIN ROUND OF RAINFALL.

THE NATIONAL WATER MODEL SOIL MOISTURE ANALYSIS AND FLASH FLOOD
GUIDANCE INDICATE THAT SOIL CONDITIONS WILL LIKELY BE MOST
FAVORABLE FOR FLOODING FROM NRN NJ AND SRN NY TO THE
NORTHEAST...AT LEAST INITIALLY. SOME OF THE LOWEST FFG VALUES ARE
IN THE URBAN CORRIDOR OF N/NE NEW JERSEY AND THE NEW YORK CITY
METRO AREA. REGIONAL RADARS ALSO SHOW THAT THE LARGER AREA OF RAIN
IS BEGINNING TO REACH SOME OF THE INLAND TERRAIN FEATURES...AND
RATES COULD BEGIN TO INCREASE WHERE INCREASINGLY MOIST LOW-MID
LEVEL FLOW IS ANGLED FAVORABLY TO INTERCEPT THE TERRAIN GRADIENT.
THIS SEEMS MOST LIKELY IN THE POCONOS AND CATSKILLS. LIGHT TO
MODERATE RAIN OVER THE NEXT SEVERAL HOURS MAY ALSO BEGIN TO PRIME
OTHER LOCATIONS IN THE REGION FOR FLASH FLOODING WHEN THE MAIN
ROUND OF RAIN ARRIVES IN THE EVENING.

REGARDING THE ARRIVAL OF DEEPER TROPICAL MOISTURE...EXTRAPOLATION
OF OFFSHORE TRENDS FROM THE PAST SEVERAL AMSU 89GHZ PASSES
(02-14Z) SUGGEST THE SURGE OF MOISTURE SHOULD ARRIVE IN THE
COASTAL PORTIONS OF THE REGION AROUND 22-00Z...ACCOUNTING FOR SOME
SLIGHT EXPECTED ACCELERATION. THIS CLOSELY MATCHES AVAILABLE
HI-RES MODEL GUIDANCE...WHICH ALL INDICATE AN INCREASE IN RAIN
RATES AROUND THAT TIME IN THE MPD AREA WITH MORE ORGANIZED BANDING
OF HEAVY RAIN. THIS IS WHEN THE FLASH FLOOD THREAT SHOULD NOTABLY
INCREASE ACROSS THE ENTIRE REGION.

GIVEN THE SHALLOW NATURE OF ANY CONVECTIVE RAIN BANDS...AND THE
INCREASING LOW-MID LEVEL MOISTURE...WARM RAIN PROCESSES SHOULD BE
DOMINANT AND RADARS MAY UNDERESTIMATE RAIN RATES ACROSS THE REGION.

LAMERS

ATTN...WFO...ALY...BGM...BOX...OKX...PHI...

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

LAT...LON   42767271 42137264 41297277 40877247 40607266
            40447326 40457368 39987387 39567401 39077452
            38797490 38987516 39507512 39697501 40097508
            40587542 40877584 41547573 42207477 42567379