Mesoscale Precipitation Discussion 0649
NWS Weather Prediction Center College Park MD
1150 AM EDT Mon Aug 13 2018

Areas affected...Northeast PA...Southern & Southeast NY...Northern
NJ...Southern CT...

Concerning...Heavy rainfall...Flash flooding likely

Valid 131549Z - 132145Z

SUMMARY...Persistent Strong and Moist Warm-air advection into
Mountainous Terrain Continues Severe Flash Flooding Event.  Deeper
Convection Rooted Nearer the Surface Also Supports Up to 2"/hr and
Likely Flash Flooding.

DISCUSSION...Goes-WV suite depicts center of strong upper low well
west over W PA with broad upper level divergent/diffluent pattern
over the Delaware River Valley.  This also supports a deepening
surface low in the Hudson Canyon Region offshore of NJ and S of
NYC metro.  The west to east vertical tilt of the cyclone supports
strong synoptic ascent and strong TROWAL from NYC across N NJ into
N Central PA/S Central NY, instability within the TROWAL supports
strong convective elements at best convergence points, near the
Surface Low nearing Sand Hook, NJ as well as near the 850mb low
inflection just NE of PA.  The strong convergence, access to
deeper instability near the warm sector/TROWAL supports rain rates
in excess of 2"/hr with slow motions, so totals of 2-4" are
likely.
This threat will expand into N NJ/Long Island, S New York and
possibly S CT over the next few hours.  The lack of instability
north of the warm front in CT may limit coverage and intensity,
though still possibly lead to flash flooding given urban
environment and rate of rainfall (cells will propagate west in
CT/LI ahead of the front, with some speed).

Given the orientation of the flow, the warm conveyor is well east
with a western branch supporting the TROWAL and deep moisture
flux.  Modest "cool" conveyor belt across Southern New England
into the Hudson Valley, also supports deepening the moisture and
supporting strong mst convergence overlapping with the 850-7H
TROWAL...currently across Steuben/Chemung, NY to a new developing
850mb low/triple point near Lehigh county, PA).  This low will
continue to track along the convergence band will very slow NNE
shift of the band with .5-1"/hr , compounding already ongoing
flooding in complex terrain.  Given the breadth of the swath of
heavy rain, nearly all rivers/drainage basins are filling, and
backing up at hydrological convergence points, further compounding
flash flooding with inundation/longer term rise flooding making it
quite severe across a large area.

Gallina

ATTN...WFO...ALY...BGM...CTP...OKX...PHI...

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

LAT...LON   42597683 42397542 41627398 41397251 40947199
            40517341 40207378 39697384 39467417 39877506
            40557625 41307730 42037771