AWUS01 KWNH 150859
FFGMPD
MAZ000-RIZ000-CTZ000-NYZ000-NJZ000-151400-

Mesoscale Precipitation Discussion 0929
NWS Weather Prediction Center College Park MD
459 AM EDT Tue Aug 15 2023

Areas affected...Southern New England...Southeast NY & Long Island...Northeastern NJ...

Concerning...Heavy rainfall...Flash flooding possible

Valid 150900Z - 151400Z

SUMMARY...Upstream redevelopment in very high moisture flux
environment. Efficient rainfall production with 2-2.5"/hr rates,
should continue potential for flash flooding near lifting warm
front.

DISCUSSION...GOES-E WV suite depicts shortwave at the leading edge
of the larger scale dry slot jet continues to rapidly advance
eastward into the Hudson valley and central New England. CIRA LPW
continues to denote the confluence of two deeper moisture streams
through southern New England, with a narrowing stream originally
from the Gulf and Eastern Pacific coming through the central
Appalachians into the Lower Hudson Valley while strong southerly
LLJ surges western Atlantic low level moisture northward. 08z
surface analysis shows the associated wave near Trenton, moving
quickly; while the warm front continues to lift northward. Strong
sfc to 850mb speed and directional convergence (upstream near
surface wave) along instability gradient supports updrafts and
given proximity to deep layer moisture up to 2.25 through SE NY,
across Long Island; moisture flux convergence is supporting highly
efficient rainfall production with 2-2.5"/hr rates likely. Deep
layer steering is angled just north of parallel to the frontal
boundary so some training remains possible; however, it is the
directional veering upstream that should support some
back-building cells through NYC into central Long Island over the
next few hours, maintaining flash flooding risk. North of the
strong boundary layer convergence, instability rapidly diminishes
and moderate to heavy stratiform should dominate. Duration across
urban corridors may cause flooding concerns, but rises should be
more gentle in nature and occur in typical poor drainage areas.

Of note: GOES 3.9um SWIR shows a north-south convergence band with
even higher moisture/theta-E air along/just south of the tip of
Long Island, this seems to mark sfc-850mb PW over 1.2" and total
values of 2.5". Stronger updrafts, possible supercells appear to
have developed on the line. While trends are for continued ENE
track, as the base of the shortwave advances, steering may back
just enough that a cell could near Block Island or even SE MA and
the Islands after day-break. While this is less likely and
remains uncertain; extreme rates of 3"/hr are probable given
localized isallobaric influences to low level moisture convergence
for the stronger rotating updrafts. So even though these islands
are less prone to flash flooding due to sandy soil types, if they
were to track through rapid inundation flooding becomes possible.

Gallina

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

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

LAT...LON 42277010 41266991 41217038 41197100 40897215
40677286 40457380 40527444 40917454 41307400
41727302 41967216 42147126

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