Weather Prediction Center issues Mesoscale Precipitation Discussion #834 concerning HEAVY RAINFALL...FLASH FLOODING LIKELY713
AWUS01 KWNH 311755
FFGMPD
RIZ000-CTZ000-NYZ000-NJZ000-312345-
Mesoscale Precipitation Discussion 0834
NWS Weather Prediction Center College Park MD
155 PM EDT Thu Jul 31 2025
Areas affected...Northern NJ...Southern NY & Long Island...CT...
Concerning...Heavy rainfall...Flash flooding likely
Valid 311755Z - 312345Z
SUMMARY...Slow moving thunderstorms with rates up to 2"/hr and
multiple rounds pose local 2-5" totals and possible flash
flooding.
DISCUSSION...17z Surface analysis shows the stationary front has
been further reinforced by southerly sea-breeze off the NY Bight
as well as outflow boundary and easterly flow north of the front.
Temperatures into the low 90s and Tds in the low to mid-70s across
N NJ/Long Island with solid deep layer moisture profiles supports
a very unstable environment in the warm sector with 1500-2000 J/kg
of MLCAPE analyzed; combined with 1.9-2" total PWats that remain
near .8-1" in the Sfc-850mb layer noted in CIRA LPW. The solid
directional convergence at the surface has resulted in a few
clusters of cells developing from N NJ across LI. The combination
will allow for efficient rainfall production with 2-2.5"/hr.
Deep layer flow suggests slow east-northeast cell motions, but
convergence from outflow and frictional effects near coasts
suggest somewhat chaotic motions that may allow for some cells to
redevelop in place allowing for increased duration. 17z WoFS
members denote this sort of interaction with slow motions across E
Long Island resulting in some increased overall totals up to 2-5"
between the 50th and 90th 6hr percentiles. This is in agreement
with recent HRRR and 12z Hi-Res CAM solutions to provide moderate
confidence toward this cell motion environment. Proximity to
urban locations and limited infiltration is likely to result in
localized flash flooding/rapid inundation conditions, across LI, S
NY and southern CT, though precise locations of enhanced totals
will remain elusive to lock down, resulting in a scattered nature
to the flooding pattern across the area of concern through the
evening hours.
Mid to upper-level shortwave feature upstream will continue to
press eastward across Upstate NY; this should keep southerly flow
backed in the low levels and increased DPVA/divergence aloft may
allow for additional thunderstorms to develop north and west
toward 23z and may seek out remaining unstable air parcels across
the area of concern and may track through areas already affected
with the initial rounds.
Gallina
...Please see www.wpc.ncep.noaa.gov for graphic product...
ATTN...WFO...ALY...BOX...OKX...PHI...
ATTN...RFC...RHA...TAR...NWC...
LAT...LON 41967296 41937223 41747190 41277185 41007190
40747261 40607306 40557340 40537441 40927478
41337457 41617423 41857369
Source:
Weather Prediction Center issues Mesoscale Precipitation Discussion #834 concerning HEAVY RAINFALL...FLASH FLOODING LIKELY----------------
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