584
AWUS01 KWNH 082120
FFGMPD
MAZ000-RIZ000-CTZ000-NYZ000-NJZ000-PAZ000-DEZ000-MDZ000-090300-

Mesoscale Precipitation Discussion 0623
NWS Weather Prediction Center College Park MD
519 PM EDT Tue Jul 08 2025

Areas affected...Eastern PA and DE northeast into Southern New
England

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

Valid 082118Z - 090300Z

Summary...Convection blossoming along a southward advancing cold
front will exhibit short term training of 2-3"/hr rain rates. This
could produce 1-3" of rainfall with locally higher amounts. Flash
flooding is possible, especially within urban areas.

Discussion...The regional radar mosaic this afternoon shows
clusters of thunderstorms advancing eastward along and ahead of a
cold front. This cold front is dropping slowly southward in
response to height falls downstream of an approaching trough axis
centered near the Great Lakes, with weak vorticity impulses
embedded within the flow. As the front becomes strung out more
west-east this evening, it will likely slow, continuing to be the
impetus for thunderstorm development as it moves into a more
robust thermodynamic airmass.

PWs across the area are 2-2.2 inches according to the SPC RAP
analysis, which is well above the 90th percentile and nearing
daily records. The accompanying instability is equally impressive,
with a narrow ribbon of 3000 J/kg SBCAPE emerging from the
DelMarVa northward into southern CT and RI. This CAPE is
characterized of the "tall skinny" variety as well as reflected by
normalize dCAPE values of just 0.1 to 0.15, indicating moist
adiabatic lapse rates through a deep column suggestive of an
efficient rainfall environment. Thunderstorms that have already
developed across the area have radar-estimated rain rates of more
than 2"/hr according to KDIX, with FFWs already in effect across
eastern PA.

During the next few hours, the CAMs are in relatively good
agreement that showers and thunderstorms will expand and spread
eastward into the Tri-State area of NJ/NY/CT immediately
downstream of the cold front. As the front aligns generally west
to east, mean 0-6km winds of 15-20 kts parallel to the front
combined with bulk shear increasing to 20-25 kts suggest storms
will organize into clusters and repeatedly track/train along this
front and the attendant instability gradient. With rainfall rates
potentially ereaching 2-3"/hr at times (20-40% chance), leading to
1-hr rainfall that could be up to 1.5-2.0 inches according to HREF
and REFS PMM, any short term training or repeating could produce
1-3" of rain with locally higher amounts possible.

FFG across the area is generally 2-3"/3hrs, for which the HREF
indicates has a low probability (10-15%) chance of exceedance.
However, urban areas will likely need much lower rainfall to cause
rapid runoff where 1-hr FFG is only around 1 inch. Additionally,
the low-level SW flow could force some locally enhanced
convergence/training, especially from northeast NJ, across Long
Island, and into southern CT where FFG exceedances peak. This
indicates that the greatest risk for flash flooding will be in
this vicinity this evening, but anywhere short-term training of
these intense rain rates can occur, impacts are possible.


Weiss

...Please see www.wpc.ncep.noaa.gov for graphic product...

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

ATTN...RFC...RHA...TAR...NWC...

LAT...LON   42257332 42257234 42217137 42037070 41797041
            41497061 40957191 40857222 40607284 40257346
            39857383 39537416 39027463 38527517 38447556
            38877591 40027610 40627639 41257647 41677621
            42087475