Mesoscale Precipitation Discussion 0828
NWS Weather Prediction Center College Park MD
322 PM EDT Wed Sep 12 2018

Areas affected...SE NY...Long Island...Southern New England

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

Valid 121921Z - 130000Z

Summary...Showers and thunderstorms with rainfall rates up to
2"/hr will continue along a stationary front this afternoon. Flash
flooding is possible.

Discussion...GOES-16 visible imagery shows a clear demarcation of
clear sky conditions along and south of a surface boundary draped
along the coast from near Philadelphia, PA northeast towards
Chatham, MA. Within this clear area, MLCape has risen to 500-1000
J/kg which has allowed thunderstorms to develop and persist across
the threat area. PWATs across the region are over 2 inches, and
latest 18Z upper air soundings from KOKX have freezing levels
nearing 15 kft, above the 90th percentile for the date. This deep
warm cloud depth and high PWAT is allowing efficient rain
processes, with rainfall rates over 2"/hr estimated recently by
KOKX radar.

Although instability weakens north of the front, warm advection
atop the boundary is driving lift, aided by weak PVA and
diffluence within a 300mb jet entrance region. 18Z soundings from
CHH and OKX have 20kts of 0-6km shear, so multicell development is
possible, which could train thanks to boundary parallel flow and
corfidi vectors aligned with the 0-6km mean wind.

The high res guidance is varied in continuing or intensifying
convection through the remainder of the afternoon, but the
sufficient lift in a very moist environment supports at least a
continuation of a heavy rain threat through this evening. Many
places within the threat area have received 2-4" of rainfall in
the past 24 hours, and these antecedent conditions lead to an
enhanced flash flood threat. An additional 1 to as much as 3
inches of rainfall is possible, and flash flooding will be
possible where rain rates become 2"/hr.

Weiss

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

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

LAT...LON   42167056 42107018 41796985 41606989 41467027
            41417066 41327102 41197130 41147157 41007189
            40857228 40707277 40557326 40627366 40777386
            41077400 41407434 41797436 42037345 42047261
            42047216 42017167 42017105