Mesoscale Precipitation Discussion 0603
NWS Weather Prediction Center College Park MD
239 PM EDT Wed Jul 10 2024
Areas affected...Upstate New York, Vermont
Concerning...Heavy rainfall...Flash flooding possible
Valid 101837Z - 110030Z
Summary...Showers and thunderstorms will expand and train to the
northeast through this evening. Rainfall rates of 2"/hr or more
will be common, which through training could produce 2-4" of rain
with locally higher amounts. Flash flooding is possible.
Discussion...The regional radar mosaic this afternoon indicates
widespread showers and thunderstorms expanding downstream of post
tropical cyclone Beryl which is located over southern Michigan. A
warm front extending east from this system draped across Upstate
NY and into northern New England is providing a focus for ascent
within extreme thermodynamics. PWs as measured by GPS are 2-2.25
inches across Upstate NY, daily records for both BUF and ALY, with
accompanying SBCAPE peaking above 3000 J/kg south of the warm
front into PA. Within the warm sector, 850mb inflow is reaching
25-35 kts, drawing these impressive thermodynamics northward, and
isentropically ascending the warm front to enhance ascent. At the
same time, modest RRQ upper diffluence is providing additional
synoptic forcing, with mesoscale convergence along the nose of the
surging 850mb winds also resulting in enhanced lift. A pre-frontal
surface trough is pivoting across PA and driving additional
ascent, along which supercells are lifting northward within the
warm sector, and rainfall rates within these supercells and in
convection just north of the warm front have been estimated via
local radars above 1.5"/hr already today.
During the next few hours, there is good agreement that the warm
front will pivot northward to approach the Canadian border while
the surface trough lifts east. Together these will compress the
warm sector leading to enhanced thermodynamic transport northward
to fuel even more intense and widespread convection. Storms
developing south of the warm front will likely transit northward
rapidly on NE 850-300mb winds of 30-40 kts, with additional
development occurring along and just north of the boundary. The
overlap of intense thermodynamics and ascent will result in
rainfall rates for which the HREF and REFS both indicate will
exceed 2"/hr, and the HRRR suggests short-term 4"/hr rates are
possible as reflected by 15-min accumulations reaching 1". Storm
organization through bulk shear of 40-60 kts is also expected,
which will prolong and enhance rain rates, and the environment
supports training of echoes to the E/NE, especially where the
greatest confluence of moisture transport vectors occurs. This
could result in 2-4" of rain with locally higher amounts as
reflected by HREF probabilities for 3"/6hrs reaching 10-20%.
These intense rain rates could produce instances of flash flooding
across any urban areas of sensitive terrain features through this
evening. However, the greatest risk for flash flooding will be
where any training occurs across locally compromised FFG which is
as low as 1.5-2"/3hrs, where the HREF indicates a 50-70% chance of
FFG exceedance. While instances of flash flooding are possible the
next several hours, the concern may increase later this evening as
primed soils from this afternoon's rainfall get impacted by
multiple rounds of heavy rain, and additional MPD issuances may be
needed.
Weiss
ATTN...WFO...ALY...BGM...BTV...BUF...
ATTN...RFC...NERFC...NWC...
LAT...LON 45337295 45287213 45057194 44677228 44387246
43897315 43437386 43197514 42917628 42767703
42707771 42827802 43097806 43597738 43947687
44127659 45057552 45267457
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