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WPC Met Watch

Mesoscale Precipitation Discussion: #0972
(Issued at 128 PM EDT Wed Sep 15 2021 )

MPD Selection

Graphic for MPD #0972

Mesoscale Precipitation Discussion 0972
NWS Weather Prediction Center College Park MD
128 PM EDT Wed Sep 15 2021

Areas affected...Central Appalachians through Western New England

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

Valid 151730Z - 152330Z

Summary...Showers and thunderstorms ahead of a cold front will
expand in coverage and train to the northeast through the evening.
Rainfall rates may exceed 1"/hr at times, producing 1-3" of
rainfall. Isolated flash flooding is possible.

Discussion...The regional radar mosaic this afternoon depicts
increasing coverage of showers and thunderstorms from southeast
Ohio into northern New England. This convection is blossoming
ahead of a surface cold front analyzed draped from Quebec, Canada
to near Cincinnati, Ohio, which is moving into a robust
thermodynamic environment. PWs measured by GPS are 1-1.25", above
the 75th percentile for the date, within a plume of SBCape
exceeding 1500 J/kg across the Northeast. Forcing for ascent is
being provided by low-level convergence along the front,
increasing upper diffluence within the RRQ of a potent jet streak,
and mid-level height falls as the mean trough axis digs

This overlap of ascent and thermodynamics is already producing
widespread showers and thunderstorms with rain rates estimated by
local radars as high as 1.5"/hr. As the front continues to dig
southward and instability climbs, even more widespread and intense
convective development is likely, with storm organization
occurring through 0-6km bulk shear of 35-50 kts. This organization
could lead to more intense rain rates, and the HREF probabilities
peak above 25% for 2"/hr early this evening.

Storm motions are likely to remain quick as progged by 0-6km mean
winds around 30 kts, and dry air noted on CIRA LPW fields and
within the 12Z U/A soundings between 700-400mb indicates a
somewhat isolated flash flood risk. However, propagation vectors
aligned with the mean wind and the front suggest a high potential
for training, and as rain rates reach towards 2"/hr this could
produce rainfall accumulations of around 3" in a short period of
time. This is reflected by many of the available CAMs, and low-end
HREF probabilities for 3"/3hrs.

40cm soil moisture across this region remains above the 90th
percentile due to 14-day rainfall that is more than 200% of normal
in some places. This has led to compromised FFG as low as
0.75-1"/1hr and 1.5-2"/3hrs and the HREF exceedance probabilities
for these thresholds reach above 30%. This suggests flash flooding
is possible despite fast storm motions, but should be confined to
any urban areas or where the most significant training can occur.




LAT...LON   44327289 44097234 43747198 43117205 42507247
            42117335 41697420 40937585 40337733 39747869
            39557946 39318039 39268072 39408103 39998106
            40798051 41547957 42227829 42837703 43477578

Last Updated: 128 PM EDT Wed Sep 15 2021

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Page last modified: Wednesday, 13-May-2015 19:29:02 GMT