Thunderstorm Threat

The National Weather Service’s Storm Prediction Center is hitting the thunderstorm threat quite hard for eastern New York State and parts of Southern New England today. While the threat is not as high locally, a few storms can’t be ruled out in our neck of the woods this evening.

Here is the latest write-up from the SPC:

” …Northeast…
Thunderstorms are expected to develop and grow upscale from midday
through afternoon, sweeping eastward across the region with the main
risk being severe wind. The potential for a well-organized swath of
damaging wind — including a few gusts around hurricane force —
appears greatest over the enhanced-risk area. Large hail also is
possible, especially in the first few hours of the convective cycle
when storm modes can be more favorably discrete or semi-discrete. A
few tornadoes also may occur, whether from supercells or
QLCS-embedded vortices.

A nonsevere area of convection now moving across western NY and
northern PA should reinforce the baroclinic zone across parts of
south-central and eastern NY, which should act as a fairly sharp
northern delimiter for substantial severe potential, given strongly
stable air to its north. The UVV field related to the MCV appears
well-timed to encounter the destabilizing warm sector across
northern PA today, along and south of the frontal zone, in support
of initial development, which then should expand/intensify to severe
levels as it moves rapidly eastward toward parts of southern NY, NJ
and New England. Whether the resulting convective wind event
qualifies as a derecho may be a semantic exercise; impacts could be
of that caliber in the area affected.

One source of uncertainty in this scenario is an area of outflow to
the south, across parts of VA/MD/Delmarva and eastern PA,
originating from yesterday’s MCS, and sampled peripherally by the
12Z IAD sounding. Airmass recovery is expected from the southwest,
around the northwest rim of that outflow pool and south of the
morning convective/frontal baroclinic zone. Expect midday to
afternoon preconvective destabilization arising from both theta-e
advection and diabatic surface heating. 68-70 F surface dew points,
such as forecast by the NAM, may be overdone considering the
available recovery trajectories, and the nearest dew points that
large are 300-400 nm away over NC, on the other side of the outflow

Regardless, a plume of EML air advecting over this region will
foster steep midlevel lapse rates, overlying strengthening
boundary-layer lapse rates and low/mid-60s F surface dew points.
That combination still supports peak MLCAPE in the 1500-2500 J/kg
range, amidst strong west-southwesterly mean-wind and deep-shear
vectors. Forecast soundings suggest that, despite a nearly
unidirectional vertical wind profile, effective-shear magnitudes of
45-55 kt may be realized. Downward momentum transfer from strong
flow above 700 mb, into a well-mixed preconvective boundary layer,
should offer favorable conditions for severe thunderstorm winds.
Given the strong westerly component of the near-surface flow,
more-unstable inland air may be shunted eastward to very near the
coast across much of the region, extending the severe threat
accordingly, before the MCS encounters too much stable marine-layer
air and weakens.”

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