Day 1 Severe Weather Outlook

Valid 041200Z – 051200Z


Strong to severe thunderstorms capable of producing mainly damaging
winds will be possible this afternoon across parts of the
Mid-Atlantic and New England. Isolated tornadoes may occur primarily
along the coast of South Carolina late tonight through early
Thursday morning in association with Hurricane Dorian.

An upper trough will move eastward across the Northeast and
Mid-Atlantic today. At the surface, a low will develop northeastward
across Quebec through the period, with a trailing cold front
shifting quickly eastward over these areas. Strong forcing for
ascent associated with the upper trough along with low-level
convergence along the front will likely foster robust convective
development by early afternoon across NY into southern New England.
50-60 kt of mid-level southwesterly flow associated with the upper
trough will overspread the warm sector, and strong shear will
support organized updrafts. Generally mid 60s to lower 70s surface
dewpoints should advect northeastward ahead of the front in a
low-level warm air advection regime. Even though mid-level lapse
rates will likely remain modest, the increasing low-level moisture
and at least modest diurnal heating should promote about 1000-2000
J/kg of MLCAPE. The greatest instability should reside across the
Mid-Atlantic, with gradual lessening with northeastward extent into
the Northeast. By contrast, relatively stronger shear should be
present farther north in New England.

Regardless, there appears to be a sufficient overlap of instability
and shear to support organized storms, especially from northeastern
PA/northern NJ into southern New England. Given the strength of the
mid-level flow and some orthogonal component to the surface front,
some supercells could occur initially, with clustering potential
increasing later in the afternoon. Damaging winds will probably be
the main threat given the strength of the low/mid-level flow, but
isolated marginally severe hail could also occur. A tornado or two
cannot be ruled out, although low-level flow will have a tendency to
veer to southwesterly through the afternoon, generally limiting
low-level hodograph length.

Farther south into southern PA/NJ and MD/DE, low-level convergence
should be weaker along the front and mid-level height falls will be
more subdued. Resultant overall storm coverage will probably be more
isolated. Still, some threat for damaging winds should exist with
any storms that can form across this area as low-level lapse rates
steepen and moderate instability develops.

…Southeast Coast…
Hurricane Dorian is forecast by the National Hurricane Center to
continue moving north-northeastward near the coast of FL/GA/SC
through the period. The overall tornado threat is expected to remain
quite limited today along the east coast of FL and coastal GA as
this region will remain within the western semicircle of Dorian’s
circulation, limiting low-level curvature and effective SRH. A
somewhat greater potential for isolated tornadoes may develop late
tonight into early Thursday morning across parts of coastal SC,
where strengthening northeasterly low-level winds will substantially
increase low-level hodographs. Low-topped supercells within one or
more outer rainbands could approach the immediate coast of SC late
in the period, with this isolated tornado threat likely continuing
into parts of coastal NC into Day 2/Thursday.

…Deep South Texas…
Tropical Storm Fernand, now over the far western Gulf of Mexico, is
forecast by the National Hurricane Center to move west-northwestward
across parts of northeastern Mexico through the period. Low-level
shear on the northern periphery of Fernand’s circulation may be
sufficient to support a low risk for a brief tornado or two as
low-topped cells move onshore across deep south TX.

…North Dakota/Minnesota…
A low-amplitude shortwave trough now over WA/southern BC will move
eastward along the international border through the period. A
south-southwesterly low-level jet should strengthen across the
northern/central Plains in response this evening/overnight. Elevated
storms may evolve southeastward from southern MB/SK into
northeastern ND and northern MN late in the period. Forecast MUCAPE
appears too marginal at this time to include any probabilities for
large hail, although shear is forecast to be strong across this

..Gleason/Squitieri.. 09/04/2019


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