Day 1 Severe Weather Outlook

Valid 221200Z – 231200Z


Isolated severe wind gusts are possible this afternoon and evening
from Oklahoma eastward across the Ohio Valley and into parts of the
mid-Atlantic and New England. Meanwhile, hail or wind, occasionally
reaching severe levels, will also be possible over the central and
northern High Plains area.


A couple of upper level troughs and associated surface cold fronts
will result in a few areas of strong, occasionally severe storms
today. The first trough will pivot eastward from the upper Great
Lakes to the Northeast. A band of stronger southwesterly deep-layer
flow will overspread much of the northeastern U.S. from around PA
northward while a surface cold front shifts eastward. The western
extent of the front will drop more slowly south/southeast across the
Midwest, extending from southern MO toward Chesapeake Bay by Friday
morning. Several weak shortwave impulses and remnant MCVs will float
through westerly flow from OK to the Mid-Atlantic ahead of the
southward-sagging front, aiding in the development of sporadically
strong thunderstorm clusters. Further northwest, a shortwave trough
will migrate eastward across the northern Rockies. A surface cold
front will progress eastward across MT/WY during the afternoon and
evening, while a surface trough strengthens southeasterly low level
flow in a moist upslope flow regime across the northern High Plains.
This will focus another area for strong storm potential during the
afternoon and evening across parts of the northern and central High

…Mid-Atlantic and New England…

A warm and very moist airmass will reside ahead of the surface cold
front. Overall, west/southwesterly flow will be rather modest, with
the strongest flow residing over the Northeast. A corridor of modest
destabilization will overlap with this relatively stronger flow, and
a few fast moving storm clusters or line segments are possible from
PA northward through coastal ME. A few strong wind gusts could
accompany this activity, though the overall threat should remain
limited by poor midlevel lapse rates and in the absence of stronger
shear. Further south toward the Chesapeake and VA/NC Piedmont
vicinity, thunderstorm clusters are expected to develop in vicinity
of a lee trough as subtle shortwave impulses migrate across the
Appalachians. The airmass across this area will be very moist with
PW values greater than 2 inches forecast. Strong heating will result
in moderate destabilization and some strong gusts in wet microbursts
will be possible.

…Oklahoma toward the Ohio Valley…

Deep-layer westerly flow will remain weak across the region.
However, several shortwave impulses are forecast to migrate through
westerly flow while a myriad of outflow boundaries and MCVs from
prior convection influence thunderstorm development during the
afternoon. Steep lapse rates and moderate instability will be
sufficient for severe storms, but weak flow will limit overall
organization. Nevertheless a few strong to severe storms are
possible with strong downburst winds being the main threat. Should
an adequate cold pool develop, very steep low level lapse rates
could support a surging cluster/bow with an increased threat for
damaging wind, but confidence in this scenario is low, precluding
higher probabilities.

…Portions of the Northern/Central High Plains…

Strong southeasterly upslope flow in the vicinity of a pre-frontal
lee trough will bring upper 50s to low 60s surface dewpoints across
the High Plains of MT/WY/CO. Strong heating and very steep midlevel
lapse rates will result in 2000-3000 J/kg MLCAPE by afternoon. While
southeast flow through around 2 km will be strong, mid and
upper-level flow will remain rather weak. Nevertheless, as forcing
increases with the approach of a surface cold front and the upper
trough, scattered thunderstorms are expected. A deeply mixed
sub-cloud layer will support strong downburst winds, especially as
clusters/line segments are favored. However, very steep midlevel
lapse rates also will support some potential for hail initially. In
the absence of stronger shear, will maintain Marginal risk.

Further west across the higher terrain of south-central into
southwest MT, a couple of storms could produce gusty winds and hail
as the cold front moves through during the early afternoon.

..Leitman.. 08/22/2019


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