Day 1 Severe Weather Outlook

Valid 051200Z – 061200Z


Some risk for a couple of tornadoes could develop across parts of
the coastal Carolinas today through tonight in association with
Hurricane Dorian. A couple of strong thunderstorms may also impact
parts of the Upper Midwest, and portions of western and central
Oregon, accompanied by at least some risk for severe hail and wind.

The stronger westerlies will remain confined to the northern tier of
the U.S. and Canada through this period. Models suggest that some
amplification within this regime is possible across the northeast
Pacific through the Canadian Rockies, with short wave ridging
building inland of the British Columbia coast, downstream of a
significant progressive short wave trough over the northeast
Pacific. Preceding the latter feature, a weaker short wave is
forecast to advance inland of the northern California and Oregon
coast, while subtropical ridging (centered near/east of the Colorado
Rockies) otherwise maintains a prominent influence across much of
the remainder of the West/Rockies and Plains.

Downstream of the ridging, the most prominent impulse within the
westerlies appears likely to continue rapidly progressing through
larger-scale mid/upper troughing, across Quebec/Newfoundland and
Labrador and the Canadian Maritimes. Meanwhile, within much weaker
troughing at lower latitudes, Hurricane Dorian is expected to, at
least initially, continue a slow northeastward migration near the
Carolina coast. However, by tonight, an acceleration toward the
North Carolina Outer Banks vicinity is forecast, as at least one
short wave impulse within the westerlies digs across the
Canadian/U.S. border through the Upper Midwest/Great Lakes region.

Seasonably moist air has largely become confined to portions of the
Southwest, Deep South Texas, and Southeast coastal areas, to the
north and west of Dorian, and in the wake of an initial surface
front which has advanced off the New England and northern Mid
Atlantic coast, as well as into the lower Ohio/lower and middle
Missouri Valleys. Considerable further drying into and through the
Southeast and Gulf Coast appears likely today through tonight, in
the wake of Dorian. However, substantive northward moisture return
does appear possible ahead of the troughing advancing inland of the
Pacific coast.

…Carolina coastal areas…
Potential for convection capable of producing tornadoes will largely
hinge on the inland advection of tropical boundary layer moisture,
now present offshore, as evolving clockwise curved low-level
hodographs become characterized by strong to extreme shear in the
right front quadrant (with respect to storm motion) of Dorian.
Based on the most probable track of the low-level cyclonic
circulation center, this may come close to coastal areas, but could
remain mostly offshore through this period. There may be some
window of opportunity for increasing potential for tornadoes near
northeastern South Carolina and southern North Carolina coastal
areas this morning into midday, then mainly near the North Carolina
Outer Banks vicinity later today through tonight, as the hurricane
turns northeastward and roughly parallels the coast.

…Upper Midwest…
Scattered thunderstorm development, in the presence of steep
lower/mid tropospheric lapse rates and strong vertical shear within
the convective layer, may pose a risk for marginally severe hail and
wind. This will largely be focused along a zone of enhanced
lower/mid tropospheric warm advection, associated with a couple of
digging shortwaves within increasingly northwesterly mid-level flow.

…Near/east of the Oregon Cascades…
Stronger mid/upper forcing for ascent associated with the inland
progressing short wave, in combination with strengthening
southwesterly mid-level flow (30-40 kt around 500 mb), deep boundary
layer mixing and destabilization aided by mid-level moistening, may
provide support for scattered afternoon storms which could pose a
risk for marginally severe hail and strong wind gusts.

..Kerr/Bentley.. 09/05/2019


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