Day 1 Severe Weather Outlook

Valid 031300Z – 041200Z


A marginal severe-wind threat exists from thunderstorms today over
parts of eastern Ohio into western Pennsylvania.

A deep, mid/upper-level, synoptic-scale trough over the extreme
northeastern Pacific will shift southeastward and move ashore
tonight over the U.S. Pacific Coast. Associated cold midlevel air
and steep low/midlevel lapse rates will foster isolated/episodic
thunder potential across portions of the near-coastal Northwest.
Meanwhile, ridging will amplify downstream over the Rockies, then
move eastward over the Great Plains, behind a cyclone now centered
near the MN Northwest Angle. The latter cyclone will change to a
positively tilted, open-wave, increasingly progressive trough,
reaching northeastern ON, Lake Superior and northern WI by 00Z. By
12Z, the trough should extend from northwestern ME across the lower
Great Lakes area.

11Z surface analysis showed a frontal-wave low near the northwestern
IN shoreline, with warm front east-southeastward over southeastern
PA and the Delmarva Peninsula, and cold front to central IL, the
MO/northwest AR Ozarks, southwestern OK, and east-central NM. The
low should ripple eastward across northwestern PA early this
afternoon, reaching western NY by 00Z. The cold front then should
extend southwestward across eastern KY, northwestern MS, and
north-central through far west TX. By 12Z, the low should be
offshore from New England, with cold front over southeastern VA,
northwestern SC, central AL, and central TX, becoming diffuse
farther west amidst the onset of High Plains return flow.

Isolated to widely scattered thunderstorms may develop this
afternoon and persist long enough to produce damaging gusts, before
moving into greater stability and weakening. Coverage, spatial
extent and longevity of convection are in question, precluding a
greater unconditional risk.

The mid/upper ridge will be suppressed through the day, ahead of the
approaching trough. Beneath associated height falls, low-level
convergence will strengthen near the surface low, in the peripheral
mass response to that trough. Meanwhile, as the warm front ahead of
the low shifts northward on the mesoscale, the warm sector will
overspread the outlook area. This should support convection
initiation across parts of eastern OH and western PA, moving quickly
eastward through (while outpacing) a narrow corridor of weak but
still supportive instability. Diabatic surface heating and rich
moisture — with dew points generally upper 60s to near 70 F — will
offset weak midlevel lapse rates to yield surface-based prefrontal
buoyancy. Model forecast soundings, most of which appear slightly
under-mixed (under-heated) without modification, suggest 500-1000
J/kg MLCAPE is possible in a small, southwest/northeast-oriented

Strong mid/upper-level flow will contribute to favorable deep-layer
speed shear, yielding warm-sector effective-shear magnitudes in the
45-55-kt range. Convection may assume the form of small bows and/or
supercell structures before activity moves out of its optimal
source-region buoyancy. Potential will be limited on the north,
west, and east ends by lack of CAPE related to frontal air masses,
and with southward extent by weak lift.

..Edwards/Broyles.. 10/03/2019


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