Day 1 Severe Weather Outlook

Valid 091300Z – 101200Z


Severe storms are possible across parts of South Dakota, Oregon, and
eastern Virginia.

…Eastern VA area this afternoon…
A subtle mid-upper speed max will progress east-southeastward from
WV this morning across VA this afternoon. Surface heating in the
wake of dissipating morning convection and lingering low-level
moisture will contribute to moderate buoyancy (MLCAPE near 1500
J/kg) this afternoon across central/eastern VA. A pre-frontal
surface trough will help focus storm initiation by early afternoon
across central/northeast VA, and storms will subsequently spread
southeastward through the afternoon. Straight hodographs with
effective bulk shear of 30-35 kt and steep low-level lapse rates
will support some organized (possible supercell) storms capable of
producing damaging gusts and large hail.

…SD area this afternoon/evening…
An embedded shortwave trough will crest the midlevel ridge over WY
and move eastward across SD today, along with an associated weak
surface low. Convection is ongoing this morning in advance of this
wave across southwestern SD, and some form of this convection may
persist well into the day into central SD. Surface heating on the
southern-western flank of the ongoing convection, within a corridor
of 65-70 F boundary-layer dewpoints, will contribute to MLCAPE near
2000 J/kg this afternoon. Surface-based thunderstorm development
will become more probable by early afternoon near the weak surface
low as convective inhibition diminishes. Deep-layer shear/buoyancy
will support supercells capable of producing damaging winds and
large hail. A tornado or two will also be possible in the narrow
zone of enhanced low-level shear (0-1 km SRH near 200 m2/s2) near
the surface low and the north edge of the unstable warm sector.

…Interior Northwest this afternoon/evening…
Embedded shortwave troughs will rotate northward from CA to OR
around the eastern periphery of a closed low off the northern CA
coast. Typically steep lapse rates over the northern Great Basin
and interior Northwest, along with sufficient low-midlevel moisture,
will support weak-moderate buoyancy this afternoon across parts of
OR, ID, and northern NV. Scattered thunderstorm development is
expected this afternoon, focused by ascent preceding the ejecting
midlevel waves and surface heating over the higher terrain. The
more concentrated storms are expected over the OR Cascades and the
higher terrain in northeastern OR, where isolated large hail and
damaging gusts will be possible with high-based storms in an
environment supporting clusters and marginal supercells. Gusty
outflow winds will also be possible farther east-southeast into
parts of northern NV and ID.

..Thompson/Kerr.. 08/09/2019


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