Day 1 Severe Weather Outlook

Valid 092000Z – 101200Z



Scattered severe thunderstorms will likely occur across parts of
South Dakota this afternoon and evening. A few tornadoes, some of
which may be strong, isolated large hail, and scattered severe winds
will all be possible. Other isolated to widely scattered severe
thunderstorms will remain possible across parts Oregon and Virginia
through this evening.

…20Z Update…
19Z surface analysis shows a weak low over southwestern SD.
Low-level convergence to the east of this low has encouraged recent
convective development across central SD. A strong baroclinic zone
also extends northwest-southeast across SD. The environment
along/near this boundary appears increasingly favorable for
significant severe weather this afternoon and evening. Supercell
thunderstorms are expected to be the dominant storm mode initially
given 45-50 kt of effective bulk shear, with a couple supercell
structures observed already. Isolated large hail should occur with
these storms. MLCAPE up to 2500 J/kg will likely develop across the
warm sector to the southwest of the boundary across parts of western
into south-central SD. A very strongly veering low-level wind
profile will provide ample effective SRH along the surface boundary,
perhaps upwards of 250-450 m2/s2. The potential for a few tornadoes
in this environment with any supercells warrants an increase in
tornado probabilities. A strong tornado may also occur if a
supercell can move southeastward along the surface boundary given
the very favorable low-level shear expected. A small cluster of
storms should eventually develop across south-central into
southeastern SD and perhaps far northern NE later this evening as a
southerly low-level jet strengthens across this area. Scattered
severe wind gusts, some potentially significant, will likely occur
with these storms through mid to late evening.

The Marginal Risk has been expanded westward across western VA into
parts of WV to account for thunderstorm development that has
occurred this afternoon along a weak front. Both isolated large hail
and strong/gusty winds will be possible with these storms as they
develop southeastward through the remainder of the afternoon and
into the early evening.

No changes have been made to the severe probabilities across the
northwestern CONUS.

..Gleason.. 08/09/2019

.PREV DISCUSSION… /ISSUED 1130 AM CDT Fri Aug 09 2019/

An embedded shortwave impulse is cresting the midlevel ridge and
will move east from eastern WY across SD with an attendant surface
cyclone. Elevated convection is ongoing in advance of this
impulse across western/central SD and the northern extent of this
activity should persist east near the SD/ND border. Surface heating
on the southern-western flank of this ongoing convection, within a
corridor of 65-70 F boundary-layer dewpoints, will contribute to
MLCAPE from 1500-2500 J/kg between 21-00Z across west-central into
south-central SD.

Surface-based thunderstorm development will become more probable in
the next few hours, either emanating from the southwest flank of the
elevated convection and/or developing westward towards the surface
cyclone. These storms should spread southeast along a pronounced
differential heating corridor across central to southeast SD. Backed
surface winds east of the cyclone, beneath strengthening
low/mid-level flow should yield a rather enlarged hodograph towards
early evening in a spatially narrow corridor from around Pierre to
Mitchell. If a discrete supercell or two can be maintained through
this time frame, the tornado risk may be greater than currently
forecast. Otherwise, supercells may evolve into small-scale bows
with a risk for significant severe wind. A cluster with severe wind
as the primary hazard may progress into southeast SD/northeast NE
before waning later in the evening given the dearth of instability
with eastern extent in western IA. This area will be monitored for a
possible upgrade in the 20Z outlook.

…Eastern VA…
A subtle mid-upper speed max will progress east-southeastward from
eastern WV across VA. Downstream of the Appalachians, robust
diabatic heating is well underway and this will contribute to
moderate buoyancy with MLCAPE of 1000-2000 J/kg. A pre-frontal
surface trough may help focus storm initiation in the next few hours
across central/northeast VA, with storms potentially spreading
southeastward into early evening. Straight hodographs with effective
bulk shear of 30-35 kt and steep low-level lapse rates will support
some transient supercell structures capable of producing damaging
gusts and marginally severe hail.

Embedded shortwave impulses 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 later this afternoon across
parts of OR, ID, and northern NV. Scattered thunderstorm development
is expected to be focused by ascent preceding the ejecting midlevel
waves and surface heating over the higher terrain. The more
concentrated storms are anticipated over the OR Cascades and the
higher terrain in northeastern OR, where isolated severe wind gusts
and marginally severe will be possible with high-based storms in an
environment supporting clusters and marginal supercells. Locally
strong to severe gusts will also be possible farther east-southeast
into parts of northern NV and ID.


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