Valid 091630Z – 101200Z
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SD…OR…AND
EASTERN VA…
…SUMMARY…
Scattered severe thunderstorms are possible across parts of South
Dakota, Oregon, and eastern Virginia mainly through early evening.
…SD…
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.
…Northwest…
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.
..Grams/Bentley.. 08/09/2019
$$