Day 1 Severe Weather Outlook

Valid 061300Z – 071200Z


Isolated severe thunderstorm winds and hail are possible this
afternoon and evening over parts of the northern Intermountain
region, northern/central Rockies and adjacent High Plains.

In mid/upper levels, an initially strong anticyclone centered over
the south-central Rockies will shift southwestward and weaken as a
strong shortwave trough crosses the northern Rockies. That trough
— now evident in moisture-channel imagery over western OR and
northern CA — is progged to overtake/absorb lesser vorticity lobes
initially to its east across eastern OR/ID. The combined, somewhat
stronger perturbation will reach the MT/ID border region and
southern ID by 00Z. Thereafter, the trough will pivot eastward and
east-southeastward, reaching central MT, western WY and northwestern
CO by 12Z.

Meanwhile, farther east, shortwave troughs now over the upper Great
Lakes and northern MB will move southeastward across the
Mid-Atlantic and upper Great Lakes, respectively. In response,
Hurricane Dorian has commenced accelerating northeastward over/off
the Outer Banks, and should eject away from the Tidewater region
through the remainder of the period, per NHC forecasts. With the
most thermodynamically and kinematically favorable sector for
supercellular tornadoes shifting offshore, and forecast to stay
southeast of the Cape Cod/Martha’s Vineyard region tonight,
unconditional overland tornado probabilities have been dropped.

Otherwise, the surface analysis showed a low near GRB with occluded
front to southeastern IA, and slow-moving cold front to a weak
frontal-wave/lee low over western KS, then extending across parts of
southeastern CO. The low should move southeastward across OH
overnight and become poorly defined in the broader field of low
pressure surrounding Dorian. Meanwhile the front should cross the
Ohio Valley and stall across the southern KS/northern OK/
southeastern CO corridor.

…Northern Rockies/Intermountain region to Front Range/central High
Widely scattered to scattered thunderstorms are expected to develop
this afternoon into evening in three primary regimes, described
below from west to east, each offering isolated severe gusts/hail,
with enough spatial overlap to result in a unified outlook area:

1. A “cold-core” regime near the midlevel/500-mb thermal trough,
from eastern OR across much of southern ID, potentially forming a
quasi-linear band of strong/isolated severe convection over ID
before weakening tonight in western WY where regime 2 below will
have produced enough convection to stabilize the air mass. Diurnal
heating and midlevel cooling should overlap well, steepening low/
middle-level lapse rates considerably, atop 50s F low-elevation
surface dew points. This should result in MLCAPE 1000-1500 J/kg
atop a well-mixed boundary layer supporting strong/locally severe
gusts and survival of hail to ground level. Organization should be
tempered somewhat by weakening deep-layer flow with time and
proximity to the mid/upper trough, and related lack of shear.
However, a conditional cold-pool aggregation potential exists,
especially across parts of the Snake River Plain and southern
mountains west of I-15.

2. A band of relatively maximized deep-layer ascent shifting
eastward from southeastern ID and northern UT across western/central
WY. A combination of lift from large-scale DCVA/cooling aloft,
frontal forcing near a low-level baroclinic zone preceding that
trough, and pre-frontal warm advection will support convection in
this area. Buoyancy should be less in both coverage and magnitude
than in regime 1, with MLCAPE mainly around 500 J/kg. Still,
localized well-mixed boundary layers and marginal deep shear will
promote a few strong-severe cells in the broader convective plume.

3. Heating of higher terrain, favorable low-level moisture, and
resultant weak MLCINH should foster convection initiation this
afternoon over the Front Range, Laramie Range, adjacent foothills,
and perhaps the Bighorns. Activity then should move eastward across
a diabatically destabilized strip of the central High Plains,
offering damaging gusts and isolated hail before weakening. Strong
surface heating and deep, strongly mixed boundary layers will
support isolated severe potential with this activity before it
weakens this evening in a stabilizing boundary layer, at lower

..Edwards/Smith.. 09/06/2019


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