Day 1 Severe Weather Outlook

Valid 081300Z – 091200Z


The greatest concentration of potentially severe storms today should
be from the lower Great Lakes and north-central Appalachians to the
mid/upper Ohio Valley.

In mid/upper levels, the CONUS subset of the large-scale pattern
will continue to feature a mean ridge in the West, from a TX
anticyclone to the northern Rockies, and troughing from eastern
Canada down the Appalachians. However, some breakdown of the
ridging will occur as a series of variably amplified shortwave
impinges on its northwestern parts. Two of those perturbations
follow each other in quick succession and are evident in moisture
channel imagery: the first from over northwestern NV to
southwestern UT, the second over the Pacific, approaching the CA
Channel Islands. Both of these features will move northeastward
across the Great Basin through the period, with the leading one
reaching the northern Rockies by 12Z.

A series of shortwaves also will traverse the extensive cyclonic-
flow field over the Great Lakes and Northeast regions, anchored by a
large cyclone over far northern ON and Hudson Bay. Among those is a
convectively enhanced vorticity lobe, now apparent in satellite and
composited radar imagery from Lake Huron to northern/eastern IN.
This perturbation will pivot across the Lower Great Lakes and
adjoining states through tonight, following a leading shortwave now
ejecting across New England. Meanwhile, in a zone of weaker,
difluent flow aloft, convectively induced vorticity lobes will move
slowly southeastward over a corridor from KS to the Mid-South.

At the surface, 11Z analysis showed a cold front from northwestern
Lake Huron across central Lake Michigan, central IA and northern NE,
becoming a warm front over southeastern MT to a low near GTF. The
eastern portion of this front should proceed eastward across the
lower Great Lakes, much of New England, and parts of the Mid-
Atlantic through the period. An outflow-reinforced baroclinic zone
was evident over northern MS, northern AR, and northern OK,
quasistationary except for a southward shunting by convective
processes over OK and parts of northern AR.

…Lower Great Lakes to Ohio Valley…
Widely scattered to scattered thunderstorms will develop from late
morning through the afternoon over western parts of the outlook
area, moving somewhat southeastward across the central Appalachians
region and eastward to east-northeastward over NY, in close
accordance with the cyclonic ambient-flow pattern. Convection
should increase in coverage/intensity as it moves into a favorably
moist and destabilizing boundary layer. Occasional large hail and
damaging to severe gusts will be the main concerns.

Activity may form along but mainly ahead of the surface cold front,
in areas of weak pre-frontal surface troughing and low-level
convergence/heating where MLCINH is locally minimized. Sever hours
of strong, direct heating of the near-surface layer with 60s F dew
points should occur over most of the area, given the lack of clouds
evident in satellite imagery. Meanwhile, the large-scale lift/DCVA
field of the shortwave trough should provide supportive
destabilization aloft. This should permit a patchy prefrontal
corridor of 1000-1500 J/kg MLCAPE to develop, atop steep boundary-
layer lapse rates and a well-mixed subcloud layer. Lack of stronger
shear area-wide will be a limiting factor for an even better-
organized event, though pockets of 30-40 kt effective-shear
magnitudes appear possible. Severe potential should weaken with
eastward extent and time this evening, into a stabilizing
thermodynamic regime.

…Eastern Rockies to central/southern Plains…
Two physically distinct regimes will contribute to strong/severe
thunderstorm potential today into this evening, with activity moving
generally eastward to southeastward around the upper high/ridge.
These areas have been split up spatially due to convective/precip
trends since the prior outlook:

1. A large area of precip and embedded, non-severe thunderstorms in
evident initially from central KS across northeastern OK to the
Ozarks of northern AR, supported by warm advection and related lift
to LFC northeast of the surface front. A brief/isolated severe-hail
event cannot be ruled out in this regime for a few more hours.
Additional, widely scattered to scattered thunderstorms are expected
to form this afternoon along associated outflow/differential-heating
boundaries, offering the threat for isolated large hail and damaging
to severe gusts. MCVs now apparent over the northeastern TX
Panhandle and west-central KS may provide mesoscale mass-response
enhancement in support of storm organization as they shift
east-southeastward to southeastward. More-sustained diurnal heating
of the favorably moist boundary layer over eastern OK and AR will
yield 2000-3000 J/kg MLCAPE, transitioning to 1000-2000 J/kg under
more cloud cover to the west. Weak low/middle-level winds and lack
of stronger shear should limit overall organization, though
localized clustering of damaging gusts may occur where driven by
aggregation of multicellular cold pools.

2. Farther west, scattered thunderstorms should develop this
afternoon over the eastern WY mountains southward over the CO Front
Range and Sangre de Cristos, as well as adjoining foothills and
eastward-extending topographic ridges. Sporadic/isolated damaging
gusts and severe hail will be possible from the most intense cores.
Formation of one or two small MCSs with locally maximized
wind-corridor potential also cannot be ruled out as convection moves
onto the adjoining High Plains, and into a well-heated/mixed
boundary layer supporting a transition from mixed hail/wind threat
to mostly wind. In the absence of substantial upper forcing or LLJ
activity, convection generally should weaken this evening in keeping
with stabilization of the nocturnal boundary layer.

…Great Basin to extreme southern ID…
Isolated thunderstorms already have been noted over the southern
Sierra and northwest of LAS during the pre-dawn hours, beneath the
leading shortwave perturbation. Episodic, widely scattered to
scattered thunderstorms are possible across a large part of the
Great Basin today in association with both mid/upper level troughs.
At least marginal unconditional risk for strong/locally severe gusts
is apparent from midday through afternoon — mainly over areas of
western UT and eastern NV that can diabatically destabilize through
pre-existing cloud cover, ahead of the densest cloud/precip areas.

Thermodynamic support also will be provided by a plume of low-level
monsoonal moisture. Preconvective, lower-elevation surface dew
points commonly in the 40s over western parts of the outlook area
50s in UT will support MLCAPE mainly in the 500-1000 J/kg range,
locally a bit lower or higher. Slight enhancement to the ambient
southwest flow is possible in mid/upper levels near each trough,
leading to modest but sufficient deep shear for storm organization.
Some clustering and cold-pool aggregation may occur as well in
support of a more-concentrated wind swath within this broader area,
but this depends on mesoscale to storm-scale processes still
unresolved. The overall threat should diminish with time this
evening int ID and central UT as nocturnal/diabatic stabilization
and outflow stabilize the boundary layer.

..Edwards/Broyles.. 08/08/2019


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