SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0758 AM CDT Sun Jul 03 2022
Valid 031300Z - 041200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM
SOUTHERN/CENTRAL MONTANA TO PARTS OF WESTERN NORTH DAKOTA...
...SUMMARY...
Scattered severe thunderstorm gusts are expected this afternoon
through the evening from southern/central Montana to parts of
western North Dakota. Isolated large hail also may occur.
...Synopsis...
In mid/upper levels, a persistent/high-latitude blocking pattern
over western Canada will transition from an omega shape toward a Rex
configuration through the period, with slow progress of a basal
cyclone now located just offshore from the Pacific Northwest. The
associated 500-mb low is forecast to pivot inland over the
southwestern WA/northwestern OR area by around 12Z tomorrow, with a
trough southward along the OR coastline and offshore from
central/southern CA. Several vorticity maxima and shortwaves --
embedded in the cyclone's southern semicircle -- should eject
northeastward toward the northern Rockies and through the
central/northern High Plains mean ridge.
The 11Z surface analysis showed a cold front from near Cape Cod
across NJ, northern VA, south-central WV, and northeastern KY,
becoming quasistationary over southern parts of IN/IL, then a warm
front across western IA, eastern SD and southwestern ND, to a low
over southeastern MT. The low should remain over southeastern MT
for much of the day before migrating eastward overnight, while the
warm front decelerates and perhaps becomes quasistationary to its
east over ND, then southeast across southwestern MN. A lee trough
will extend from the low across eastern parts of WY and the NE
Panhandle.
...MT to western ND...
Scattered to numerous thunderstorms are possible, developing in
multiple episodes and clusters across southwestern, central,
south-central and eastern MT this afternoon into evening. This
activity should develop over higher terrain of southern MT, as well
as near the boundary, as shots of DCVA/ascent aloft eject over the
area and out of the northwestern mid/upper trough. Activity should
track along and north of the baroclinic zone, close to an axis of
low-level moisture, and with the easterly ambient low-level wind
component maximizing storm-relative flow in the inflow layer. This
potential will be supported by a corridor of strong veering of flow
with height -- supporting 50-60 kt effective-shear magnitudes and
enlarged boundary-layer hodographs. Forecast soundings reasonably
show steep low/middle-level lapse rates with MLCAPE in the 1500-3000
J/kg range as well.
The full spectrum of severe threats is possible, given that
sufficient shear and buoyancy will exist for supercells. However,
clustering and upscale growth of convection, including bow-echo
evolution, are expected. With potential for downward momentum
transfer in downdrafts from the faster flow aloft, cold-pool
aggregation and rear-inflow-jet development, severe gusts should be
the most common effect with this activity overall.
...Northern/central Plains...
See SPC mesoscale discussion 1336 for more on the short-term,
marginal-hail threat across parts of SD and southwestern MN.
Otherwise, isolated to scattered thunderstorms are possible over a
broad swath of Nebraska and the Dakotas, east and southeast of the
more strongly synoptically influenced MT regime.
Specific potential foci for initiation are varied and saddled with
mesoscale uncertainties, including:
1. The main synoptic front across ND, where the greatest flow
aloft, surface wind backing, hodograph size, and deep shear will be
in place for conditional supercell/large-hail and perhaps even
tornado potential, but with least confidence in storm location/
coverage due to ridging aloft and recovery concerns around outflow
to the south.
2. Outflow/differential-heating boundaries over SD from prior
overnight and ongoing convection, and their shaping of theta-e
fields. This regime still is being affected by ongoing/elevated
activity over the region that itself has at least marginal hail
potential. The related MCV -- now apparent in radar animations over
northeastern SD -- should be well east of the area by prime
destabilization time this afternoon. A choppy field of favorable
low-level moisture should develop by mid/late afternoon in a
corridor about 100-200 nm wide from southeastern/south-central NE
across central SD, beneath midlevel lapse rates steep enough to
support 2000-3000 J/kg MLCAPE.
3. The surface low and trailing lee trough, and orographically
aided lift over the Black Hills. This regime probably is the most
certain for initiation today, ahead of a weak perturbation apparent
in moisture-channel imagery over portions of WY/CO. Activity should
move into a well-heated/mixed boundary layer and offering a severe-
gust threat. However, this area will be under nebulous large-scale
forcing (also near the ridge aloft) and weaker bulk shear than
farther north. Some activity moving out of the northern part of
this regime may persist well into the evening over parts of SD,
supported by increasing low-level moisture and the LLJ.
...Carolinas, Hampton Roads...
Isolated to scattered thunderstorms are possible along and south of
the front today, offering occasional damaging winds that may reach
marginal severe levels on an isolated basis. Difluent westerly to
northwesterly flow aloft will exist over the region, well southwest
of a mid/upper cyclone moving eastward across northeastern QC and
Labrador. Following the remnants of T.C. Colin, the airmass over
the region will remain richly moist, with surface dew points
commonly in the upper 60s to mid 70s F. This, in tandem with
diurnal heating, will offset modest midlevel lapse rates enough to
support 1000-2000 J/kg MLCAPE. Low/middle-level winds and deep
shear will be weak, with multicells the dominant mode and localized,
water-loaded downdrafts offering the greatest gust potential, until
activity diminishes this evening.
..Edwards/Goss.. 07/03/2022
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