ACUS01 KWNS 131259
SWODY1
SPC AC 131257

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0757 AM CDT Sat Jul 13 2024

Valid 131300Z - 141200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF
THE NORTHERN PLAINS INTO THE UPPER MISSISSIPPI VALLEY...

...SUMMARY...
Multiple clusters of strong to severe thunderstorms are possible
across portions of the Northern Plains into the Upper Mississippi
Valley today and tonight.

...Synopsis...
The basic middle/upper-level pattern will change little over the
CONUS this period, and will feature:
1.  A strong anticyclone centered over the Four Corners region, and
covering most of the landmass from the Sierra Nevada to the
south-central High Plains, and MX border to southern parts of ID/WY.

2.  Extensive but fairly weak synoptic troughing from the Great
Lakes to the mid Mississippi Valley, TX and north-central MX.
3.  A belt of west-northwest to northwest flow across the northern
Rockies, northern Plains and upper Mississippi Valley, becoming
weaker and strongly difluent over the Upper Great Lakes and lower
Missouri Valley.

A northern-stream shortwave trough -- evident in moisture-channel
imagery over southern AB -- should move across southern SK today and
reach northern MN and adjoining parts of ON overnight.  Downstream,
MCVs and accompanying small, convectively generated/enhanced
shortwave perturbations are apparent over the ND/MB border,
west-central/southwestern MN, and the NE Sandhills.  The MN feature
may be most influential for severe potential, and is expected to
move southeastward over southern WI through this evening.

At the surface, 11Z analysis showed a low over southern MB near
Winnipeg, with cold front southwestward over central/southwestern ND
and southeastern MT.  A wavy, warm to quasistationary front was
drawn from the low southeastward across central/southeastern MN to
central IL.  The latter boundary should move little through the
period, except where shifted/reinforced on the mesoscale by
convective processes, such as currently across parts of southern MN.
 The cold front should move to northwestern MN and northern SD by
00Z, decelerating or perhaps stalling over
southeastern/south/central MT.

...North-central CONUS...
A multi-episode, somewhat conditional, but potentially significant
severe threat exists across the north-central CONUS through tomorrow
morning.  Although individual severe thunderstorms and a few
clusters/complexes are possible across the broader swath from MT to
the Upper Great Lakes, two corridors of greatest convective/severe
potential are apparent, with some spatial overlap in the middle,
represented for now by the 15% "slight risk" probabilities.  If/as
confidence increases in especially wind potential with either of
these, based on succeeding guidance and mesoscale environmental
trends, more tightly focused/greater probabilities may need to be
introduced.

1.  Eastern Dakotas to Lake Michigan vicinity:
An ongoing, arc of convection and precip, with scattered embedded
thunderstorms, is moving southeastward across southern MN.  This
complex earlier was responsible for generating the aforementioned
MCV.  That feature may aid in additional development and maintenance
of thunderstorms later this morning, along either the leading-edge,
low-level pressure theta-e perturbation of the earlier complex, or
the trailing outflow/differential-heating boundary.  The main
concern will be severe gusts -- perhaps in an organized swath --
with isolated large hail possible.  Until then, isolated severe
hail/gusts may occur with the more isolated strong-severe convection
in the ongoing complex.

Additional activity may form along or north of the resulting
boundary almost anytime this afternoon or evening, lending timing
uncertainty to the onset of any convection that may grow upscale
into an MCS with primary wind concern.  However, the process appears
probable enough at some point that the corridor of severe
probabilities should be maintained for now.  Rich low-level moisture
will persist south of the front/outflow in this corridor, with upper
60s to mid 70s F surface dewpoints common.  Combined with diurnal
heating, this should contribute to afternoon MLCAPE ranging from
around 3000-4000 J/kg over the eastern Dakotas, southwestern MN and
northwestern IA south of the baroclinic zone (where lapse rates will
be greatest) to around 2000-2500 J/kg in southern WI.  Although
low-level flow and hodographs generally will be weak, boundary-aided
enhancements to low-level vorticity and shear may contribute some
tornado risk.

2.  Northern High Plains to eastern Dakotas:
Widely scattered to scattered thunderstorms are expected to develop
close to the MT/SK border, on either or both sides, late this
afternoon into evening.  This activity should move southeastward
into a substantial post-frontal easterly flow component near the
surface, aiding both storm-relative winds in the inflow layer and
low-level shear.  In the regime of enhanced mid/upper gradient flow
between the border shortwave to the east and the Four Corners
anticyclone, and with near neutral large-scale forcing, deep shear
should be favorable as well, despite weak absolute speeds in the
boundary layer.

As such, high-based supercell and multicell activity is possible
early amidst 35-45-kt effective-shear magnitudes, and atop a still
well-mixed boundary layer remaining from diurnal heating.  Large
hail and localized severe/downburst gusts will be possible while
activity still is relatively discrete.  Any upscale aggregation of
outflow and related forced ascent would move into a progressively
moister environment with continued steep lapse rates above the
surface -- all supporting 1500-2500 J/kg MLCAPE and enough remaining
well-mixed low-level air and related DCAPE to sustain an organized,
potentially significant wind threat with any forward-propagating
complex(es) into tonight.

..Edwards/Broyles.. 07/13/2024

$$
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