SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1248 AM CDT Sat Sep 17 2022
Valid 171200Z - 181200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE UPPER
MISSISSIPPI VALLEY INTO THE MID MISSOURI VALLEY...
...SUMMARY...
Isolated severe thunderstorms are expected Saturday across the upper
Mississippi Valley region, south into the Mid Missouri Valley. All
hazards are possible across the north with primarily hail/wind into
the Missouri Valley.
...Discussion...
A notable short-wave trough is currently ejecting across the
northern High Plains and should advance into the eastern Dakotas by
18z, before shifting into the upper Great Lakes/northwestern ON by
daybreak Sunday. A weak surface low is forecast to evolve along the
lee trough in response to this feature, then track into northeast SD
by 18z. Some deepening is expected, which should result in a
sharpening baroclinic zone across central MN by peak heating. Latest
model guidance suggests strong surface heating should occur across
eastern SD into west-central MN immediately ahead of the surface
low/front. Resultant thermodynamic profiles could yield SBCAPE in
excess of 2500 J/kg where dew points hold in the mid 60s.
Additionally, forecast shear favors supercells as the approaching
short wave is expected to contribute to strengthening wind profiles.
NAM 20z sounding for AXN exhibits 40kt surface-6km bulk shear,
veering winds with height and 2800 J/kg SBCAPE. Convection is
expected to develop near the ND/SD/MN border ahead of the surface
low, possibly as early as 19-20z. This activity should track
northeast along a warm front that will advance north ahead of the
short wave. All hazards are possible with these supercells,
including the risk for a few tornadoes. Convection should spread
toward the upper Great Lakes late in the period.
Farther south, a weakening corridor of convection is spreading
across the central Plains ahead of a disturbance at 0530z. LLJ is
currently strengthening across KS into southern NE early this
morning, and this should aid continued eastward propagation into the
beginning of the day1 period. This activity is not forecast to
completely dissipate, and some early-day hail/wind threat appears
possible as LLJ veers into western IA and strengthens. It's not
entirely clear how concentrated this activity will be, as some
uncertainty exists regarding downstream heating and the influence of
convective debris/precipitation. Additionally, timing of the
early-day disturbance is not particularly favorable for supporting
renewed convection along the primary surface boundary that should
remain draped across southeast NE into northern KS. Even so, strong
surface heating in the wake of this activity, along with convective
outflow, may result in secondary development later in the afternoon.
While the early convection will likely be multicell clusters, some
risk for supercells appears possible where isolated convection could
evolve along the trailing boundary. Hail/wind are the primary risks
with this activity.
..Darrow/Karstens.. 09/17/2022
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