ACUS01 KWNS 121249
SWODY1
SPC AC 121248

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0748 AM CDT Thu Sep 12 2024

Valid 121300Z - 131200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
THE NORTHERN GREAT PLAINS...AND FROM THE FLORIDA PANHANDLE TO
NORTH-CENTRAL ALABAMA...

...SUMMARY...
A few tornadoes are possible today, mainly in the morning and
afternoon from the Florida Panhandle to north-central Alabama.
Scattered large hail and severe gusts (some significant) are
expected across the eastern Montana vicinity during the afternoon to
early evening.

...Synopsis...
Over the CONUS, in mid/upper levels, a highly amplified initial
pattern will become more blocky through the period.  A strong
northern-stream trough with an intermittently closed cyclone is
apparent in moisture-channel imagery from the Canadian Rockies
southward over ID to the central Great Basin.  The 500-mb low is
forecast to move northeastward across western and northern MT to
near the southwestern corner of SK by the end of the period.  By
then, the attached trough will deamplify slightly and become more
positively tilted, extending from the low to central UT.  A
baroclinic leaf with strongly difluent flow aloft precedes the
trough in satellite imagery over southeastern ID, western WY and
central MT, with an eastward shift and some reinforcement expected
today as small vorticity maxima pivot northeastward out of the
trough.

Meanwhile, an initially separate, weak 500-mb low now over the Red
River near PRX, and the remnants of Tropical Storm Francine aloft,
are progged to merge over the upper Delta/Mid-South region today
into tonight.  The result after 06Z should be a slow-moving
mid/upper-level low over AR, nearly stacked above the weakening
low-level vortex just to the east around MEM.  [See NHC advisories
for latest forecast track/intensity guidance on Francine.]  An
anticyclone aloft should form later today and expand tonight
northeast of that low, over the Upper Great Lakes region.

At the surface, 11Z analysis showed a warm front across northernmost
FL to the coastal FL Panhandle, then slightly inland over the
western Panhandle, southwestern AL and southeastern MS.  This
boundary will move slowly northward/inland today, demarcating the
north rim of mid-70s F surface dewpoints.  Meanwhile, a surface low
was drawn over eastern MT west of GDV, with slow-moving cold front
south-southwestward across northeastern/central WY.  The low and
front will continue slow/intermittent eastward progress today, then
move into the western Dakotas around 22-00Z, then eastward to the
central Dakotas late overnight.

...Northern Great Plains...
Strong-severe thunderstorms are possible this afternoon into
evening, along and behind the front, with the most probable severe
potential arising from activity moving north-northeastward from
northern WY.  High-based supercells and multicells are possible,
offering large hail and severe gusts.  Some of the activity may
aggregate upscale into one or two bowing clusters, racing northward
accompanied by a swath of better-organized, potentially significant
(65+ kt) gust potential.

Thunderstorms may develop along the front in early-mid afternoon, as
well as in and near the Bighorns, where diurnal heating of elevated
terrain preferentially removes MLCINH earliest.  A corridor of
favorable surface moisture will remain near and behind the front and
ahead of this convection.  Meanwhile, midlevel temperatures will
cool and steepen low/middle-level lapse rates, as large-scale ascent
spreads over the area, both ahead of the ejecting mid/upper trough
and in the left-exit region of a cyclonically curved 300-mb jet
segment.  This will occur atop a deep and well-mixed boundary layer
containing nearly dry-adiabatic lapse rates.  A northerly flow
component west of the front will enhance storm-relative winds in the
inflow layer, as well as low-level and deep-layer shear,
strengthening lift and helping to maintain and further organize the
convection.  MLCAPE of 500-1200 J/kg and 45-55 kt effective-shear
magnitudes will support the severe potential.

Convection forming later and farther east near the front over the
western Dakotas, from late afternoon into evening, also may pose a
strong to locally severe gust threat for a few hours, and the
marginal outlook area has been expanded eastward somewhat to give
more room for that process.  Flow aloft will bear a substantial
component parallel to the convective axis, indicating a largely
training or linear configuration is possible, along the western rim
of a 50-60-kt LLJ fostering warm advection and moisture transport
above the surface.  However, the near-surface airmass over the
central Dakotas should stabilize with time and eastward extent
tonight, limiting duration and eastward shift of the threat.

...Southeastern CONUS...
Tropical Storm Francine is forecast to continue moving northward
over MS, and to lose definition as it interacts more closely with
the mid/upper-level low.  Accordingly, associated deep-layer winds
will continue to weaken, but low-level shear will remain favorable
through most of the day well east-southeast to southeast of center.
A destabilizing airmass along and south of the warm/marine front
will support a continued, slow inland shift of supercell potential
amid enlarged hodographs.  The favorable environment will be bounded
on the west by a dry slot and veering low-level flow, on the
northeast by unsuitably stable air in heavy precip and north of the
front, and on the south by a gradual weakening of low-level winds
and shear as the cyclone center pulls away and fills.  In the
favorable corridor, where inflow parcels are surface-based, expect
effective SRH in the 200-500 J/kg range, very rich moisture, low
LCL, and weak MLCINH.  The threat should diminish overnight as the
airmass slowly stabilizes inland, and the system continues to weaken
overall.

..Edwards/Kerr.. 09/12/2024

$$
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