LIVE MAP (ABOVE) ... SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0747 AM CDT Mon Sep 04 2023
Valid 041300Z - 051200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
NORTHERN PLAINS...
...SUMMARY...
Severe gusts and large hail are expected over parts of the northern
Plains this afternoon and evening.
...Synopsis...
West of a cut-off cyclone lingering south of Atlantic Canada, the
mid/upper-level pattern has become substantially more progressive.
High-amplitude ridging will shift eastward through the period, from
a Great Lakes/Tennessee Valley axis to the Appalachians and St.
Lawrence Valley. This will occur as heights fall across much of the
central CONUS and Mississippi Valley in relation to two ejecting
perturbations:
1. A strong trough (itself a formerly cut-off Pacific Coast
cyclone), now readily apparent in moisture-channel imagery from
southwestern MT southward over western UT to near LAS. Although
this trough will lose some amplitude through the period, it still
will present a seasonally formidable wind profile aloft as it moves
east-northeastward across the north-central Rockies toward the
northern Great Plains today, with a strongly cyclonically curved jet
core of 60-80 kt at 250 mb and 40-50 kt at 500 mb still apparent by
00Z from the central Rockies to the Plains in WY and the western
Dakotas. By 12Z tomorrow, the trough should become more positively
tilted, extending near a MOT-PHP-LIC axis.
2. A weak, presently cut-off cyclone centered near Lake Texoma,
devolving to an open wave today as the larger perturbation
encroaches upon the central/northern Plains. The southern trough
will shift east-northeastward, reaching near a line from SGF-TXK-LFK
by 00Z, and near MLI-UNO-SHV by 12Z.
At the surface, 11Z analysis showed a weak cold front from southern
MB to a low near MBG, south-southwestward over eastern CO. A
stronger cold front/surge was apparent over eastern MT into central
WY, and will catch up with the leading boundary later today. The
low should reach southeastern MB by 00Z, connected by the cold front
to a secondary/newer low over the SD Badlands region, with front
southeastward over south-central CO. By 12Z, the SD low should move
to northeastern SD or adjoining extreme western MN. This low will
anchor a small wave cyclone along a cold front extending
northeastward over the MN/ON Boundary Waters region, and southward
to southwestward over eastern NE, central KS, northwestern OK, the
TX Panhandle, and northeastern NM.
...Northern Plains...
Scattered to locally numerous thunderstorms are expected to form
mainly behind the cold front, but along and perhaps slightly ahead
of it as well, depending on laterally expansive extent of a complex
that may evolve from initial activity over WY and move rapidly
northeastward. Large hail and sporadic severe gusts are possible
from early-stage, relatively discrete convection, with the threat
becoming wind-dominant when and where cold pools aggregate from the
early convection and build a forward-propagational process.
Timing/location of that evolution from more discrete elements
(including supercells) to an MCS is uncertain, but the 15%/"Slight"
area still appears to represent the greatest unconditional wind
threat. A fairly sharp gradient of severe-wind potential in
particular may evolve, and some part of the current 15% area may
need an upgrade today if mesoscale details become clearer and
12Z/onward numerical guidance focuses potential better. In the
meantime, the 15%/wind area has been expanded somewhat to allow more
room for MCS growth, especially on the north and east ends.
Activity initially should be somewhat high-based atop 40s and 50s F
surface dewpoints over WY, moving into increasing moisture and
buoyancy with northeastward extent. Forecast soundings reasonably
indicate well-heated/mixed, deep subcloud layers and 500-800 J/kg
MLCAPE in the immediate preconvective environment over WY and
western SD. This will develop amidst strengthening deep shear, with
40-50-kt effective-shear magnitudes possible along and behind the
front. This environment will support strong-severe downdrafts,
maintenance of hail from relatively discrete storms to the surface,
and outflow-aggregation potential. 60s F surface dewpoints
downshear over northern SD and central/eastern ND will support a
corridor of MLCAPE in the 1000-1500 J/kg range, enabling maintenance
of any forward-propagating cluster into a nominally stabilizing
near-surface environment that still may support severe-gust
potential. Isolated significant-severe (65+ kt) gusts are possible
conditionally almost anywhere in the corridor. However, such a
hazard now appears most probable in a zone bounded by greatest
confidence of maturing upscale organization on the southwest side
(northwestern SD) and most-impactful evening stabilization on the
northeast end (central ND).
...Portions of Ozarks/Mid South...
Widely scattered to scattered, multicellular thunderstorms should
develop today over portions of AR and move northeastward, with
localized wind damage, isolated hail near severe limits, and perhaps
a marginally severe gust or two possible.
Though low-level forcing appears modest over his region, a
combination of antecedent weak MLCINH, rich low-level moisture
(upper 60s to mid 70s F surface dewpoints), and strong diabatic
heating will support development, with aid from outflow/
differential-heating boundaries. Some midlevel cooling is also
expected over the area as the trough approaches, resulting in a net
diurnal steepening of low/middle-level lapse rates, supporting
preconvective MLCAPE commonly in the 2000-2500 J/kg range (locally
higher). In a nearly unidirectional deep-layer wind profile, a
channel of relatively fast (40-50-kt) southerly flow may develop
east of the trough over parts of AR this afternoon, aiding in storm
organization despite modest (25-35-kt) effective shear. Convective
organization should diminish this evening amidst regional
stabilization from both outflow and diabatic near-surface cooling.
..Edwards/Leitman.. 09/04/2023
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