ACUS01 KWNS 281253
SWODY1
SPC AC 281252

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0752 AM CDT Wed Aug 28 2024

Valid 281300Z - 291200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE OHIO
VALLEY TO THE NORTHERN MID-ATLANTIC COAST...AND ACROSS PARTS OF
NORTH AND SOUTH DAKOTA...

...SUMMARY...
Severe thunderstorms are expected from the Ohio Valley to the
northern Mid-Atlantic Coast, and across parts of North and South
Dakota.

...Synopsis...
In mid/upper levels, a progressive northern-stream pattern will
prevail through the period, with the main feature being a cyclone
now apparent in moisture-channel imagery near the southeastern
corner of BC.  The associated 500-mb low should move eastward
roughly along, or just north of, the international border through
the period, with the accompanying trough extending south-
southwestward.  By 00Z, the low should be near the southwestern
corner of SK, with trough to northern UT.  By 12Z, the low should
reach extreme south-central SK, with trough across central MT to
southwestern WY.

Downstream, a complex but much lower-amplitude shortwave trough was
evident over Lake Superior and adjoining portions of ON and western
U.P. of MI.  One part of this perturbation should break eastward to
the St. Lawrence Valley by the end of the period, while the trailing
part drifts eastward over Lake Superior and the U.P.  Of perhaps
greater direct influence on convective potential, an MCV was
apparent in composited radar imagery over southeastern IA.  The
associated perturbation should move across much of central IL today,
reaching western OH by 12Z tomorrow.

The 11Z surface analysis showed a cold front from northern ME west-
southwestward across Lake Ontario and southern Lower MI, becoming
quasistationary for the time being across southern parts of Lake
Michigan and WI, to northeastern NE, then a warm front over
southwestern SD, to a low over southeastern MT.  This front is
expected to move southeastward by 00Z to coastal NS, Cape Cod, Long
Island, northern parts of MD/WV, and southern parts of OH/IN/IL,
becoming a diffuse warm front northwestward over central IA,
southwestern MN and central ND.  There it will be overtaken by
another cold front -- initially analyzed through the low over
eastern MT and across northwestern WY.  By 00Z, this second,
stronger cold front should extend from central ND across central/
southwestern SD to east-central WY.  By 12Z, the western cold front
should extend from a low over southeastern ND or northeastern SD
across south-central SD, western NE and northeastern CO.

...Ohio Valley to Mid-Atlantic...
Widely scattered to scattered thunderstorms are expected along and
south of the front today from IN to the Mid-Atlantic Coast, and
perhaps the southern New England Coast/Long Island vicinity.
Damaging gusts and isolated large hail will be possible.

Coverage of convection is uncertain, but relative concentrations may
develop downshear from what is now disorganized/nonsevere convection
over OH (in the same UVV field), and ahead of the MCV over parts of
IL/IN, becoming organized across parts of IN/OH. Diurnal heating of
a favorably moist airmass is expected to weaken already modest
ambient MLCINH along the outlook corridor by late morning.  With
lift along the front and prefrontal outflow/differential-heating
boundaries,  growth of morning to early afternoon development into
one or more complexes appears to be a plausible scenario, rendering
a greater wind than hail threat.  Surface dewpoints commonly in the
mid 60s to low 70s F will offset modest mid/upper-level lapse rates
enough to contribute to peak/preconvective MLCAPE in the 800-1500
J/kg range from OH eastward, and 1000-2000 J/kg back into portions
of IN.  Weak, generally veered low/middle-level flow will limit
vertical shear.  Still, nearly unidirectional westerly flow through
a deep layer will foster movement of convection parallel to the
low-level moisture-instability axis.

...Dakotas...
A line or arc of scattered thunderstorms should develop along/near
the cold front during late afternoon into evening, perhaps initially
over ND where deep-layer forcing will be stronger, then southward
into greater capping in SD, as the front impinges on greater
moisture and lingering boundary-layer instability.

Ahead of the cold front, enough easterly component to near-surface
flow will develop to support moist advection into the convective
genesis zone, with surface dewpoints into the 60s F becoming common.
Capping at the base of a modest EML will suppress development for
much of the afternoon, especially over SD, but once frontal lift
overcomes it, a corridor of 2000-3000 J/kg MLCAPE should be
available southwest of (and parallel to) the warm front.  Though the
strongest flow aloft and large-scale ascent will remain behind the
cold front, the eastern rim of stronger height falls will spread
over the frontal zone, with enough mass response to both foster
favorable vertical shear for supercells and enhanced storm-relative
flow in low levels.  Effective-shear magnitudes of 30-40 kt (locally
higher) and broad low-level hodographs along and south of the warm
front should foster supercell development, until mergers and upscale
growth occur.  Large to very large hail and severe gusts will be the
main concerns, though a tornado is possible.  However, relatively
weak midlevel flow may encourage a transition to messier, quasi-
linear mode with embedded LEWP/bow formations for a few hours.
Activity should weaken overnight as it moves into stabilizing
boundary-layer air, though a 45-55-kt LLJ may encourage persistence
of a cluster or two tonight into the eastern Dakotas.

..Edwards/Bentley.. 08/28/2024

$$
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