ACUS01 KWNS 261300
SWODY1
SPC AC 261258

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0758 AM CDT Mon Aug 26 2024

Valid 261300Z - 271200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
SOUTH DAKOTA...EXTREME NORTHERN NEBRASKA...MINNESOTA...AND WISCONSIN...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
ENHANCED RISK...AND ACROSS PORTIONS OF NEW ENGLAND...

...SUMMARY...
Severe thunderstorm gusts, large to very large hail and a few
tornadoes are possible today and tonight across parts of South
Dakota, extreme northern Nebraska, Minnesota, and Wisconsin.

...Synopsis...
In mid/upper levels, a persistent and large anticyclone has shifted
its center across the southern Plains to the lower Missouri Valley,
with ridging northeastward across the Lake Superior and James Bay
regions.  The ridge will shift eastward across the Upper Great Lakes
and southern QC through the period.  This will occur in response to
these developments on either side:

1.  A broad but not very intense cyclone -- initially centered near
the western ME/QC border -- will pivot roughly eastward to the
adjoining parts and the Canadian Maritimes through the period.  As
this occurs, a shortwave trough/vorticity lobe -- now in the western
semicircle of the cyclone over the St. Lawrence Valley -- should dig
southward across eastern NY and extreme western New England today,
then weaken and move offshore from the southern New England Coast
tonight.

2.  A strong shortwave trough -- initially over western MT,
southeastern ID and northern UT -- should move to eastern MT and
central WY by 00Z, then to the western Dakotas within 12 more hours.
This will foster the northeastward ejection of two weak
perturbations, now apparent in moisture-channel imagery over the
western NE Panhandle/southeastern WY region into southwestern SD,
the other over central/eastern CO.  By 00Z, these features should
reach eastern ND and eastern/southern SD, respectively.

At the surface, 11Z analysis showed a low near FSD, with cold front
arching southwestward across the NE Sandhills then northwestward to
another low over east-central WY near CPR.  A quasistationary
frontal zone was drawn from the low northeastward over western/
northern MN.  The low should move slowly eastward today while the
poleward frontal extension moves little, and the cold front moves
southeastward into northwestern IA and eastern/central NE.  The
western segment of that front should hang up over northwestern NE
and east-central WY amid mass response to the approaching shortwave
trough, which also should induce a surface trough and perhaps
secondary/reinforcing baroclinic zone across the lee of the
north-central Rockies over eastern WY and into southeastern MT.

...North-central Plains to Upper Midwest...
Two main rounds of severe-thunderstorm potential are apparent today,
each crossing portions of the main risk area, which represents an
aggregated severe potential from both complexes.  From east to west:

1.  Strong-severe thunderstorms should develop over eastern SD near
or north/northwest of the surface low this afternoon -- perhaps
arising as early and far west as central SD during midday.  This
activity will be related to maximized low-level lift associated with
the low, and a plume of large-scale ascent preceding the weaker
shortwave troughs, and already supporting convection at this time
over western SD.  Whether the ongoing convection will be continuous
with later activity is uncertain.  In either event, this eastern
complex should intensify substantially over the southeastern
SD/southwestern MN/northwestern IA vicinity as it crosses the
low-level baroclinic zone and encounters a strongly unstable, richly
moist boundary layer, with upper 60s to mid 70s surface dewpoints.
Beneath steep lower-middle-level lapse rates, this should contribute
to a corridor of 3500-5000 J/kg MLCAPE near I-90 from the FSD area
to near the Mississippi River, decreasing to a still-favorable
2000-4000 J/kg in central MN.  Supercells with destructive hail,
potentially significant-severe gusts, and a few tornadoes are
possible, though so is fairly quick upscale expansion.

2.  An area of convection may develop as early as late morning to
midday well behind the surface cold front, in higher terrain of
northern/central WY, initially with a marginal severe threat.  This
convection should be supported by at least modest low/middle-level
moisture, and strong cooling/destabilization aloft, related to the
swath of strong large-scale DCVA/ascent immediately preceding the
northern Rockies shortwave trough.  This activity may be an
outgrowth or extension of nonsevere thunderstorms now over extreme
western WY near the Star and Hams Fork Valleys and adjacent ranges.
These thunderstorms should organize into an MCS and expand/intensify
as activity proceeds into greater moisture, instability and
storm-relative/post-frontal low-level flow to the east, on the High
Plains and near the Black Hills.

Northeasterly to easterly flow behind the front and near the trough,
beneath strengthening southwest flow aloft, will support 45-60 kt
effective-shear magnitudes.  Buoyancy will increase eastward as the
convection -- which may already have evolve into an outflow-dominant
arc by the time it gets into greater moisture east of the Black
Hills -- further intensifies into the northern part of a corridor of
1500-2000 J/kg MLCAPE and still-favorable storm-relative boundary-
layer flow.  Some of this convection may be supercellular to the
extent it can remain at least somewhat discrete, with severe wind
and large to very large hail possible. With time, wind will dominate
as the main threat.  This activity should encounter the outflow from
the other complex somewhere over southeastern SD or southern MN
tonight.

Mesoscale uncertainty over the positioning and strength of the
outflow (and thus threat of severe to its poleward side) preclude
greater unconditional probabilities at this time.  Still, a focused
corridor of destructive winds remains a possibility across southern
parts of SD/MN, some of it potentially affecting areas struck by the
first complex.

...Northeastern CONUS...
Widely scattered to scattered thunderstorms should develop over
portions of eastern NY and New England, as well as parts of
western/central PA and northwestern MD -- each moving southward to
southeastward through a favorable airmass for organized convection.
Strong to locally severe gusts are possible, along with severe hail.

Over the New England part of the outlook, cold air aloft, overlying
low-level moisture represented by mid/upper 50s F surface dewpoints,
should combine with limited diurnal heating to remove MLCINH, and
support peak/preconvective MLCAPE ion the 1000-1500 J/kg range.
Though low-level flow will be modest, relatively straight/lengthy
hodographs will support some hail potential.  Further southwest,
greater low-level moisture (dewpoints mostly mid/upper 60s to near
70 F at lower elevations) but weaker mid/upper-level lapse rates
will be present, yielding similar CAPE values but with different
vertical distribution of buoyancy (more skewed to low levels).
Still, enough deep/speed shear will be present to support organized
multicells and at least transient/isolated supercells.

..Edwards/Bentley.. 08/26/2024

$$
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