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Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0753 AM CDT Sat Aug 27 2022

Valid 271300Z - 281200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
IOWA...SOUTHERN MINNESOTA AND WESTERN WISCONSIN...

...SUMMARY...
Severe thunderstorm potential appears most concentrated today over
parts of Iowa, southern Minnesota and western Wisconsin.

...Synopsis...
The middle/upper-level pattern will beco*e more zonal and
shortwave-dominated this period, with large-scale height rises
filling a prior eastern mean trough, and nearly co*pleted breakdown
of former western CONUS ridging.  A leading, convectively enhanced,
northern-stream perturbation over parts of eastern ND and MN will
eject northeastward over the Northwest Angle and Boundary Waters
regions to northwestern ON through the afternoon, while losing
amplitude.  The next shortwave trough in the series -- apparent in
moisture-channel imagery over southern SK and the MT/ND border area
-- is a former cut-off low emanating from the Pacific Northwest. 
This feature should move east-northeastward to southern MB and
eastern ND by 00Z, then eject across northwestern ON and weaken.

The third and strongest northern-stream trough -- initially located
over central/southern BC and inland WA/OR -- will amplify through
the period as it moves east-southeastward.  By 00Z, the trough
should extend from southern MB across ID.  By 12Z, it should extend
from a newly formed closed low over southern SK, southward across
east-central MT to central WY.  At somewhat lower latitudes, a
trough now apparent over the central High Plains will move east-
northeastward by 00Z, to a position from south-central MN to central
KS, with a weak vorticity lobe trailing toward the TX Panhandle.  By
12Z, this trough should reach western WI, central IA, eastern KS,
and northern OK.

On the 11Z surface analysis, a cold front was drawn from near Cape
Cod southwestward over central VA, beco*ing wavy/quasistationary
near a line from TRI-MVN-UIN, then a warm front northwestward to a
triple point near FAR, and an occluded front to a low over southern
SK (stacked beneath the remnant mid/upper low).  A weak,
quasistationary front and surface trough extended from that triple
point southwestward over western KS, and should move little today. 
The warm front will move northeastward across central/southern MN,
eastern IA, and western WI through tonight.

...Upper Midwest to southern High Plains...
An elevated field of warm advection, moisture transport, and lift to
LFC -- related to the leading mid/upper-level shortwave perturbation
-- is contributing to a swath of nonsevere thunderstorms from
northeastern MN to parts of western WI.  Through the remainder of
the morning, this activity should proceed eastward to northeastward
and weaken, as both the LLJ and large-scale ascent fields preceding
the trough lessen in intensity.

This afternoon, isolated to scattered thunderstorms with at least
marginal severe hail/gust potential are expected to develop in a
corridor of relatively maximized low-level lift -- arising from a
co*bination of subtle low-level convergence, diurnal heating to
convective temperature, and large-scale ascent/DCVA preceding mainly
the central Plains shortwave trough.  This should occur as the lift
aloft overlaps a diabatically heated plume of favorable boundary-
layer moisture in the outlook area -- characterized by 60s F surface
dewpoints from MN to portions of central/eastern KS, and patchy 50s
to low 60s over the southern High Plains amid diurnal mixing.

The greatest overlap of favorable parameters and relatively dense
convective concentration should be from central IA and southern MN
area to the warm-frontal zone in western WI, where deep lift, low-
level shear and moisture each will be relatively maximized. 
Confidence in convective coverage in this area has increased enough
for a 15% unconditional probability.  Despite some moderate to dense
mid/upper-level cloud cover, sufficient heating and warm advection
should occur to support essentially uncapped surface-based parcels
with steep low/middle-level lapse rates, and peak/preconvective
MLCAPE in the 2000-3000 J/kg range.  Buoyancy and lift each will
diminish southwestward into southeastern NE and northern KS, which
may be a relative minimum in severe potential (but will keep a
marginal risk for the time being, given at least isolated strong-
severe convection possible).  Farther southwest, despite weaker
moisture and shear, enough diurnal heating is expected, through
thinner mid/upper cloud cover, to yield a well-mixed boundary layer
and enough buoyancy for isolated severe potential into early evening
(mainly gusts, but hail cannot be ruled out).

...Southern/eastern MT and vicinity...
Isolated to widely scattered, high-based thunderstorms should
develop this afternoon over the higher terrain around Yellowstone
and the Beartooth Mountains.  Strengthening large-scale ascent --
amidst height falls that precede the Northwest trough -- will
overlap favorable diurnal heating and weak low-level moisture to
support this activity.  Convection should move east-northeastward
then eastward over well-mixed, deep subcloud layers at lower
elevations, offering sporadic strong to isolated severe downbursts
as it expands laterally/upscale.  Lack of greater moisture will
limit buoyancy substantially, with peak/preconvective MLCAPE
generously forecast to 300-500 J/kg.  Deep-layer flow should be
nearly  unidirectional/westerly, encouraging activity to move
quickly eastward before it encounters prohibitively stable air over
the western Dakotas area this evening.

..Edwards/Goss.. 08/27/2022