SPC AC 271253

   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 become more zonal and
   shortwave-dominated this period, with large-scale height rises
   filling a prior eastern mean trough, and nearly completed 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, becoming 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
   combination 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

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