SPC AC 270711

   Day 3 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0211 AM CDT Mon Jul 27 2020

   Valid 291200Z - 301200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS WEDNESDAY
   AFTERNOON AND EVENING ACROSS THE BLACK HILLS VICINITY SOUTHWARD INTO
   THE CENTRAL HIGH PLAINS...AS WELL ACROSS PARTS OF INTERIOR OREGON...

   ...SUMMARY...
   Strong thunderstorms may impact parts of western South Dakota and
   Nebraska, ajacent portions of the Great Plains, as well as parts of
   central Oregon, late Wednesday afternoon and evening, accompanied by
   at least some risk for severe weather.

   ...Synopsis...
   Initially broad mid-level troughing, centered over Ontario and
   Quebec, may begin undergoing considerable deformation while being
   gradually forced eastward within the slowly progressive mid-latitude
   westerlies during this period.  Upstream, models indicate that there
   will be some further southeastward progression of a broad and
   relatively deep mid-level low over the northeastern Pacific.  Ahead
   of this feature, a short wave trough emerging from the subtropical
   eastern Pacific may accelerate northeastward, and inland of the
   northern California coast by late Wednesday night.  As this occurs,
   downstream mid-level ridging may continue to build through the
   northern intermountain region, to the north of an increasingly
   prominent mid-level high over the Southwest.  Meanwhile, within weak
   flow trending increasingly northwesterly to the east of the ridging,
   an elongated cyclonic circulation may slowly dig to the lee of the
   northern Rockies, through the Black Hills vicinity and adjacent
   portions of the higher plains.

   ...Western South Dakota/Nebraska vicinity...
   A corridor of stronger daytime heating/deeper boundary-layer mixing
   may develop across the region by late Wednesday afternoon, in
   advance of the digging perturbation, with low-level moisture
   sufficient to support CAPE in excess of 1000 J/kg.  Aided by
   favorable large-scale lift, and orographic forcing across the Black
   Hills, considerable thunderstorm development appears possible. 
   Particularly south of the Black Hills, modest northwesterly
   mid-level flow may allow for the evolution of organizing clusters
   capable of producing strong to severe wind gusts into Wednesday
   evening.

   ...Oregon...
   At least some model output suggests that moistening in advance of
   the subtropical perturbation may become sufficient to support
   mixed-layer CAPE up to around 1000 J/kg, to the lee of the Cascades
   by late Wednesday afternoon.  As the occurs, orographic forcing,
   coupled with lift associated with the approaching impulse by
   Wednesday evening, probably will support at least scattered
   thunderstorm development.  It is possible that strengthening
   mid/upper flow will contribute to sufficient deep-layer shear to
   support supercell structures, which could pose a risk for severe
   hail and strong surface gusts, before boundary-layer instability
   wanes Wednesday night.

   ..Kerr.. 07/27/2020

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