SPC AC 280455

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1155 PM CDT Mon Jul 27 2020

   Valid 291200Z - 301200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS WEDNESDAY
   AFTERNOON AND EVENING ACROSS PARTS OF THE LOWER GREAT LAKES
   REGION...NORTH CENTRAL PORTIONS OF THE HIGHER PLAINS AND PARTS OF
   CENTRAL OREGON...

   ...SUMMARY...
   Strong thunderstorms may impact parts of lower Great Lakes region,
   north central portions of the higher plains, and parts of central
   Oregon Wednesday afternoon and evening, accompanied by at least some
   risk for severe weather.

   ...Synopsis...
   Models continue to indicate that initially broad mid-level
   troughing, with an embedded closed low over Ontario and Quebec, will
   begin undergoing considerable deformation during this period.  While
   being gradually forced eastward within the mid-latitude westerlies,
   it appears that the troughing will elongate along a positively
   tilted axis while approaching the northern Atlantic coast Wednesday
   through Wednesday night.

   Upstream, the relatively deep center of a broad mid-level low over
   the northeastern Pacific may slowly redevelop southeastward. 
   However, associated amplified larger-scale troughing, within a
   southern branch of split westerlies, may move little while awaiting
   the approach of a significant upstream perturbation.  

   Ahead of the eastern Pacific troughing, a short wave trough emerging
   from the subtropics is forecast to continue accelerating toward the
   northern California/Oregon coast.  As this occurs, downstream
   mid-level ridging likely will continue to build through the northern
   intermountain region, to the north of an increasing prominent
   subtropical high center over the Southwest.  

   Within weak flow trending increasing northwesterly, to the east of
   the amplifying ridge, an elongated cyclonic circulation is forecast
   to dig to the lee of the Wyoming/Colorado Rockies.  It appears that
   this impulse will be preceded by at least a couple of additional
   convectively generated or enhanced perturbations within a belt of
   weak, broadly cyclonic flow extending across south central portions
   of the Great Plains and Ozark Plateau into the Southeast.

   ...Lower Great Lakes...
   Relatively cool mid-level air and seasonably strong (30-50 kt at 500
   mb) mid-level flow, associated with a perturbation progressing
   through the base of the large-scale troughing, may enhance
   convective potential to the lee of Lakes Erie and Ontario by
   Wednesday afternoon.  As insolation contributes to CAPE on the order
   of 500-1000 J/kg, the lake breezes may provide a focus for
   thunderstorm initiation, before activity propagates
   east-southeastward.  The environment may become favorable for the
   production of small to marginally severe hail and locally strong
   wind gusts in stronger convection.

   ...North central portions of the higher plains...
   Beneath elevated mixed-layer air edging eastward mostly to the lee
   of the Front Range, models indicate a corridor of stronger daytime
   heating and deeper boundary-layer mixing, with sufficient low-level
   moisture to support CAPE on the order of 1000-2000 J/kg.  Aided by
   at least modest deep-layer shear (largely due to pronounced veering
   of wind profiles with height, beneath westerly to northwesterly
   mid-level flow), and large-scale ascent associated with the digging
   mid-level impulse, the environment may become conducive to upscale
   growing clusters of storms capable of producing strong to locally
   severe wind gusts late Wednesday afternoon into early evening.

   ...Oregon...
   Models now appear at least a bit slower with the approach of the
   mid-level short wave emerging from the subtropical eastern Pacific. 
   However, the NAM and NCEP SREF in particular, suggest that moisture
   may be sufficient, with strong daytime heating along/east of the
   Cascades, to support CAPE of 500-1000 J/kg by late Wednesday
   afternoon.  Coupled with strengthening of south-southwesterly
   mid/upper flow, and aided by orographic forcing, the environment
   could still become conducive to a couple of strong storms which may
   pose some risk for severe wind and hail.

   ..Kerr.. 07/28/2020

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