SPC AC 191256

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0756 AM CDT Wed Aug 19 2020

   Valid 191300Z - 201200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
   INTO EARLY TONIGHT ACROSS NORTHERN MN...FROM THE NORTHERN ROCKIES TO
   THE CENTRAL HIGH PLAINS...AND FROM CENTRAL AZ TO SOUTHEASTERN CA...

   ...SUMMARY...
   Isolated large hail and damaging winds will be possible across
   northern Minnesota through early tonight.  Isolated strong/locally
   severe wind gusts will be possible within an arc from the northern
   Rockies to the central High Plains, and from central Arizona into
   southeastern California.

   ...Northern MN through tonight...
   A subtle midlevel trough over southern MB will continue
   east-southeastward over northern MN to Lake Superior by tonight.  In
   advance of this midlevel trough, a west-southwesterly low-level jet
   segment and associated warm advection will develop eastward toward
   the MN Arrowhead through the day.  Given a plume of steep midlevel
   lapse rates from the west, atop boundary-layer dewpoints in the mid
   60s, there will be some potential for the ongoing convection across
   northwest MN to persist into the day with isolated large hail. 
   Additional convection (either ongoing or new development) is
   expected to move southeastward into the MN Arrowhead later today
   into early tonight from western ON, in association with the midlevel
   trough.  Effective bulk shear near 30 kt on the east edge of the
   steeper lapse rates and MLCAPE of 1500-2000 J/kg will support a mix
   of supercells and multicell clusters capable of producing large hail
   and damaging winds.  The influence of the morning convection on
   afternoon destabilization, and the track/evolution of the ON storms
   both add uncertainty to the forecast.  Thus, will maintain MRGL in
   this update, but a SLGT may become necessary in later day 1 updates.

   ...Northern Rockies to the central High Plains this
   afternoon/evening...
   Scattered clusters of high-based storms are again expected in an arc
   from the northern Rockies to the central High Plains this afternoon
   into this evening, around the northern and eastern periphery of the
   persistent Great Basin midlevel high.  Low-midlevel moisture will be
   rather modest with PW values generally near or below 1", which will
   tend to limit CAPE.  However, typical inverted-v profiles will
   support strong outflow winds.  Somewhat greater chances for storms
   are expected in conjunction with diurnal heating and embedded speed
   maxima, across southwestern MT and adjacent ID, and near/east of the
   CO Front Range.

   ...Central AZ to southeast CA this afternoon/evening...
   The strong midlevel high over the Great Basin is shifting gradually
   southward as height falls occur over the Pacific Northwest.  This
   has shunted the slightly stronger midlevel easterly flow and
   low-midlevel moisture a bit southward, which will tend to limit the
   potential for more intense/organized storm clusters to move off the
   higher terrain and into the lower deserts.  Still, deep inverted-v
   profiles with at least weak surface-based buoyancy could support
   very isolated instances of strong downburst winds.  Thermodynamic
   profiles will also favor isolated strong downbursts farther west
   into southeast CA, in association with a diffuse MCV moving westward
   over the CO River Valley this morning.

   ..Thompson/Gleason.. 08/19/2020

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