SPC AC 180537

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1237 AM CDT Tue Aug 18 2020

   Valid 181200Z - 191200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE NORTHERN INTER-MOUNTAIN REGION...NORTHERN/CENTRAL GREAT
   PLAINS...AND OVER ARIZONA...

   ...SUMMARY...
   Isolated severe thunderstorms -- accompanied by some risk for
   damaging winds -- are forecast to spread across parts of the
   northern inter-mountain region, northern/central Great Plains, and
   over portions of Arizona. Gusty winds are the primary threat.

   ...Discussion...

   Dominant upper ridge centered over the Great Basin will move little
   over the next 24 hours. Several weak mid-level disturbances will be
   forced around this feature; most will at least enhance the prospect
   for convection capable of producing gusty winds. As a result, no
   appreciable day-to-day changes are expected between Monday and
   Tuesday.

   While large-scale subsidence will suppress convection across the
   Great Basin region, strong boundary-layer heating around the
   periphery will prove instrumental in destabilization necessary for
   isolated high-based thunderstorm development. The strongest signal
   for potential organized convection will be across central/eastern OR
   into central ID. Thunderstorms are expected to develop beneath a
   belt of strong mid-level flow characterized by 40 kt at 500 mb.
   Forecast soundings suggest organized, potentially rotating updrafts
   are possible. Latest CAMs suggest robust convection is once again
   expected to develop over central/eastern OR, then spread
   east-northeast atop the Great Basin ridge. Damaging wind gusts are
   the primary threat with this activity.

   Downstream, weak low-level warm advection combined with strong
   surface heating across the western Dakotas should be adequate for
   isolated, potentially severe convection by late afternoon. While
   surface parcels may approach their convective temperatures over the
   western Dakotas, thunderstorms may actually be slightly elevated
   downstream, driven partially by weak isentropic ascent.

   Another region where a few strong storms may produce a few severe
   wind gusts will be across AZ. High-based convection developed over
   the Mogollon Rim Monday afternoon which spread toward lower
   elevations during the evening. This activity did not produce an
   expansive cloud-debris field and strong boundary-layer heating is
   once again expected later today. Large-scale steering currents will
   change little and convection should once again spread off the Rim
   toward the lower deserts during the early evening.

   ..Darrow/Bentley.. 08/18/2020

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