SPC AC 181245

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0745 AM CDT Tue Aug 18 2020

   Valid 181300Z - 191200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THIS
   AFTERNOON/EVENING FOR PARTS OF THE INTERIOR NORTHWEST...AZ...AND
   FROM THE CENTRAL DAKOTAS TO THE CENTRAL HIGH PLAINS...

   ...SUMMARY...
   Isolated damaging winds will be possible this afternoon/evening from
   eastern Oregon into parts of the northern Rockies, and in Arizona
   and vicinity.  Isolated large hail and damaging winds may also occur
   in a north-south corridor from the central Dakotas into the central
   High Plains.

   ...Northern Plains to the central High Plains through tonight...
   A north-south band of elevated thunderstorms, associated with warm
   advection in the 850-700 mb layer, may persist into the day across
   SD/NE while continuing to move toward the south-southeast.  There
   may be enough CAPE rooted aloft for a marginal hail threat with the
   continuation of the morning storms.  Otherwise, some increase in
   low-level moisture is expected today with southerly low-level flow
   and evapotranspiration along the baroclinic zone across the central
   Dakotas/NE.  Uncertainty is fairly high regarding details of any
   afternoon thunderstorms along the baroclinic zone, given a lingering
   cap and only subtle perturbations aloft moving around the northeast
   periphery of the strong Great Basin high.  If storms form across the
   central Dakotas, the storm environment appears conditionally
   favorable for supercells (MLCAPE up to 1000 J/kg and effective bulk
   shear of 40-50 kt) with an attendant threat for isolated damaging
   winds and large hail.

   Farther southwest into the central High Plains, strong surface
   heating and deep mixing may support isolated high-based thunderstorm
   development along the surface lee trough later this afternoon. 
   Inverted-v profiles will favor a threat for isolated strong outflow
   gusts.  There is also some potential for a small cluster or two to
   persist into tonight, in association with low-level warm advection
   on the nose of a 30+ kt low-level jet, and in advance of an embedded
   speed max moving over WY (over eastern OR/ID this morning). 

   ...Eastern OR to southwestern MT this afternoon/evening...
   The relatively static large-scale pattern will maintain a plume of
   midlevel moisture and embedded perturbations from northern CA into
   the northern Rockies through tonight.  The most prominent embedded
   speed max will move from eastern OR this morning to southwestern MT
   by midday/early afternoon, with accompanying clouds and a bit ahead
   of the diurnal heating cycle, so storm development with this initial
   speed max is somewhat in question.  Still, the lingering moisture
   plume and stronger surface heating from west-to-east as clouds thin,
   as well as other subtle embedded speed maxima, should support
   thunderstorm development this afternoon in a few clusters from OR
   into ID/MT.  Inverted-v profiles with SBCAPE near 500 J/kg and PW
   values near 1", along with 30+ kt midlevel flow, could again support
   at least isolated strong-severe outflow gusts with high-based
   convection. 

   ...AZ and vicinity this afternoon/evening...
   Similar to the prior two days, a strong midlevel high will remain
   anchored over NV/UT, with a belt of 20-30 kt east-northeasterly flow
   around its periphery over AZ.  Meanwhile, overnight convection was
   not widespread and with only a small amount of cloud debris, strong
   surface heating will again drive hot surface temperatures and deep
   inverted-v profiles.  PW values remain in the 1.1-1.25" range this
   morning with boundary-layer dewpoints in the low-mid 50s off the
   Mogollon Rim, which will be sufficient for afternoon MLCAPE near 500
   J/kg.  The somewhat modest PW/buoyancy will again keep storm
   coverage somewhat scattered, but DCAPE in excess of 1500 J/kg will
   favor isolated strong downburst/outflow potential with clusters of
   storms moving southwestward off the higher terrain this
   afternoon/evening.

   ...Portions of the Southeast this afternoon...
   Scattered-numerous storms are expected this afternoon along a
   diffuse front across southern GA and vicinity.  Thermodynamic
   profiles could support strong downbursts with isolated tree damage,
   but severe outflow winds should remain very isolated.  Somewhat
   stronger flow is expected farther north across KY with a weak
   shortwave trough.  Storms with gusty outflow winds may occur, but
   the threat for severe storms will likewise remain quite low.

   ..Thompson/Gleason.. 08/18/2020

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