SPC AC 301947

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0247 PM CDT Sat May 30 2020

   Valid 302000Z - 311200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
   OF CENTRAL WASHINGTON AND NORTH-CENTRAL OREGON...

   ...SUMMARY...
   Scattered to numerous severe thunderstorms, offering destructive
   winds and hail, are expected to continue through roughly 10 PM PDT
   over parts of the interior Pacific Northwest, east of the Cascades.

   ...Discussion...
   Little change is being implemented to the ongoing outlook at 20Z, as
   the current forecast remains reflective of expectations.  

   Very isolated/low-end severe risk may persist over southeast VA and
   the coastal Carolinas into early evening, with no adjustments to the
   existing outlook needed.

   In the Pacific Northwest, an initial severe storm has developed over
   northern Klamath county, with additional development expected with
   time.  Latest CAM runs show a bit less tendency for upscale growth
   into an MCS later this afternoon and evening as storms spread
   northward.  As such, current wind probability appears sufficient at
   30%.  A northward nudge in 5% tornado probability is being effected
   due to this more cellular trend in forecast storm mode, though
   overall tornado risk still appears limited -- tempered by the
   general high-based nature of the convection.

   ..Goss.. 05/30/2020

   .PREV DISCUSSION... /ISSUED 1131 AM CDT Sat May 30 2020/

   ...Interior Northwest...
   Primary change is to increase tornado probabilities across parts of
   central OR. Categorical outlook is largely unchanged. Consideration
   was given for an upgrade to Moderate Risk in the western portion of
   the Columbia Basin. Concerns over the spatial extent of the severe
   wind coverage preclude further upgrading at this time.

   Thunderstorms will likely intensify after 19Z in a zone of strong
   deep lift across south-central OR, ahead of an ejecting shortwave
   trough. Here, robust diabatic heating is underway amid a plume of
   low to mid 50s surface dew points. This will support a narrow
   corridor of modest buoyancy with MLCAPE reaching 750-1500 J/kg from
   central to northeast OR. Greater buoyancy is expected to develop
   towards 00Z farther downstream in eastern WA to northwest MT where
   mean-mixing ratios are higher.

   Surface-based effective-inflow parcels will exist on the west side
   of the surface front, where low-level winds will be veering with
   height, hodographs strongly curved, and deep shear relatively
   maximized. With low-level vorticity also maximized in the frontal
   zone, multiple supercells are expected. Though the environment
   generally favors higher-based convection, initial supercells should
   have the best potential for large hail and a couple tornadoes
   between about 21-23Z. As the supercells spread rapidly
   north-northeast, at peak boundary-layer heating over the Columbia
   Basin, upscale growth into a forward-propagating MCS appears
   probable. This will increase the potential for severe wind gusts,
   some of which may be significant, given the favorable combination of
   steep lapse rates and moderately large buoyancy. Overall setup
   appears likely to yield a swath of severe wind in the Columbia Basin
   from north-central OR across parts of central and eastern WA before
   the MCS moves into British Columbia.

   ...Northern Rockies to central High Plains...
   Isolated to scattered thunderstorms will develop this afternoon near
   the large-scale ridge, primarily over higher terrain. Convection
   should spread slowly off the mountains in a loosely fan-shaped
   pattern across the outlook corridor: to the north-northeast in the
   ID/MT region, and east-southeast across the central High Plains.
   Isolated severe is possible as the convection moves across a
   marginally moist, but strongly heated boundary layer supporting
   maintenance of hail/gusts to the surface. The lack of substantial
   upper support and ridging-related weaknesses in low/middle-level
   wind speed will be limiting factors to a greater severe risk.

   ...Carolinas/Southeast VA...
   Isolated to scattered thunderstorms are expected this afternoon over
   portions of the Piedmont and coastal plain, moving east to
   southeast. Weak winds through most of the troposphere will limit
   organization of the severe threat. However, sporadic tree damage
   from localized strong gusts will be possible.

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