SPC AC 300539

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1239 AM CDT Sat May 30 2020

   Valid 301200Z - 311200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE PACIFIC
   NORTHWEST...

   ...SUMMARY...
   Scattered severe thunderstorms are forecast this afternoon/evening
   across the Pacific Northwest. A few strong storms are also possible
   across the northern Rockies and southeastern US.

   ...Pacific Northwest...

   Late-evening water-vapor imagery depicts a well-defined upper low
   off the CA Coast, lifting north-northeastward in line with 00z model
   guidance. Strong mid-level 12hr height falls will spread across the
   Pacific Northwest during the day as 500mb speed max translates
   across western NV into southeastern OR by late afternoon. Strong
   large-scale forcing for ascent will be noted within the exit region
   of this speed max, and latest data suggests clusters of strong
   convection will likely develop around 18z over south-central OR,
   then spread north toward BC during the mid evening hours. While PW
   is not expected to be particularly high, it appears a corridor of 1+
   inch values should result in MLCAPE on the order of 1500 J/kg where
   surface-based convection develops ahead of the speed max. Forecast
   profiles suggest supercells are possible, especially early in the
   convective cycle. However, clustering may ultimately lead to an MCS
   that could prove efficient in generating strong winds, especially if
   a bow-type squall line evolves.

   Downstream across the northern Rockies, diurnal heating and
   orographic forcing will be the primary drivers in thunderstorm
   development. However, short-wave ridging and the lack of large-scale
   support suggest this activity will remain isolated.

   ...Southeastern US...

   Southern influence of northeastern US upper trough will affect the
   southern Middle Atlantic into the Carolinas later today. Weak
   low-level convergence along the synoptic front should settle into
   southeastern VA - central Carolinas. This boundary is expected to
   aid isolated thunderstorms that could produce gusty winds and
   marginally severe hail.

   ...Central CA...

   Very cold mid-level temperatures will spread across central CA early
   this afternoon with 500mb readings north of the jet expected near
   -20C. As a result, steep lapse rates should prove adequate for a few
   robust thunderstorms with the potential for small hail. At this time
   it appears instability will be too meager to warrant severe
   probabilities for hail >1 inch.

   ..Darrow/Nauslar.. 05/30/2020

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