SPC AC 280712

   Day 3 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0212 AM CDT Thu May 28 2020

   Valid 301200Z - 311200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE
   INTERIOR PACIFIC NORTHWEST EASTWARD/SOUTHEASTWARD ACROSS THE
   NORTHERN AND CENTRAL HIGH PLAINS...

   ...SUMMARY...
   Isolated severe thunderstorms are possible from the interior Pacific
   Northwest eastward/southeastward across the northern and central
   High Plains on Saturday.

   ...East Coast into the Southeast...
   At the beginning of the period, a cold front is forecast to extend
   from a surface low over far eastern Quebec southward to Delmarva
   then back southwestward to the central Gulf Coast then arc
   westward/northwestward to a low over the central High Plains. This
   front represents the leading edge of a dry and stable continental
   air mass that will build southeastward from the northern Plains
   through the MS Valley into the Southeast. Showers and thunderstorms
   are possible along and ahead of this front, with the highest
   coverage expected from the Carolinas into the FL Panhandle. However,
   weak vertical shear should preclude storm organization and minimize
   any severe potential across much of the region. The only exception
   is across eastern ME, where stronger vertical shear could support
   more organized storms and higher severe potential. However,
   uncertainty regarding frontal position and storm coverage precludes
   including any severe probabilities with this outlook. 

   ...Pacific Northwest into the High Plains...
   Farther west, compact cyclone over the northern CA coast early
   Saturday is expected to devolve into an open shortwave trough as it
   moves quickly northeastward through the interior Pacific Northwest
   and northern Rockies. Surface low attendant to this upper system is
   expected to deepen in the lee of the Cascades before moving into
   southern British Columbia early Sunday morning. Southeasterly
   surface winds will strengthen across the northern High Plains,
   contributing to aggressive northwestward low-level moisture
   advection. By late Saturday afternoon, mid 50s dewpoints could be in
   place across much of central and eastern MT. Dewpoints closer to 60
   deg F could develop where less boundary-layer mixing takes place
   near the surface low over eastern WA. 

   All of these factors contribute to a fairly anomalous pattern for
   this part of the country. Warm temperatures aloft will keep the area
   capped until the late afternoon/early evening. Therefore, forcing
   for ascent near the surface low and shortwave trough is expected to
   result in thunderstorms. Given the environment, some of these storms
   may be severe. While guidance has been relatively consistent
   regarding this scenario, combination of factors needed to produce
   this environment (i.e. near 60s dewpoints, surface low location,
   late afternoon/early evening shortwave timing) are still uncertain
   enough at this forecast range to only introduce 5% severe
   probabilities. 

   Aforementioned moisture advection combined with a sharpening lee
   trough will likely result in scattered convective initiation along
   the northern and central High Plains. The deeply mixing boundary
   layer and veering wind profiles could contribute to a few stronger
   storms capable of strong wind gusts and hail.

   ..Mosier.. 05/28/2020

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   NOTE: THE NEXT DAY 3 OUTLOOK IS SCHEDULED BY 0730Z