SPC AC 270724

   Day 3 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0224 AM CDT Wed May 27 2020

   Valid 291200Z - 301200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
   OF THE NORTHERN MID-ATLANTIC AND NORTHEAST STATES AND ACROSS
   PORTIONS OF OREGON...

   ...SUMMARY...
   Isolated severe thunderstorms are possible across portions of the
   northern Mid-Atlantic and Northeast States Friday afternoon through
   Friday evening. Isolated strong to severe storms are also possible
   across portions of Oregon Friday evening.

   ...Northern Mid-Atlantic into the Northeast States...
   A pair of shortwave troughs will contribute to broad cyclonic flow
   over the central and eastern CONUS early Friday morning.
   Southernmost shortwave initially over the lower OH Valley is
   expected to quickly eject northeastward through the Lower Great
   Lakes/Northeast. This shortwave will eventually merge with the more
   northerly shortwave moving through the Upper Great Lakes and into
   the Northeast/Quebec. At the surface, a cold front is forecast to
   move into the region Friday evening, continuing eastward overnight
   and reaching the coastal areas by early Saturday morning.

   A relatively moist air mass is expected to be in place over the
   region Friday, with afternoon dewpoints in the mid to upper 60s.
   Temperatures are forecast to climb into the upper 70s and low 80s
   ahead of the front. This combination of warm temperatures and ample
   low-level moisture will contribute to moderate instability. At the
   same time, enhancement of mid-level flow attendant to the
   approaching upper trough will spread eastward/northeast over the
   region. The resulting combination of instability and vertical shear
   supports the potential for some strong to severe storms. The highest
   storm coverage is anticipated along the front, but some pre-frontal,
   warm-sector storms are also possible. Primary severe threat is
   currently expected to be damaging wind gusts. Guidance differs on
   the surface wind direction ahead of the front, which leads to
   varying forecast of low-level shear and resulting tornado potential.
   Current expectation for surface winds to be more southwesterly than
   southerly. Even so, a non-zero tornado threat exists, particularly
   in areas where more southerly surface winds are realized.

   ...Pacific Northwest...
   A compact upper cyclone is forecast to move northeasterly toward the
   central CA coast from Friday evening/overnight into Saturday
   morning. The progression of this system will result in mid-level
   moisture advection into the Pacific Northwest, beginning on Friday
   morning. By Friday afternoon, the combination of this mid-level
   moisture and deep boundary-layer mixing could support moderate
   instability across portions of OR. Large-scale forcing for ascent
   will be weak, but convergence and orographic effects on the western
   edge of a deepening surface low could result in isolated
   thunderstorms. Enhanced southwesterly mid-level flow will result in
   a enough vertical shear to support a few stronger storms.

   ..Mosier.. 05/27/2020

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