SPC AC 041257

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0757 AM CDT Thu May 04 2017

   Valid 041300Z - 051200Z

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

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE
   SOUTHEAST U.S....

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

   ...SUMMARY...
   Strong to severe thunderstorms will be possible today into tonight
   across portions of the Pacific Northwest, in addition to the
   Southeast States and Tennessee Valley to mid-Atlantic region.

   ...Synopsis...
   A highly amplified pattern will exist over the CONUS throughout the
   period. A closed trough over the Ozarks and lower/middle MS River
   Valley this morning will move toward the southern
   Appalachians/Southeast States through tonight and early Friday,
   while an additional upper trough gradually approaches the Pacific
   Northwest. 

   ...Southeast States/Tennessee Valley to mid-Atlantic States...
   The key uncertainty factor for much of the region relates to an
   early-day band of outflow-focused showers and thunderstorms
   extending from GA southwestward into the northeast and central Gulf
   of Mexico early this morning. An extensive cloud shield is
   associated with and precedes this convection, casting some
   uncertainty regarding diurnal destabilization in pre-cold front
   warm-sector areas. An increased confidence in thinning clouds and
   somewhat more appreciable destabilization may warrant a categorical
   Slight Risk upgrade in subsequent Day 1 Outlooks, potentially for
   parts of GA into SC/NC.

   Initially, at least some severe risk will exist early today across
   the FL panhandle into other parts of north FL and south GA. For
   additional short-term details, see Mesoscale Discussion 645. Even
   with a cloud canopy persisting today, gradual low-level
   moistening/destabilization through the afternoon could allow
   outflow-focused storm development to occur into east GA and parts of
   SC with at least some damaging wind/tornado risk. Other strong to
   severe thunderstorms may develop this afternoon across eastern AL,
   middle/eastern TN, and north GA in a north-south corridor near the
   cold/occluded front.

   Into this evening, it is possible that an intensification of
   relatively low-topped/increasingly linear storms will gradually
   occur near the east/northeastward-moving cold front, initially
   across parts of GA into the Carolinas. This scenario would be aided
   by a northeastward-ejecting vorticity maxima with the base of the
   trough, while a surface low continues to deepen across the TN Valley
   and southern Appalachians vicinity. Even with just modest
   surface-based destabilization, dramatically strengthening
   low/mid-level winds, accentuated by 250+ m2/s2 0-1 km SRH, will at
   least conditionally support a tornado risk and/or damaging
   thunderstorm winds this evening into the overnight, potentially as
   far northeast as portions of NC/VA.

   ...Pacific Northwest...
   An upper trough centered along -140W early this morning will
   continue steadily eastward today, with preceding height
   falls/forcing for ascent reaching the Pacific Northwest later today
   and especially into tonight. With a relatively moisture-rich air
   mass across the region, steep mid-level lapse rates will support
   moderate destabilization across the region into mid/late afternoon.
   Diurnally enhanced orographic circulations and trough-preceding
   height falls/ascent will support storm development, with convection
   spreading north-northeastward this afternoon and evening. The
   aforementioned instability, combined with 30-45 kt of effective
   shear, may support organized convective structures capable of
   isolated severe hail and wind into this evening.

   ..Guyer.. 05/04/2017

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