SPC AC 290455

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1155 PM CDT Tue Jul 28 2020

   Valid 301200Z - 311200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THURSDAY INTO
   THURSDAY  NIGHT ACROSS PARTS OF THE SOUTH CENTRAL GREAT PLAINS...THE
   TENNESSEE VALLEY INTO SOUTHERN APPALACHIANS...AND SOUTHERN NEW
   ENGLAND INTO ADJACENT PORTIONS OF THE NORTHERN MID ATLANTIC COAST...

   ...SUMMARY...
   Strong thunderstorms may impact parts of the south central Great
   Plains, the Tennessee Valley into the southern Appalachians, and
   southern New England into adjacent portions of the northern Mid
   Atlantic coast Thursday into Thursday night, accompanied by at least
   some risk for severe weather.

   ...Synopsis...
   Mid-level ridging may continue to build across much of the  northern
   U.S. intermountain region and Rockies through much of western Canada
   during this period, downstream of an initially broad and deep
   mid-level low over the northeastern Pacific.  As this occurs, it
   appears that the mid-level low may begin to undergo considerable
   deformation, as it is forced eastward and northeastward with the
   approach of a vigorous upstream impulse within the mid-latitude
   westerlies.  Models continue to suggest that this will include a
   transition to negatively tilted troughing, which may pivot toward
   the British Columbia and Pacific Northwest coast by late Thursday
   night.

   Meanwhile, within one branch of westerlies downstream of the
   amplified ridging, a significant upper trough is forecast to  
   gradually progress into and across the northern Atlantic Coast
   region, as consolidating upstream perturbations begin to dig into
   the Hudson Bay vicinity.  

   Another weaker branch of westerlies will linger to the southwest and
   south, with perhaps the most significant embedded troughing
   continuing to gradually dig across the central Great Plains.  This
   likely will be preceded by at couple of convectively generated or
   enhanced perturbations across the Ozark Plateau and Tennessee Valley
   into the southern Mid Atlantic coast vicinity.

   Highest thunderstorm probabilities Thursday through Thursday
   night probably will remain focused with the higher moisture content
   along this weaker southern branch.

   ...South central portions of the Great Plains...
   Uncertainties linger concerning the evolution and progression of the
   digging larger-scale trough, and associated embedded convective
   perturbations, which could impact the convective potential for this
   period.  However, latest model output generally remains suggestive
   that a zone of stronger differential surface heating will develop to
   the north of the Red River by late Thursday afternoon, beneath a
   plume of elevated mixed-layer air advecting southeastward across
   much of the southern Great Plains.

   This baroclinic zone is forecast to become the focus for moderate
   boundary-layer destabilization beneath northwesterly mid-level flow
   strengthening to 30+ kt.  Aided by forcing for ascent associated
   with the digging upper trough, it is possible that this environment
   could support the evolution of an organized, southeastward
   propagating cluster of storms, with the potential to produce severe
   wind gusts late Thursday into Thursday night.

   ...Mid South to southern Appalachians vicinity...
   To varying degrees, there has been a signal within considerable
   model output that a convectively enhanced or generated perturbation
   will migrate east-southeast of the Ozark Plateau toward the southern
   Appalachians during the day Thursday.  It appears that this may be
   accompanied by a belt of 30-40 kt westerly lower/mid tropospheric
   flow, while also providing a renewed focus for upscale growing
   convection.  Although inflowing air may be characterized by somewhat
   modest CAPE, seasonably high moisture content will contribute to
   heavy precipitation loading, and perhaps enhance the risk for
   potentially damaging wind gusts.

   ...Southern New England/Long Island vicinity...
   At least moderate boundary-layer destabilization (CAPE of 1000-2000+
   J/kg) appears possible just ahead of a slow moving surface front. 
   This is expected to occur in the presence of strengthening
   deep-layer shear, as 30-50 kt westerly mid-level flow in the base of
   the large-scale trough overspreads the region.  Although stronger
   mid-level forcing for ascent may remain displaced to the north of
   this region, guidance suggests that at least isolated to widely
   scattered thunderstorm development is possible Thursday afternoon. 
   Given the environment, this may include supercell structures with
   some risk for severe hail and locally strong surface gusts.

   ..Kerr.. 07/29/2020

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