SPC AC 250549

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1249 AM CDT Fri Sep 25 2020

   Valid 251200Z - 261200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON AND
   EVENING ACROSS PARTS OF THE UPPER GREAT LAKES REGION...

   ...SUMMARY...
   A few severe thunderstorms are possible this afternoon and evening 
   across parts of northern Wisconsin and adjacent portions of Upper
   Michigan.  This may include the risk for a couple of tornadoes, in
   addition to large hail and potentially damaging wind gusts.

   ...Synopsis...
   A belt of strong westerlies may begin to undergo amplification
   across the northeastern mid-latitude Pacific, but appear likely to
   remain largely zonal across the Pacific Northwest into the northern
   Great Plains and Upper Midwest vicinity through this period.  On the
   leading edge of this regime, a fairly significant short wave trough
   is forecast to progress east of the Canadian Prairies through the
   southern Hudson/James Bays vicinity.  It appears that this feature
   will be trailed by another perturbation across the northern Great
   Plains, through and east-northeast of the Upper Great Lakes region,
   as a more substantive upstream short wave trough progresses inland
   of the Pacific Northwest through the northern Rockies.

   In response to these developments. to the south of a deep cyclone
   migrating across northern Saskatchewan at the outset of the period,
   an initial intrusion of colder air may be ongoing to the lee of the
   northern U.S. Rockies.  It appears that this will trail a modestly
   deep secondary low, initially migrating across the mid Missouri
   Valley, before continuing northeastward across the Upper Midwest and
   adjacent portions of the Great Lakes region by early this evening,
   then into southeastern Ontario overnight.

   Meanwhile, in advance, and to the southeast, of this regime, a short
   wave impulse now digging toward the lower Ohio Valley will maintain
   mid-level troughing within a weaker branch of westerlies, which is
   forecast to continue shifting toward the Mid Atlantic coast. 
   Embedded within this regime, models indicate that the low-level
   remnants of Beta will weaken substantially further across the upper
   Tennessee Valley/southern Appalachians region.

   Higher moisture content air will remain confined to a broad plume
   across the southeastern Gulf of Mexico, into and east of southern
   Atlantic coastal area, to the south and east of the southern/lead
   mid-level troughing.  However, modest moisture lingers in a corridor
   across parts of the eastern Great Plains and lower Missouri Valley
   into the Upper Great Lakes region, as a plume of warm elevated
   mixed-layer air noses across the middle Missouri Valley, toward the
   Upper Midwest/Great Lakes region.

   ...Upper Midwest/Great Lakes region...
   Large-scale forcing for ascent, including lower/mid tropospheric
   warm advection, may focus most convective development above/to the
   cool side of a residual frontal zone across the Upper Great Lakes
   region.  However, steepening mid-level lapse rates and moistening
   above the frontal zone may contribute to thermodynamic profiles
   supportive of at least marginally severe hail in stronger storms.

   The front is expected to advance northward to the Wisconsin/Upper
   Michigan border vicinity with the approach of the modest surface
   cyclone, near the nose of the warmer and more strongly capping
   elevated mixed-layer air.  Beneath this elevated mixed-layer, a
   corridor of stronger daytime heating may contribute to
   boundary-layer CAPE on the order of 1000-2000 J/kg across parts of
   northern Wisconsin (near the surface low and warm front) by late
   afternoon.

   Coupled with strengthening lower/mid-tropospheric wind fields
   (30-50+ kt) within the warm sector, the environment is expected to
   become conducive to organized severe storm development.  Based on
   model output, this may tend to focus near or just north of the warm
   front, but could include supercells accompanied by the risk for a
   tornado or two, in addition to large hail.  Eventually, activity may
   transition to an upscale growing cluster with potentially damaging
   wind gusts becoming the more prominent hazard before weakening this
   evening.

   ...Georgia/Carolinas...
   Low-level wind fields associated with remnants of Beta are forecast
   to continue to weaken, with stronger flow continuing to tend to
   spread northeastward into initially drier, potentially cold/stable
   air across the southern Appalachians.  However, modest westerly
   mid-level flow could enhance thunderstorm development where
   substantive boundary-layer destabilization is able to take place,
   which currently seems most probable across parts of eastern Georgia
   through the South Carolina Piedmont and coastal plain, and adjacent
   portions of North Carolina, this afternoon.

   ..Kerr/Bentley.. 09/25/2020

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