SPC AC 210539

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1239 AM CDT Fri Aug 21 2020

   Valid 211200Z - 221200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE
   CENTRAL/NORTHERN PLAINS INTO THE UPPER GREAT LAKES...OVER PARTS OF
   THE NORTHEAST...AND ACROSS THE SOUTHEAST...

   ...SUMMARY...
   Thunderstorms associated with marginally severe wind gusts will be
   possible across parts of the Southeast and Northeast today. A few
   storms with strong wind gusts and hail may occur across parts of the
   northern and central Plains into the upper Mississippi Valley.

   ...Northern/Central Plains to Upper Great Lakes...

   Early-morning water-vapor imagery depicts a well-defined mid-level
   short-wave trough extending from AB into eastern MT. This short wave
   is ejecting east in line with latest model guidance and should
   extend from eastern SK into central ND by late afternoon. Weak
   height falls will flatten the ridge across the northern Plains and
   at least weak large-scale forcing for ascent should overspread a
   region of steep low/mid-level lapse rates. In response to this
   feature, a surface low should reorganize over north-central SD and
   an east-west boundary should become more focused from this low, east
   along the ND/SD border across southern MN into WI. While low-level
   convergence will not be that strong along the front across ND,
   strong surface heating should allow surface parcels to reach their
   convective temperatures by late afternoon. Scattered convection
   should be noted ahead of the short wave during the morning into
   early afternoon north of the international border. However, by
   20-21z isolated thunderstorms may develop south along the surface
   front across the central Dakotas into portions of western NE. It's
   not entirely clear how concentrated this activity will be but
   forecast soundings suggest fairly strong buoyancy with surface-6km
   bulk shear of 30-35k. This flow regime appears marginally supportive
   of supercells and hail/wind will be the primary threats. Depending
   on coverage, some of this activity could spread/develop downstream
   along the aforementioned east-west boundary draped across MN/WI. One
   or more thunderstorm clusters could ultimately evolve across this
   region during the overnight hours.

   ...Northeast...

   Seasonally strong west-northwesterly flow will persist across QC
   into the northeastern US during the day1 period as a mid-level speed
   max ejects off the Canadian coast. In the wake of this feature, a
   surface boundary should sag south of the international border,
   extending from northern NY-central VT/NH to southern ME during the
   afternoon. This boundary will serve as the focus for isolated robust
   convection that should form within a modestly sheared/buoyant
   environment. Forecast soundings do not exhibit particularly steep
   lapse rates and gusty winds will be the primary threat with the
   strongest convection.

   ...Southeast...

   Slow-moving upper trough, currently located over the lower MS
   Valley, may begin to gradually lift northeast over the next 36-48hr
   as strongest mid-level flow shifts downstream along a corridor from
   the eastern Gulf-GA-Carolinas. While this corridor will experience
   somewhat stronger shear than areas beneath the immediate trough,
   high-PW air mass and poor lapse rates will not permit strong
   buoyancy to develop across this region. Even so, as has been the
   case the last few day, a few storms will likely produce gusty winds.
   Hence, 5% severe wind probabilities will be maintained across this
   region.

   ..Darrow/Moore.. 08/21/2020

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