SPC AC 281259

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0759 AM CDT Wed Jul 28 2021

   Valid 281300Z - 291200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
   WISCONSIN...

   ...SUMMARY...
   This afternoon and evening, a corridor from northeastern Minnesota
   across Wisconsin to the southern Lake Michigan region is under
   threat of very large hail, severe wind (locally exceeding hurricane
   force) and a few tornadoes.

   ...Synopsis...
   In mid/upper levels, a rather stagnant large-scale pattern is
   expected over the CONUS, with a strong anticyclone covering much of
   the central/northern Rockies and central/southern Plains, to the
   Mid-South region.  To its northeast, a belt of somewhat difluent
   northwest flow aloft will remain from central Canada across the
   upper Great Lakes, becoming cyclonic over the lower Great Lakes and
   Northeast.  Within that flow belt, a subtle shortwave trough -- now
   apparent in moisture-channel imagery over parts of SK and MT -- will
   move quickly away from the northern Rockies mean-ridge position, to
   the eastern ND/northern MN vicinity by 00Z.  This perturbation
   should proceed southeastward across the upper Great Lakes overnight,
   with some likely convective vorticity augmentation/reinforcement.

   At the surface, 11Z analysis showed a frontal-wave low over eastern
   ND, with cold front across southwestern SD, becoming quasistationary
   over central to northwestern MT.  A warm front was drawn from the
   low across northwestern to southeastern MN, southeastern WI, and
   southwestern Lower MI.  The low should shift east-southeastward to
   southeastward over northern MN, WI, southern Lake Michigan, to
   southern Lower MI, by the end of the period.  The warm front should
   move northward across eastern MN and western/central WI ahead of the
   low, especially once ongoing convective influences clear out of the
   area (below).  The warm-frontal zone will move more slowly over
   southeastern WI and southern MI.

   ...Upper Midwest/Great Lakes...
   The main severe threat will arise from southeastward-moving
   supercells developing this afternoon over parts of northern MN and
   northwestern WI.  This activity will pose a threat for
   significant/destructive hail, a few tornadoes, and severe gusts.  At
   some point, likely over WI but with some lingering uncertainty how
   far southeast, activity should expand into an MCS capable of severe
   gusts and tornadoes across the remainder of southern/eastern this
   evening, shifting into the southern Lake Michigan and lower MI
   vicinity tonight.  A derecho may occur, with embedded channels of
   hurricane-force gusts.

   Isolated severe hail and gusts remain possible from an ongoing area
   of thunderstorms (with elevated low-level inflow) across portions of
   MN and into western WI, before activity weakens later this morning. 
   Associated precip has been falling into an already high-theta-e,
   moisture-rich boundary layer, with some cooling/stabilizing
   influence, but not to the extent expected with a drier antecedent
   boundary layer.  However, associated thick cloud cover can
   impede/delay substantial diurnal heating by a few hours today,
   especially over WI, with a differential-heating zone acting as the
   effective warm front.

   Airmass recovery is expected behind the morning convection during
   mid/late afternoon, by means of both boundary-layer theta-e
   advection and diabatic heating/destabilization.  This process should
   progress from west to east across the outlook area and effectively
   relocate the warm-frontal position northward.  The rich low-level
   moisture content (supporting surface dew points commonly in the 70s
   F south of the warm-frontal zone) will underlie steep midlevel lapse
   rates apparent in 12Z ABR/MPX RAOBs this morning.  That, along with
   a deep troposphere, will contribute to MLCAPE values in the
   2500-4500 J/kg range near and especially south of the warm front
   today over MN/WI, and over 2000 J/kg as far southeast as
   northeastern IL and perhaps parts of southwestern Lower MI. 
   Vertical shear will be greatest near the warm front, but favorable
   in the warm sector as well, with effective SRH 300-600 J/kg in the
   frontal zone, along with 50-70-kt effective-shear magnitudes
   area-wide.  This parameter space is quite favorable for severe of
   all types, with convective mode being the principal determinant.

   Whether or not the timing/location of the upscale storm transition
   permits the event to be classified officially as a derecho in
   hindsight, there is strong concern it will have that kind of intense
   and destructive wind impact for at least a few hours.  That said,
   preceding supercells in the northeastern MN/northern WI area may be
   more long-tracked and pose a more-sustained threat of large to very
   large hail and tornadoes than evident earlier.  The downshear side
   of the hail and wind probabilities have been shifted somewhat
   southeastward along the corridor accordingly.  Also, the 15%+
   probabilities in hail and wind have been expanded laterally to
   account for:
   1.  Uncertainty (margin for error) in both axis and width extent of
   the supercell and later MCS coverage, and
   2.  Potential for a wider wind-producing thunderstorm swath (with a
   few embedded/QLCS tornadoes possible) near the warm front, whatever
   its specific axial position.

   ...Mid-Atlantic vicinity...
   Although large-scale support aloft will be weak at best, diurnal
   heating of a richly moist air mass will reduce MLCINH through
   mid-late afternoon in support of isolated to scattered thunderstorms
   -- especially east of the Appalachians.  Damaging gusts will be the
   main concern.

   Sea-breeze/bay-breeze boundaries also may assist with convective
   initiation over near-coastal areas.  Modified RAOBs and model
   soundings suggest large low-level theta-e and vigorous diabatic
   heating will overcome modest midlevel lapse rates enough to support
   MLCAPE as high as around 2000-2500 J/kg across eastern NC and
   southeastern VA -- less with northward/northwestward extent.  Weak
   low/middle-level flow and associated small vertical shear will limit
   convective organization, though some clustering and forward
   propagation (perhaps southwestward over the eastern NC area) may aid
   wind potential locally.  Strong anvil-level flow also will
   contribute to somewhat-favorable cloud-layer shear.

   ..Edwards/Gleason.. 07/28/2021

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