SPC AC 080815

   Day 3 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0215 AM CST Wed Dec 08 2021

   Valid 101200Z - 111200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE LOWER/MID MISSISSIPPI VALLEY INTO THE LOWER OHIO VALLEY...

   ...SUMMARY...
   Severe thunderstorms capable of producing a few tornadoes and
   scattered damaging winds appear possible Friday into Friday night
   across parts of the lower/mid Mississippi Valley, Ohio/Tennessee
   Valleys, and Southeast.

   ...Synopsis...
   A highly amplified upper trough centered over the Rockies Friday
   morning should advance eastward across the Plains through the day,
   eventually reaching the MS Valley by the end of the period. Rather
   strong low/mid-level southwesterly flow will be present ahead of the
   upper trough across much of the lower/mid MS Valley into the OH/TN
   Valleys. A surface low initially over the central High Plains is
   forecast to develop eastward across the southern/central Plains
   through the day. This low should continue northeastward while
   gradually deepening over the mid MS Valley, Midwest, and Great Lakes
   from Friday evening though early Saturday morning.

   Rich low-level moisture characterized by at least mid to upper 60s
   surface dewpoints will likely be in place Friday morning from
   central/east TX into the lower MS Valley and Mid-South. As both the
   large-scale upper trough and related surface low shift eastward,
   this moist low-level airmass is forecast to spread northward in
   tandem with a warm front into the mid MS Valley, OH/TN Valleys, and
   parts of the Southeast by Friday evening. A cold front attendant to
   the surface low will sweep eastward from the southern/central Plains
   to the vicinity of the TX Coast and MS River by the end of the
   period.

   ...Lower/Mid Mississippi Valley into the Ohio/Tennessee Valleys and
   Southeast...
   Even with the low-level moisture across these regions increasing
   through the day, generally modest mid-level lapse rates will tend to
   limit the degree of instability that can develop, especially with
   northward extent into the mid MS Valley, Midwest, and OH Valley.
   Still, most guidance suggests that MLCAPE will be sufficient (at
   least 500 J/kg) across a majority of the warm sector to support
   surface-based storms. A fair amount of uncertainty remains regarding
   the timing of robust convection. The 00Z ECMWF indicates that an
   embedded shortwave trough will overspread the lower MS Valley during
   the day, potentially supporting convective initiation earlier than
   other guidance would suggest. Regardless, thunderstorms will likely
   increase in coverage and intensity along/ahead of the cold front by
   Friday evening and continue eastward through Friday night, as strong
   forcing associated with the large-scale upper trough overspreads the
   warm sector.

   Deep-layer shear is forecast to become quite strong by Friday
   evening owing to the strength of the mid-level southwesterly flow.
   50+ kt of effective bulk shear will easily support updraft
   organization. A mix of supercells and clusters may develop
   initially, but dominant convective mode should quickly trend to
   linear Friday night as thunderstorms grow upscale into one or more
   lines along/ahead of the cold front. Even though not much veering
   with height is forecast in the boundary layer due to mostly
   southwesterly flow, there should be ample speed shear, as the winds
   at 850 mb are expected to strengthen to 50-60+ kt Friday night.
   Strong 0-1-km SRH should support low-level rotation with any
   supercells or embedded QLCS circulations, and a few tornadoes may
   occur. Isolated to scattered severe/damaging winds also appear
   possible given the strength of the low-level flow, as it will not
   take much for convective downdraft accelerations to allow the
   already enhanced low-level winds to reach the surface.

   Primary uncertainties precluding greater severe probabilities at
   this time include the degree of boundary-layer instability, a large
   component of front-parallel flow in both low/mid levels, and the
   late timing of robust convective development. The northern extent of
   any severe threat in the Midwest/OH Valley is also quite uncertain,
   but most guidance suggests at least upper 50s to perhaps low 60s
   surface dewpoints may advance as far northward as east-central IL
   into IN and southwestern OH. This may be sufficient for low-topped
   convection to become surface based. The eastern extent of the severe
   threat will be tied to the location of storms along or just ahead of
   the front at the end of the Day 3 period, with expectations that at
   least a marginal/isolated severe threat will likely continue farther
   east into Day 4/Saturday.

   ..Gleason.. 12/08/2021

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