SPC AC 261730

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1230 PM CDT Wed Aug 26 2020

   Valid 271200Z - 281200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE NORTHEAST AND MID-ATLANTIC...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE LOWER MISSISSIPPI VALLEY...

   ...SUMMARY...
   Severe storms appear likely Thursday across parts of the Northeast,
   Great Lakes, and northern Plains. Damaging winds and large hail
   should be the main threats, though a few tornadoes are also
   possible. Other severe storms with a threat for tornadoes will
   continue over portions of the lower Mississippi Valley region in
   association with Hurricane Laura.

   ...Northeast/Mid-Atlantic...
   Storms will probably be ongoing Thursday morning along/near a warm
   front in a low-level warm advection regime across parts of the
   Northeast, with a limited severe threat evident. Additional
   convective development appears likely by Thursday afternoon across
   this region along both the warm front and a pre-frontal trough. The
   airmass along/south of the warm front is expected to become at least
   moderately unstable, with MLCAPE of 1500-3000 J/kg possible. Strong
   deep-layer shear of 35-50 kt will be present across this region
   owing to enhanced west-northwesterly mid-level flow. Initial
   afternoon storm development will likely be supercellular, with both
   a large hail and damaging wind threat. Some of the large hail could
   be significant (2+ inches) given somewhat steepened mid-level lapse
   rates and very favorable thermodynamic/kinematic environment.
   Clustering and upscale growth appears possible later Thursday
   afternoon as storms approach NYC and southern New England. A more
   substantial severe/damaging wind threat will likely develop as this
   mode transition occurs.

   ...Northern Rockies/Plains into the Upper Midwest/Great Lakes...
   An upper trough over the Pacific Northwest and western CONUS is
   forecast to move eastward across the Great Basin and northern
   Rockies on Thursday. Multiple shortwave troughs embedded within this
   large-scale upper trough should develop east-northeastward across
   the northern High Plains through the day, and convective development
   over the higher terrain of the northern Rockies appears likely by
   early Thursday afternoon. As this convection develops eastward
   across the northern High Plains, it should encounter a more unstable
   airmass across eastern WY and western SD, where MLCAPE of 1500-2500
   J/kg may be present. The wind profile across this area will support
   supercells with both a large hail and severe wind gust threat
   initially. One or more clusters may ultimately develop with eastward
   extent into central/eastern SD along a stalled surface front by
   Thursday evening, and the damaging wind threat would increase if
   this occurs. However, there is too much uncertainty in the
   development and placement of these potential clusters to include
   greater severe wind probabilities at this time.

   Farther east into the Upper Midwest and Great Lakes regions,
   additional robust convective development appears possible through
   period along a stalled front. The forecast combination of
   instability and shear suggests some potential for storm
   organization, with mainly clusters/multicells capable of producing
   both isolated large hail and damaging wind gusts.

   ...Lower Mississippi Valley...
   Hurricane Laura is forecast by the National Hurricane Center to be
   near the LA/TX border at the start of the period Thursday morning.
   As Laura continues northward through the day, it should gradually
   weaken. Still, a strong low-level wind field will be in place across
   the lower MS Valley. Corresponding enhanced low-level shear will
   likely remain sufficient in tandem with at least some diurnal
   destabilization on the eastern periphery of the cyclone to support a
   few tornadoes across parts of the lower MS Valley. This tornado
   threat should be maximized in any low-level convergence zones/outer
   rain bands to the northeast/east of the center of circulation.

   ..Gleason.. 08/26/2020

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