SPC AC 271730

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1230 PM CDT Thu Aug 27 2020

   Valid 281200Z - 291200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE MIDWEST...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE LOWER MS VALLEY AND TN VALLEY...

   ...SUMMARY...
   Strong to severe storms are possible Friday from a portion of the
   upper Mississippi Valley, Great Lakes and Ohio Valley with large
   hail and damaging wind the main threats. Other severe storms may
   occur across the Tennessee Valley region with a few tornadoes and
   damaging winds the primary threats.

   ...Upper Midwest into the Great Lakes, Ohio Valley, and
   Mid-Atlantic...
   An MCS will likely be ongoing Friday morning across some portion of
   the Upper Midwest (probably the central/southern MN into northern IA
   vicinity). This MCS may continue to pose at least an isolated
   damaging wind threat across parts of WI before it weakens by mid to
   late morning. In the wake of this morning activity, additional
   robust storm development appears possible Friday afternoon across
   parts of WI/IA/northern IL as an upper trough advances
   east-southeastward from central Canada and the northern Plains into
   the Upper Midwest. A weak surface low is also forecast to develop
   eastward from MN into WI and eventually Lower MI through the day,
   with a trailing cold front moving southeastward across the Upper
   Midwest.

   There is still a fair amount of uncertainty regarding the northward
   extent of appreciable surface-based storm potential across WI, which
   will largely be dependent on how quickly the morning storms
   weaken/move eastward Friday morning. Even with this uncertainty,
   strong diurnal heating of a low-level airmass characterized by upper
   60s to low 70s surface dewpoints ahead of the cold front will likely
   support the development of 2000-3000 J/kg of MLCAPE by peak
   afternoon heating. 40-50 kt of mid-level westerly flow should foster
   similar values of effective bulk shear. Redevelopment along the cold
   front in the afternoon is likely, with supercells the main storm
   mode initially. Both scattered large hail and severe wind gusts may
   occur, with a more focused corridor of severe potential evident
   across southern WI, eastern IA, and northern IL as storms grow
   upscale into a small bow/line through early Friday evening. A few
   tornadoes also appear possible along a warm front extending eastward
   from the surface low, where low-level shear will be relatively
   maximized in association with a modestly enhanced low-level jet.

   Farther east across the Great Lakes and OH Valley/Mid-Atlantic,
   mainly elevated convection may be ongoing Friday morning along a
   stalled front. The potential for additional storm development
   through the day remains unclear, although a mid-level perturbation
   associated with the morning MCS over the Upper Midwest may provide
   enough large-scale ascent for scattered storms by Friday afternoon.
   Generally 25-40 kt of effective bulk shear in combination with
   1000-2000 J/kg of MLCAPE along/south of the front should be
   sufficient for a mix of supercells and multicells, with the better
   supercell potential across Lower MI and the OH Valley where
   deep-layer shear should be somewhat stronger. Although marginally
   severe hail may occur, the main threat should be damaging winds as
   storms congeal into multiple clusters and small bows while moving
   eastward. Have expanded the Slight Risk eastward across parts of PA,
   where a greater concentration of storms appears probable.

   ...Lower Mississippi Valley into the Tennessee Valley...
   Laura is forecast by the National Hurricane Center to be a Tropical
   Depression in the vicinity of northeastern AR by the start of the
   period Friday morning. As the cyclone develops east-northeastward
   through the day, a broken band of storms may form along its eastern
   flank across parts of the Lower MS Valley into the TN Valley and
   vicinity. Although instability may remain fairly modest owing to
   poor mid-level lapse rates, around 500-1000 J/kg of MLCAPE may still
   develop. The low-level wind field associated with Laura should
   remain favorable for low-level rotation with any low-topped
   supercells that can form. The straight-line damaging wind potential
   may also increase across parts of the TN Valley Friday afternoon if
   storms can grow into one or more line segments, as some
   convection-allowing model guidance suggests.

   ...Central Plains...
   Convective initiation along the southward-moving cold front still
   appears very questionable across most of the central Plains through
   the day, as appreciable large-scale ascent should remain displaced
   to the north of this region. A better potential for convective
   initiation Friday afternoon appears across parts of the higher
   terrain of CO, where a weak low-level upslope flow regime will
   exist. This activity should spread eastward, possibly reaching the
   central High Plains while posing a risk for a few severe wind gusts
   and perhaps some hail during the evening. Additional storms may
   develop overnight across KS, mainly north of the front. Some of this
   activity may become capable of producing hail and locally strong
   wind gusts.

   ..Gleason.. 08/27/2020

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