SPC AC 260541

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1241 AM CDT Wed Aug 26 2020

   Valid 261200Z - 271200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE UPPER MISSISSIPPI VALLEY AND GREAT LAKES REGION...

   ...SUMMARY...
   A threat for a few tornadoes will exist Wednesday and Wednesday
   night primarily across parts of Louisiana and far east Texas in
   association with Hurricane Laura. Otherwise, a few strong to severe
   storms with damaging winds and marginally severe hail will be
   possible over portions of the Upper Midwest, Great Lakes, Ohio
   Valley, and Mid-Atlantic regions. A few marginally severe storms may
   also occur from the northern Rockies through the northern Plains,
   and in Arizona.

   ...Upper Mississippi Valley/Great Lakes...
   A zonal flow pattern will be in place across the northern Plains
   today as a low-amplitude upper-level ridge moves from the upper
   Mississippi Valley into the western Great Lakes. At the surface a
   cold front will advance southeastward across the north-central
   states, and should be located from central Minnesota into eastern
   South Dakota by mid afternoon. Ahead of the front, surface dewpoints
   in the lower to mid 70s F will result in a pocket of moderate to
   strong instability. Thunderstorms appear likely to develop near the
   strongest instability in central Minnesota during the late
   afternoon. At that time, the wind profile in central Minnesota is
   forecast to be veered to the southwest at the surface but have ample
   speed shear in the low to mid-levels. This will result in 0-6 km
   shear in the 30 to 40 kt range which could support supercells with
   large hail. Supercells may also be associated with a wind-damage
   threat.

   Further to the east across Northern Wisconsin and northern Lower
   Michigan, moderate instability will likely be in place by late
   afternoon. In spite of the presence of a shortwave ridge, convective
   development is expected during the early to mid evening along the
   northern edge of moderate instability where low-level convergence
   should be maximized. This convection is forecast to track eastward
   across northern Wisconsin and northern Lower Michigan. Moderate
   instability, 0-6 km shear of 35 to 45 kt and steep lapse rates
   should be sufficient for a severe threat. The current thinking is
   that the preferred storm type will be multicells that form into
   short line segments. These lines will have potential for wind damage
   along the leading edge. A wind-damage threat may affect areas as far
   southeast as northwest Pennsylvania during the mid to late evening.

   ...Louisiana/Far East Texas/Southwest Mississippi...
   Hurricane Laura will move northwestward through the western Gulf of
   Mexico today. Rainbands to the north of the center will overspread
   southern Louisiana and southeast Texas today. Surface dewpoints will
   be in the lower to mid 70s F across the Sabine and lower Mississippi
   River Valleys, which will result in moderate instability by
   afternoon. As convection embedded in the outer band of Laura moves
   through this unstable airmass, the stronger storms may generate
   marginally severe wind gusts. As Hurricane Laura approaches
   southwest Louisiana this evening, low-level shear will dramatically
   increase from far southeast Texas eastward across southern
   Louisiana. As a result, a tornado threat is expected to develop as
   the bands of the hurricane move inland this evening and overnight.
   The tornado threat will continue across much of Louisiana and far
   east Texas as the center of Laura moves inland near Lake Charles
   late in the period.

   ...Northern Rockies/Northern Plains...
   An upper-level ridge will move northeastward across the northern
   Rockies today. At the surface, a moist and unstable airmass will be
   located from Idaho eastward into parts of the Dakotas. Convection is
   expected to develop in the higher terrain of the northern Rockies
   late this afternoon and then move eastward into the northern High
   Plains during the evening. Hail and strong gusty winds will be
   possible with the more intense cells. A marginal severe threat may
   also develop further east in parts of northeast Wyoming and western
   South Dakota where an axis of moderate instability is forecast late
   this afternoon. The marginal severe threat could continue into the
   evening or into the overnight period as the more persistent storm
   clusters move eastward across the region.

   ..Broyles/Moore.. 08/26/2020

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