SPC AC 201719

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1219 PM CDT Thu Aug 20 2020

   Valid 211200Z - 221200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE NORTHERN TO CENTRAL PLAINS...UPPER MISSISSIPPI
   VALLEY...SOUTHEAST AND NORTHEAST...

   ...SUMMARY...
   Thunderstorms associated with marginally severe wind gusts will be
   possible across parts of the Southeast and Northeast on Friday. A
   few storms with strong wind gusts and hail may occur across parts of
   the northern and central Plains into the upper Mississippi Valley.

   ...Northern and Central Plains/Upper Mississippi Valley...
   An upper-level ridge will move slowly eastward across the northern
   Rockies on Friday as west-northwest flow remains established across
   the north-central U.S. At the surface, a low will deepen across the
   eastern Dakotas as a cold front advances east-southeastward across
   the northern Plains. Ahead of the front, a moist airmass in place
   from the eastern Dakotas into the upper Mississippi Valley will
   become moderately unstable by afternoon. Due to surface heating and
   increasing low-level convergence along the front, scattered cell
   initiation is expected during the mid to late afternoon. Lift
   associated with a low-amplitude shortwave trough moving eastward
   across the northern Plains, will also aid thunderstorm development.
   As storms increase in coverage during the late afternoon, a couple
   of widely spaced clusters are expected to develop and move
   southeastward across the instability corridor. This activity could
   reach far northwest Iowa, southern Minnesota and northern Wisconsin
   by early evening. MLCAPE along the instability corridor should
   generally be in the 2000 to 4000 J/kg range. This combined with
   low-level lapse rates approaching 8.0 C/km will likely be sufficient
   for marginally severe wind gusts. Mid-level lapse rates are also
   expected to be steep suggesting a hail threat may also accompany the
   stronger updrafts.

   ...Northeast...
   A low-amplitude upper-level trough will move east-southeastward
   across the lower Great Lakes region on Friday. At the surface, a
   slow moving cold front should be located from northern New York into
   central New England. Near the front, surface dewpoints in the lower
   60s F, should result in pockets of moderate instability by
   afternoon. As surface heating takes place along the front during the
   day, thunderstorms are expected to develop and move
   east-southeastward across the region. NAM forecast soundings near
   the front in northeast New York and Vermont at 21Z show MLCAPE near
   1000 J/kg, 0-6 km shear of 30 to 35 kt, 0-3 km lapse rates near 7.5
   C/km and unidirectional westerly flow. This should be favorable for
   short line segments capable of producing marginally severe wind
   gusts. The threat should be concentrated in the mid to late
   afternoon.

   ...Southeast...
   A slow-moving high-amplitude upper-level trough will be in place
   over the lower to mid Mississippi Valley on Friday. Beneath the
   trough, a moist airmass will be located from the lower to mid
   Mississippi Valley eastward to the Carolinas. The strongest
   instability is expected to develop from near the trough eastward
   across the Gulf Coast States where strong surface heating should
   take place during the day. As low-level convergence increases along
   outflow boundaries or other pre-existing boundaries, scattered
   thunderstorms will be likely in the afternoon. A few of these storms
   could produce marginally severe wind gusts as low-level-lapse rates
   become steep during the mid to late afternoon.

   ..Broyles.. 08/20/2020

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