SPC AC 250059

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0759 PM CDT Mon Aug 24 2020

   Valid 250100Z - 251200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE EASTERN GULF COAST AND UPPER MIDWEST...

   ...SUMMARY...
   Isolated severe storms will be possible this evening from the
   central Gulf Coast into the eastern Gulf Coast states associated
   with Tropical Storm Marco. One or more clusters of storms capable of
   producing large hail and damaging winds are expected across parts of
   the Upper Midwest this evening into tonight.

   ...Gulf Coast States...
   Tropical Storm Marco is currently located near the mouth of the
   Mississippi. Marco is forecast to move northwestward into southeast
   Louisiana tonight. Scattered convection is ongoing to the east of
   the center from parts of southern Alabama into northern Florida and
   southern Georgia. WSR-88D VWPs in southwest Alabama and southern
   Georgia have strong directional shear in the lowest 3 km AGL which
   is resulting 0-3 km shear near 30 kt. This combined with surface
   dewpoints from the mid 70s to the lower 80s F and moderate
   instability across the eastern Gulf Coast will be favorable for
   rotating cells capable of producing tornadoes. The tornado threat is
   expected to continue for much of the evening as storms gradually
   move northward across eastern Alabama and south-central Georgia.

   ...Upper Midwest...
   The latest water vapor imagery shows a west-northwest mid-level flow
   pattern across the north-central U.S. A subtle shortwave trough
   appears to be moving through the flow in the northern Plains. At the
   surface, a front is located across southern Minnesota with a
   corridor of maximized low-level moisture located along the front.
   Surface dewpoints along this corridor are in the lower to mid 70s F
   which is contributing to moderate instability. The RAP is estimating
   MLCAPE in southern Minnesota near 3000 J/kg. As the shortwave trough
   moves east-southeastward into southern Minnesota this evening,
   thunderstorms are expected to develop along the leading edge of
   large-scale ascent and along the northern edge of the stronger
   instability. This cluster should move east-southeastward along the
   instability gradient during the mid to late evening.

   In addition to the instability, the RAP is showing moderate
   deep-layer shear across most of southern Minnesota. The Minneapolis
   WSR-88D VWP has 40 kt of 0-6 km shear and the Minneapolis 00Z
   sounding shows steep lapse rates from 700 to 500 mb. This
   environment should be favorable for supercells with large hail. An
   isolated potential may exist for hailstones greater than 2 inches in
   diameter. The hail threat will be greatest early in the storm
   development when cells are most likely to be discrete. As cell
   coverage increases and a cluster of storms moves from southeast
   Minnesota into southern Wisconsin, a wind damage threat will also be
   likely. The severe threat is expected to persist into the overnight
   period as cells approach Lake Michigan later tonight.

   ...Southeast Oregon/Nevada...
   Southwest mid-level flow is currently evident on water vapor imagery
   across parts of the Intermountain West. Thunderstorms have developed
   this afternoon across parts of western and central Nevada where the
   RAP is analyzing several pockets of instability with SBCAPE as great
   as 1500 J/kg. This combined with steep lapse rates and moderate
   deep-layer shear may be enough for strong multicells capable of
   marginally severe wind gusts. The threat should continue for a few
   more hours this evening.

   ..Broyles.. 08/25/2020

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