SPC AC 041728

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1228 PM CDT Fri Sep 04 2020

   Valid 051200Z - 061200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   SOUTHERN MN AND NORTHERN IA...

   ...SUMMARY...
   Isolated to scattered severe thunderstorms may occur over parts of
   southern Minnesota and Iowa Saturday night. Large hail and damaging
   winds should be the main threats.

   ...Minnesota/Iowa Vicinity...
   An upper high will remain centered over the Great Basin on Saturday,
   with large-scale upper troughing over the Northeast and eastern
   Canada forecast to move slowly eastward through the period. A
   mid-level perturbation initially over MT should rapidly move
   east-southeastward across the Plains, reaching parts of the Upper
   Midwest by late Saturday night. At the surface, a low is forecast to
   deepen over SD as the mid-level perturbation approaches, with a warm
   front expected to lift north-northeastward from the central Plains
   into parts of the Upper Midwest through Saturday night.

   A plume of very steep mid-level lapse rates (8-9 C/km in the 700-500
   mb layer) emanating from the northern/central High Plains should
   overspread the Dakotas and parts of MN/IA through the period. Latest
   models remain consistent in depicting the northward advance of
   richer low-level moisture across the MO Valley and into parts of
   western IA by Saturday evening, with low to mid 60s surface
   dewpoints probable. Low-level warm/moist air advection is also
   expected to substantially strengthen Saturday night into early
   Sunday morning with the development of a 45-55 kt southwesterly
   low-level jet across parts of NE/SD into IA/MN.

   Large-scale ascent associated with both the mid-level perturbation
   and increasing low-level jet will likely aid the development of
   scattered elevated thunderstorms from eastern SD into parts of
   southwestern MN. MUCAPE of 2500-3000 J/kg should exist across this
   region owing to the presence of very steep mid-level lapse rates and
   increasing low-level moisture centered around 850 mb. Very strong
   effective bulk shear values (50+ kt) will likely foster initial
   supercells capable of producing isolated large hail. Some potential
   may also exist for a MCS to develop and subsequently move
   southeastward into parts of southern MN/northern IA late in the
   period along the elevated instability gradient. Given the forecast
   strength of the low-level jet, large reservoir of MUCAPE, and dry
   sub-cloud layer, there is some potential for strong/damaging
   downdraft winds to penetrate the near-surface stable layer and reach
   the surface. Have included 15% wind probabilities to account for
   this possibility, even though time of day (early Sunday morning)
   would otherwise suggest unfavorable severe wind potential.

   ..Gleason.. 09/04/2020

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