SPC AC 021632

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1132 AM CDT Tue Jun 02 2020

   Valid 021630Z - 031200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS SOUTHERN
   PORTIONS OF MN/WI INTO WEST-CENTRAL LOWER MI...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS EAST-CENTRAL WY TO
   THE WESTERN NE PANHANDLE...

   ...SUMMARY...
   Severe thunderstorms are most likely across parts of the Upper
   Midwest centered on southern portions of Minnesota and Wisconsin
   beginning after 4 PM CDT, eventually spreading into at least Lower
   Michigan overnight. Damaging to destructive winds and large hail
   will be the primary hazards.

   ...Upper Midwest...
   Primary changes are to add a significant severe wind area within a
   portion of the category 3/Enhanced risk for the potential of wind
   speed damage estimates exceeding 75 mph and to expand category 3
   severe risk into west-central Lower MI.

   Convective initiation is expected by late afternoon as low-level
   convergence strengthens along a surface front extending southwest 
   and east-northeast of a cyclone expected to be over southeast SD.
   The NAM appears overdone with mean mixing ratios and resultant progs
   of extreme buoyancy given 12Z observed soundings and GOES PW
   imagery. RAP/ARW-SREF guidance and 16Z surface obs suggest a hot and
   well-mixed boundary layer will instead be prevalent. A plume of
   MLCAPE from 2500-3500 J/kg is anticipated by peak heating. 

   With a low-amplitude shortwave trough moving east-southeast from
   Manitoba across northern Ontario, the peripheral belt of moderate
   mid-level westerlies will largely remain north of the MN/IA to WI/IL
   border area. Thus, the most likely corridor for a few initial
   supercells should be confined to southern MN with some risk for very
   large hail. Upscale growth into outflow-dominated clusters appears
   probable, increasing the potential for severe wind gusts across
   southeast MN into southern WI this evening. Given the steepness of
   low to mid-level lapse rates and a strongly unstable air mass,
   potential will exist for isolated significant severe gusts as well.
   The modest deep-layer wind fields appear to be a limiting factor to
   a more widespread/organized damaging wind risk. However, guidance is
   consistent in strengthening deep-layer westerlies tonight into early
   Wednesday, supporting an MCS progressing east across Lower MI
   overnight. While instability and buoyancy will be weaker, the
   increase in shear should sustain a risk for at least strong gusts
   capable of damaging winds.

   ...Lower Great Lakes...
   Primary change is to expand category 2/Slight risk for the potential
   of a few supercells this evening and a separate MCS approaching the
   region near the end of the period.

   Strengthening low-level warm/moist advection tied to a shortwave
   impulse moving southeast from northeast to far eastern Ontario
   should yield convection spreading east of the Lower Great Lakes
   after 00Z. While the NAM appears overdone with the degree of
   boundary-layer moistening, RAP soundings suggest convection will be
   rooted from parcels around 850 mb above a nocturnal near-surface
   inversion. Hodographs will be quite elongated given low-level
   westerlies veering to northwesterly with height amid strong speed
   shear. This setup should foster a risk for a few supercells with
   severe hail probably being the primary threat centered on western NY
   into northern PA. In addition, the propagation of the MCS from Lower
   MI might reach the region towards 12Z with a risk for strong gusts
   and locally damaging winds. 

   ...East-central WY to the western NE Panhandle.
   A front will settle south across the High Plains of interior/eastern
   WY with northeasterly low-level upslope flow expected to develop by
   late afternoon/early evening. Boundary-layer heating and orographic
   forcing suggest a few supercells should develop along the southern
   fringe of modest westerly mid-level flow. Isolated large hail should
   be the primary hazard, but some severe-caliber wind gusts could
   occur. This activity should spread east-southeastward toward western
   NE Panhandle during the evening/late night hours.

   ..Grams/Moore.. 06/02/2020

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