SPC AC 021258

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0758 AM CDT Tue Jun 02 2020

   Valid 021300Z - 031200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS SOUTHERN
   MINNESOTA/NORTHERN IOWA TO SOUTHERN WISCONSIN...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE UPPER
   MIDWEST/GREAT LAKES AND EASTERN WYOMING...

   ...SUMMARY...
   Severe thunderstorms are expected from the Middle Missouri and Upper
   Mississippi Valleys to the Great Lakes. Damaging wind gusts, large
   hail and a tornado or two are possible, with the greatest threat
   expected from southern Minnesota/northern Iowa to central/southern
   Wisconsin.

   ...Upper Midwest from South Dakota/Minnesota to Lower Michigan...
   An initial concern will be the possibility of isolated severe hail
   early today from ongoing elevated storms along the North
   Dakota/South Dakota border vicinity. Hail aside, if storms persist
   into late morning/early afternoon, some stronger wind potential
   could also materialize across far southeast North Dakota/northeast
   South Dakota into west-central/southwest Minnesota given a
   relatively dry sub-cloud layer within the post-frontal environment
   (reference 12Z observed Aberdeen SD sounding). For additional
   short-term details, see Mesoscale Discussion 780.

   To the southeast of this early day storms, of greater concern will
   the potential for deep convective development across southern
   Minnesota and possibly far southeast South Dakota by around 3-5 pm
   CDT. A very unstable air mass (3500-4500 J/kg MLCAPE) is expected
   across the region in vicinity of a surface front that will extend
   from near the southeast South Dakota/northeast Nebraska border
   east-northeastward across southern Minnesota and south-central
   Wisconsin at mid-afternoon. This development will be influenced by a
   progressive low-amplitude shortwave trough/speed max along the
   International Border toward Lake Superior vicinity.

   Deep-layer winds/vertical shear will be maximized along/north of the
   southeastward-spreading front. In conjunction with the strong
   buoyancy near the frontal zone, initial intense supercells capable
   of large hail can be expected. Storms are expected to further
   increase/cluster through late afternoon and early evening as they
   progress east-southeastward across far southern Minnesota/northern
   Iowa and southern Wisconsin, with a damaging wind risk increasing
   accordingly. These linear bands of storms may reach southern Lake
   Michigan vicinity and southern Lower Michigan by the late
   evening/overnight hours with some severe risk potentially continuing
   into the overnight.

   ...North-central/eastern Wyoming...
   A front will settle south across the High Plains of interior/eastern
   Wyoming with northeasterly low-level upslope flow expected to
   develop by late afternoon/early evening. Boundary-layer heating and
   orographic forcing suggest isolated supercells may 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 the Black Hills/northwest Nebraska Panhandle during the
   evening/late night hours.

   ...New York/Pennsylvania...
   Warm/moist advection focused to the east of a warm front will
   support scattered showers and possibly a thunderstorm or two early
   today, although this activity should tend to wane during the morning
   as a passing subtle mid-level impulse shifts away from the region.
   The region will otherwise be dry/capped much of the afternoon,
   although strong/severe thunderstorms could develop into the region
   late this evening into the overnight. 

   Storms may develop southeastward late tonight out of Ontario in
   vicinity of the warm front/nearby warm sector, including parts of
   western New York and northwest/north-central Pennsylvania. Although
   nocturnal influences may keep storms slightly elevated, strong
   deep-layer/low-level shear could support supercells with a hail/wind
   and non-zero tornado risk. There is the also the possibility that
   storm clusters emanating from the Upper Midwest/southern Lake
   Michigan vicinity will move into the region late tonight/early
   Wednesday with some wind-related risk. This region will be
   reevaluated throughout the day for a possible categorical Slight
   Risk upgrade.

   ..Guyer/Gleason.. 06/02/2020

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