SPC AC 091955

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0255 PM CDT Sun Aug 09 2020

   Valid 092000Z - 101200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS IN PORTIONS OF THE
   UPPER MIDWEST...

   ...SUMMARY...
   Scattered severe thunderstorms are expected across parts of the
   Upper Midwest through tonight.  A few tornadoes and damaging gusts
   will be possible from east-central Minnesota into northern
   Wisconsin.  Large hail and severe gusts will be possible from the
   eastern Dakotas across most of Minnesota into northern Iowa.

   ...Discussion...
   The forecast is largely on track.  The only minor changes have been
   to 1) reduce the 5-percent tornado across parts of WI based on
   recent radar and model thunderstorm development trends, and 2)
   upgrade to 5-percent tornado probabilities in a narrow corridor in
   the Red River Valley.  The current expectation is gradual upscale
   growth will lessen the tornado risk to the east of the newly added
   5-percent area.

   ..Smith.. 08/09/2020

   .PREV DISCUSSION... /ISSUED 1116 AM CDT Sun Aug 09 2020/

   ...Upper Midwest...
   Two primary corridors of severe potential are expected through
   tonight.

   The first will be associated with a well-defined MCV across central
   MN. This feature should track northeast towards Lake Superior.
   Time-series of the MPX VWP has indicated widely varying low-level
   wind profiles ahead of the MCV, but has had a consistent signal for
   around 40-kt 0-6 km shear. While 12Z guidance is insistent the
   potential for a strong to extremely unstable air mass to develop by
   late afternoon, ongoing convection across north-central IA into
   southeast MN has been poorly simulated. Based on the 12Z MPX
   sounding and 16Z mesoanalysis of ML/MUCAPE fields, the expectation
   is for more moderate buoyancy to develop in cloud breaks between the
   MCV and the convection farther south. With mid 60s to low 70s
   surface dew points, potential will exist for a mesoscale cluster to
   form near the MCV with embedded supercell structures. This setup
   should tend to favor a few tornadoes and damaging winds as the
   primary hazards across east-central MN into northwest/north-central
   WI into this evening.

   The second corridor will be focused along a progressive cold front
   that should have increasing storm coverage by mid-afternoon across
   eastern ND as mid-level cooling further occurs in association with a
   Prairie Provinces shortwave trough. With a belt of fast 500-mb
   westerlies slowly shifting south through the day, the setup will be
   favorable for initial supercells that should grow into fast-moving
   linear segments by early evening. CAM guidance varies widely with
   the evolution of convection across MN this evening. The leading MCV
   will likely hinder destabilization across north-central and
   northeast MN suggesting that severe potential from the cold front
   convection will be confined to northwest MN. Other guidance suggests
   more of a southeastward-propagation towards larger buoyancy
   developing on the backside of the leading MCV as low-level moisture
   recovery occurs. This lower confidence scenario might result in a
   long-lived MCS persisting through much of tonight down the Upper MS
   Valley, as advertised by the 12Z NSSL-ARW and 00Z ECMWF. Given the
   uncertainty, no change is warranted to the broad cat 2/SLGT risk
   from the prior outlook.

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