Mesoscale Discussion 1728
   NWS Storm Prediction Center Norman OK
   1120 PM CDT Sat Sep 11 2021

   Areas affected...Parts of southeastern Minnesota into west central
   Wisconsin

   Concerning...Severe potential...Watch unlikely 

   Valid 120420Z - 120615Z

   Probability of Watch Issuance...20 percent

   SUMMARY...The development of a few storms, with at least some
   potential to produce large hail, appears possible by 1-3 AM CDT, if
   not before.  While the need for a watch still appears unlikely,
   trends will continue to be monitored.

   DISCUSSION...Westerly to northwesterly mid-level flow across much of
   the Upper Midwest is seasonably strong, but broadly confluent,
   downstream of a substantive short wave trough straddling the
   Montana/North Dakota international border area.  However, at least a
   couple of much more subtle, lower amplitude perturbations are
   progressing through this regime, and may contribute to a developing
   area of strengthening large-scale ascent aided by lower/mid
   tropospheric warm advection.  

   During the next few hours, it appears that this may become focused
   within a corridor generally north of the Rochester, MN and La Crosse
   WI areas.  This is to the north/cool side of a slowly southward
   advancing surface cold front, where low-level moisture overrunning
   the front, beneath the northern periphery of very warm and capping
   elevated mixed-layer, appears to be contributing to sizable CAPE up
   to around 2000 J/kg.

   Latest model output varies concerning the extent to which mid-level
   inhibition may suppress thunderstorm development tonight, but given
   the potential instability and at least subtle signal for increasing
   lift in the mid-levels, there appears at least low probabilities for
    the initiation of storms.  This may occur as early as 05-06Z, but
   it appears a bit more probable toward 07-08Z.

   Of primary concern, if storms do initiate, cloud-bearing layer shear
   profiles appear conducive to supercells which could pose a risk for
   sizable (perhaps greater than golf ball) hail.

   ..Kerr/Grams.. 09/12/2021

   ...Please see www.spc.noaa.gov for graphic product...

   ATTN...WFO...GRB...ARX...MPX...

   LAT...LON   44559262 44779039 43999000 43949101 43719258 44559262