Mesoscale Discussion 1301
   NWS Storm Prediction Center Norman OK
   0143 PM CDT Tue Jul 21 2020

   Areas affected...Southeast Minnesota and western Wisconsin

   Concerning...Severe potential...Watch possible 

   Valid 211843Z - 212045Z

   Probability of Watch Issuance...60 percent

   SUMMARY...Scattered thunderstorm development is possible in the
   coming hours. While storm coverage is uncertain, any storms that
   develop may pose a threat for severe wind, hail, and perhaps a
   tornado.

   DISCUSSION...Recent surface observations and satellite imagery show
   increasingly agitated cumulus developing in the vicinity of a
   surface low north of the Twin Cities area, and southward along a
   cold front. Morning cloud cover has delayed the onset of stronger
   daytime heating and boundary layer destabilization, but recent
   surface observations and RAP mesoanalyses over the past 1-2 hours
   have shown warming temperatures (into the mid to upper 70s) and
   increasing MLCAPE over southeast MN as the thicker cloud cover
   shifts east. Continued, but gradual, destabilization is expected
   through mid afternoon when 1500-2000 J/kg MLCAPE, along with 35-40
   knots of effective bulk shear, should be in place over the region. 

   In the near term, a gradual increase in shower activity, and perhaps
   a few weak thunderstorms are expected. Given nearly 200 J/kg of 0-3
   km MLCAPE and ample ambient near-surface vorticity along the cold
   front and in the vicinity of the surface low, a non-supercellular
   tornado or two can not be ruled out. (A few funnel clouds have
   already been reported earlier across west/central MN.) Heading into
   the mid and late afternoon hours, more robust thunderstorms may
   begin to materialize along the cold front and in the vicinity of the
   surface low. Boundary-normal deep layer shear across southeastern
   MN/western WI should support initially discrete modes, while broad
   ascent along the warm frontal zone to the north and weaker
   boundary-normal shear to the south may favor more cluster/linear
   modes. Severe hail and wind will be the primary concern with any
   storms that develop, but 100-150 m2/s2 ESRH ahead of the cold front
   across west/northwest WI should support at least a low-end tornado
   threat with any discrete storm.

   While the environment appears favorable for convection, the severe
   threat will largely be conditional on storm coverage and mode.
   Recent visible imagery trends have shown a couple attempts at
   convective initiation, but these have largely failed, suggesting
   more inhibition may linger in place than analyses suggest.
   Additionally, the limited spatial extent of downstream
   destabilization decreases confidence in the longevity of the severe
   threat. Trends will be monitored through the afternoon hours and a
   watch may be needed to address any conditional threats.

   ..Moore/Hart.. 07/21/2020

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

   ATTN...WFO...DLH...ARX...MPX...

   LAT...LON   45449391 46309346 46549191 46679098 46329019 45279037
               44289079 43549125 43529288 43519434 43589474 44329408
               45449391