US Mesoscale Discussions

Storm Prediction Center Mesoscale Discussion 871

   Mesoscale Discussion 0871
   NWS Storm Prediction Center Norman OK
   1232 PM CDT Tue May 21 2024

   Areas affected...Central/Eastern IA...South-Central/Southeast
   MN...Far Southwest WI...Far Northwest IL

   Concerning...Severe potential...Tornado Watch likely 

   Valid 211732Z - 211930Z

   Probability of Watch Issuance...95 percent

   SUMMARY...Environmental conditions are expected to becoming
   increasingly supportive of a significant severe thunderstorms
   capable of all severe hazards, including very large hail up to 3"
   inches, strong gusts to 75 mph, and long-track tornadoes. A Tornado
   Watch will likely be needed around 18-19Z, and a PDS designation is
   being considered.

   DISCUSSION...Recent surface analysis places a low over southeast NE,
   with an outflow-modified warm front extending east-northeast across
   southern IA through the IA/IL/WI border vicinity. This warm front is
   expected to rapidly move northward in response to strong mass
   response ahead of the approaching shortwave trough, with upper 60s
   dewpoints likely in place across much of IA and mid 60s possibly
   reaching southern MN. As this occurs, steep mid-level lapse rates
   will also advect into the region, resulting strong buoyancy (i.e.
   MLCAPE 2500-3000 J/kg) by the early afternoon. Wind fields are
   expected to strengthen during this time frame as well, result in in
   very long hodographs and fast storm motion. Bunkers right motion for
   much of the region will be around 50 kt by the early afternoon. The
   result will be an environment very supportive of significant severe

   An initially discrete mode is anticipated, with the fast storm
   motions allowing storms to stay ahead of the cold front impinging on
   the region from the west. Any discrete storms will likely obtain
   supercellular characteristics quickly, with very large hail up to 3"
   inches, strong gusts to 75 mph, and tornadoes all possible. This
   includes the possibility of long-lived, long-track
   supercells capable of strong tornadoes and intense wind damage.
   Storm interactions with the warm front, as well as the presence of
   the deepening surface low, suggest the significant severe potential
   will likely extend into south-central/southeast MI and southwest MN,
   despite being displaced just north of the better mid-level flow and
   low-level moisture. 

   Some upscale growth is anticipated as storms continue eastward, with
   the resulting convective line capable of significant wind gusts
   around 75 mph. Given the continued strengthening of the low-level
   flow anticipated, some embedded QLCS circulations are likely as

   All of these factors suggests a significant severe weather event is
   probable, and a Tornado Watch will likely be needed around 18-19Z. A
   PDS designation is being considered for this watch.

   ..Mosier/Guyer.. 05/21/2024

   ...Please see for graphic product...


   LAT...LON   40969085 40619167 40579233 40589388 40649446 40959493
               41799502 43169492 44119422 44189139 43209038 41869013

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