Storm Prediction Center Mesoscale Discussion 875

   Mesoscale Discussion 0875
   NWS Storm Prediction Center Norman OK
   0320 PM CDT Tue May 21 2024

   Areas affected...Southwest/South-Central IA

   Concerning...Tornado Watch 277...

   Valid 212020Z - 212145Z

   The severe weather threat for Tornado Watch 277 continues.

   SUMMARY...A localized corridor favorable for strong tornadoes exists
   across southwest/south-central Iowa, including Des Moines vicinity.

   DISCUSSION...Pair of supercells on the southern end of the line of
   storms extending across western IA have recently shown notable
   intensification, with rotational velocity recently estimated around
   50 to 55 kt within the southernmost cell. Strong southerly flow
   exists ahead of this storm, contributing to strong low-level shear,
   evidenced by recent 0-500m storm relative-helicity around 200 m2/s2
   from DMX VAD. Robust deep-layer shear exists as well, with DMX VAD
   sampling 0-6 km bulk shear was over 60 kt. Recent mesoanalysis
   estimates effective-layer STP from 3 to 4 downstream. All of these
   factors suggest the downstream environment will remain conducive for
   supercell persistence. Storm motion for this southernmost storm is
   estimated to be northeasterly at around 45 kt, bring it into the Des
   Moines vicinity around 2130Z.

   ..Mosier.. 05/21/2024

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

   ATTN...WFO...DMX...OAX...

   LAT...LON   40719467 41269501 41659451 41949347 41189288 40719467