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Mesoscale Discussion 1796
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   Mesoscale Discussion 1796
   NWS Storm Prediction Center Norman OK
   0356 PM CDT Sun Sep 18 2022

   Areas affected...portions of southern Iowa...northern Missouri and
   western Illinois

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

   Valid 182056Z - 182300Z

   Probability of Watch Issuance...80 percent

   SUMMARY...Gradually decreasing inhibition should allow for scattered
   supercells capable of large to very large hail, strong wind gusts
   and tornadoes to develop later this afternoon and evening. Storm
   development may take some time as upper level forcing approaches
   from the west. However, a Tornado watch will likely be needed.

   DISCUSSION...As of 2150 UTC, increased forcing for ascent and
   decreasing inhibition were evident ahead of a deepening shortwave
   trough across the northern and central Plains. Area VAD VWPs/ACARS
   soundings have shown an increase in mid-level flow with a developing
   speed max associated with the shortwave trough. As the trough
   approaches eastern NE and western IA later this afternoon and
   evening, gradual erosion of remaining inhibition should allow for
   scattered thunderstorm development near the stalled front/surface
   trough across portions of southern IA and northern MO. 

   Moderate to strong destabilization is ongoing across the region from
   robust diurnal heating and continued low-level advection of 70s F
   surface dewpoints. As inhibition is eroded with approaching dynamic
   lift, 2000-3000 J/kg of MLCAPE should support strong updraft
   development. 45-50 kt of effective shear will favor a cluster
   supercell mode with more discrete elements possible farther south
   and west where forcing for ascent is more nebulous. The favorable
   CAPE/shear overlap, 7.5-8C mid-level lapse rates and elongated
   upper-level hodographs will favor efficient large to very large
   (2.5+ in) hail growth with the strongest supercells. While initially
   small, gradual enlargement of low-level hodographs from a 40+ kt
   low-level jet may also support tornado potential with the more
   discrete supercells. Eventual upscale growth into small MCS may also
   support a risk for strong damaging wind gusts later this evening. 

   Visible satellite and the latest hi-res guidance indicate a couple
   hours of additional heating may be required to remove lingering
   inhibition ahead of the approaching trough. Current thinking is that
   initial storms may develop as early as 22-23z, though some members
   continue to hint at development more towards 00z. Despite
   uncertainty on exact timing, the potential for robust supercell
   development will necessitate Tornado Watch issuance in the next
   several hours.

   ..Lyons/Grams.. 09/18/2022

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

   ATTN...WFO...ILX...LSX...DVN...DMX...EAX...

   LAT...LON   41809430 41979352 42029271 41979219 41739155 41089064
               40569061 40269083 39999124 39849164 39869221 39939271
               40139329 40509403 41309476 41809430 

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Page last modified: September 18, 2022
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