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Mesoscale Discussion 1380
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   Mesoscale Discussion 1380
   NWS Storm Prediction Center Norman OK
   0132 PM CDT Fri Jul 21 2017

   Areas affected...much of Iowa...northwest Illinois

   Concerning...Severe potential...Watch likely 

   Valid 211832Z - 212100Z

   Probability of Watch Issuance...80 percent

   SUMMARY...Severe thunderstorms are expected to develop in the
   19Z-21Z time frame with a risk for damaging winds, large hail, and a
   tornado or two. Short term trends will be monitored and a watch is
   likely prior to 21Z.

   DISCUSSION...A composite frontal boundary extended east-west from
   northeast Nebraska across central Iowa and northern Illinois as of
   1815Z. Elevated thunderstorms continued in the vicinity of the
   Iowa/Wisconsin border, and thunderstorms have recently developed
   over northeast Nebraska/northwest Iowa.  A very moist air mass
   (lower-mid 70s surface dew points) remains in place, and with
   continued heating moderate-strong surface-based instability will
   exist near the boundary this afternoon.  The discussion area remains
   on the periphery of the stronger mid-level flow, with deep-layer
   shear averaging 35-45 kts. 

   Surface-based thunderstorms are expected to develop in the 19Z-21Z
   time frame as convective inhibition continues to diminish due to the
   combined effects of heating and ascent associated with an
   eastward-moving MCV over southern Minnesota/northern Iowa.  An
   initial supercellular mode is expected with large hail and damaging
   winds the primary threats.  Favorable low-level hodograph curvature
   near the boundary will support a risk for a tornado or two.  With
   time, one or two convective clusters should gradually evolve with a
   more concentrated risk for damaging winds into the evening hours,
   supported by a strengthening southwesterly low-level jet.

   Given this expected scenario, one or two watches will likely be
   needed for the discussion area, with an initial watch prior to 21Z.

   ..Bunting/Dial.. 07/21/2017

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

   ATTN...WFO...LOT...DVN...DMX...FSD...OAX...

   LAT...LON   42719413 42559250 42499170 42489129 42419016 42248961
               42108930 41808921 41488943 41318993 41129069 41139176
               41329378 41449530 41819601 42249617 42639571 42719413 

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Page last modified: July 21, 2017
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