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Mesoscale Discussion 440
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MD 440 graphic

   Mesoscale Discussion 0440
   NWS Storm Prediction Center Norman OK
   0158 PM CDT Thu May 17 2018

   Areas affected...Far eastern Wyoming...Nebraska Panhandle...western
   South Dakota...far southwest North Dakota...and far southeast

   Concerning...Severe potential...Watch likely 

   Valid 171858Z - 172030Z

   Probability of Watch Issuance...95 percent

   SUMMARY...Thunderstorms will increase in the next few hours posing a
   risk for severe wind gusts and severe hail.  A Severe Thunderstorm
   Watch is likely.

   DISCUSSION...Strong heating over the area is contributing to
   thunderstorm development over the Laramie Range, along/north of the
   western portion of a decaying outflow boundary, and over the Black
   Hills.  Thunderstorms are expected to be most numerous over the
   northern portion of the area in the next hour or so, then
   develop/overspread a little later over the southern portion of the
   area as thunderstorms move off the Mountains, and potentially
   develop along the lee trough. The air mass north of the old outflow
   boundary has been somewhat slow to destabilize, but MLCAPE is now
   analyzed from 500 J/kg near the North Dakota/South Dakota border to
   over 2000 J/kg over central South Dakota, with rapidly decreasing
   MLCIN with time.  Near-surface winds have maintained somewhat of an
   easterly component north of the outflow boundary, which is
   contributing to 35-40 kt of 0-6 kt shear over that area.  The
   abundance of thunderstorms developing on recent GOES-16 imagery,
   steep mid-level lapse rates, and deeply-mixed boundary layers should
   contribute to strong cold pool development and upscale growth into
   mixed multicell/supercell clusters through the afternoon. Severe
   wind gusts and severe hail will be the primary threats.

   Over the southern area, strong heating and boundary-layer dewpoints
   holding in the mid-to-upper 50s is contributing to higher MLCAPEs of
   2500-3000 J/kg.  However, given the rather weak deep-layer shear,
   thunderstorms are also expected to consist of multicell/supercell
   modes initially, with a consolidated area of thunderstorms growing
   upscale with time in the form of a loosely-organized MCS.

   Given the expected coverage of thunderstorms, widespread steep lapse
   rates, and sufficient moisture, a Severe Thunderstorm Watch is
   likely, perhaps requiring issuance over the northern area first,
   followed by a second Watch over the southern part of the area.

   ..Coniglio/Weiss.. 05/17/2018

   ...Please see for graphic product...


   LAT...LON   43900489 45300482 46210414 46580312 46410185 46180130
               45690107 44780096 43900111 42820138 42620143 41760168
               41230198 41090254 41080384 41090456 41270505 42740491

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