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Mesoscale Discussion 893
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   Mesoscale Discussion 0893
   NWS Storm Prediction Center Norman OK
   0211 AM CDT Sun May 28 2017

   Areas affected...portions of northwestern and west-central
   GA...central/northern AL...northern MS...central/southeastern AR.

   Concerning...Severe Thunderstorm Watch 285...290...

   Valid 280711Z - 280915Z

   The severe weather threat for Severe Thunderstorm Watch 285, 290

   SUMMARY...Two primary bows or arcs of severe storms will pose the
   greatest threat across this corridor the next few hours:  one moving
   southeastward across northern AL, and another over western AR.  A
   new watch may be needed for parts of AL and perhaps western GA as
   convective and instability trends warrant.

   DISCUSSION...The leading MCS for this discussion area is evident as
   of 645Z over north-central/northwestern AL, from Limestone to Morgan
   Counties.  This activity should continue to move southeastward
   astride a convectively reinforced outflow boundary from earlier
   convection in eastern TN and GA.  That outflow boundary was analyzed
   from southern fringes of the ATL metro area to just south of CTJ and
   GAD, into Cullman County, moving slowly southward.  Convection
   developing along and north of this boundary over north-central/
   northeastern AL will continue to reinforce its baroclinicity, while
   some of the activity merges into the northern parts of the
   progressive MCS.  The preconvective warm sector south of the
   boundary is characterized by rich boundary-layer moisture with
   surface dew points low-mid 70s F and PW in the 1.25-1.5-inch range,
   contributing to MLCAPE around 1500 J/kg.  Although MLCINH is
   gradually strengthening due to diabatic surface cooling, two factors
   may permit maintenance of convective-wind potential near severe
   limits past the current watch:
   1.  Forced ascent of near-surface air into the MCS along the leading
   part of the accompanying density current, aided by lift along the
   foregoing outflow boundary, and
   2.  Nearly surface-based effective-inflow parcels being maintained
   for a few more hours, given the slow pace of surface cooling amidst
   such rich moisture. 
   Reinforcing downward momentum transfer also is possible within the
   complex, amidst roughly 50-kt west-northwesterly effective-shear
   vectors with a substantial component orthogonal to the convective

   Farther west, the complex over west-central/southwestern AR may
   continue to pose an episodic damaging-wind threat as it moves
   eastward across southern AR and into MS over the next few hours, on
   either side of the outflow boundary from the convective corridor
   over AL, northern MS and eastern AR.  That boundary is stronger and
   faster-moving than the one in eastern AL, however similarly
   favorable warm-sector air is apparent to its south as well.  A
   corridor of damaging-wind potential appears across the middle and
   southern parts of watch 290, which may require local spatial
   extensions as convective trends warrant.

   ..Edwards.. 05/28/2017

   ...Please see for graphic product...


   LAT...LON   35579218 34919071 34678916 34908832 35088766 34808621
               34238509 33588481 33088520 32858845 33609154 34139354

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