SPC AC 231953

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0253 PM CDT Sat Jun 23 2018

   Valid 232000Z - 241200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE SOUTHERN
   PLAINS TO THE TN VALLEY...ACROSS THE MID ATLANTIC AND THE CENTRAL
   HIGH PLAINS...

   ...SUMMARY...
   Scattered severe thunderstorms are expected this afternoon and
   evening from the southern Plains to the lower Mississippi Valley and
   a portion of the Southeast States. Other severe storms are expected
   over the Middle Atlantic region and central High Plains.

   Overall minor changes have been made to the Day 1 Outlook this
   update. Some trimming was done on the southern end of the Slight
   risk from TX to AL based on latest surface observation and hi-res
   guidance trends. The sig hail area was expanded north and east
   across central OK to account for the current position of the outflow
   boundary and expected storm initiation over the next couple of
   hours. See MCD 838 for more details on this threat. 

   Additional adjustments include expanding the eastern CO Slight risk
   eastward into portions of far western KS as guidance has been
   consistent in maintaining storm cluster across this area this
   evening. Damaging winds and hail will be the main threats with this
   activity. See MCD 839 for more details. 

   Finally, operational HRRR guidance suggests this cluster may persist
   overnight into parts of southwest KS and northwest OK and a marginal
   wind and hail threat could accompany this activity. The Marginal
   risk area was expanded to account for this potential. Otherwise, no
   other changes were made.

   ..Leitman.. 06/23/2018

   .PREV DISCUSSION... /ISSUED 1147 AM CDT Sat Jun 23 2018/

   ...TN and lower MS Valley region today...
   The remnants of an overnight MCS, now moving into northwest AL and
   southern middle TN, will be accompanied by some threat for damaging
   winds about as far east as southeast TN and northwest GA through
   early afternoon, given a belt of 40+ kt flow near 700 mb (enhanced
   some by the MCV with the convection).  However, the widespread
   clouds/convection ongoing farther to the south in AL casts doubt of
   the degree of destabilization possible in advance of these storms. 
   Otherwise, additional strong/isolated severe storms could evolve
   from the ongoing central AL storms into GA, or develop this
   afternoon along the residual outflow from southeast AR into MS and
   eventually spread into central AL (along the southern fringe of the
   current storms).  Damaging outflow gusts will be the main threat.

   ...Northwest TX into southern OK this afternoon into early
   tonight...
   The outflow boundary from overnight and ongoing convection has
   consolidated immediately north of I-20 across north central and
   northeast TX, and the boundary may begin to retreat a little to the
   north across northwest TX this afternoon.  An influx of low-mid 70s
   boundary-layer dewpoints into the boundary, along with daytime
   heating and midlevel lapse rates near 9 C/km, will boost MLCAPE into
   the 3000-4000 J/kg range this afternoon.  This corridor will lie
   along the southern fringe of the stronger midlevel flow (near or
   above 30 kt), with some enhancement to low-level shear along the
   boundary.  The net result will be profiles sufficient for supercells
   where deep mixing reduces convective inhibition across northwest TX,
   and in the warm advection zone to the north into southern OK this
   afternoon/evening.  The strongest storms could produce isolated very
   large hail and damaging gusts.

   ...Chesapeake region this afternoon/evening...
   A midlevel trough over the upper OH Valley will progress
   east-northeastward to southern New England tonight.  Embedded speed
   maxima will continue to rotate around the southeast periphery of the
   trough and interact with a diffuse lee trough across VA.  The richer
   low-level moisture/buoyancy should remain near Chesapeake Bay
   through the afternoon, while mid-upper flow/shear will be sufficient
   for organized storms.  A few supercells will be possible, with a
   primary threat for a few damaging gusts.

   ...Central High Plains this afternoon through early tonight...
   Another midlevel trough is beginning to amplify and will move from
   the northern Rockies to WY/CO tonight.  Low-level moisture is
   returning northward across southeast CO in the wake of overnight
   convection, and profiles will become sufficient for supercells by
   this afternoon.  A few storms should form along the Palmer Divide,
   and potentially the Raton Mesa, and then spread east-southeastward
   over the Plains this evening.  There are still some concerns
   regarding storm coverage, but any supercells that form will be
   capable of producing large hail and damaging winds, and storms could
   grow into a small cluster and persist into early tonight.

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