SPC AC 231252

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0752 AM CDT Mon Oct 23 2017

   Valid 231300Z - 241200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
   CAROLINAS AND VIRGINIA...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ELSEWHERE FROM
   NORTHERN FLORIDA TO THE MID-ATLANTIC REGION...

   ...SUMMARY...
   Severe storms are possible from northern Florida to the Mid-Atlantic
   this afternoon and evening.  The primary threat will be damaging
   wind gusts, along with potential for a few tornadoes.

   ...Synopsis...
   Mean mid/upper troughing over the eastern U.S. will be reinforced
   with major cyclogenesis late in the period and into day 2.  A
   strong, negatively tilted shortwave trough -- currently containing a
   500-mb low near the MO Bootheel -- will eject northeastward then
   northward across the mid/upper OH Valley and Lake Erie today and
   tonight.  Meanwhile, a northern-stream perturbation -- now evident
   in moisture-channel imagery over northwestern MN and SD -- will
   amplify greatly as it digs southeastward, reaching IA and MO by 00Z
   and forming a closed low over IL tonight.  By 12Z, a large cyclone
   should cover much of the U.S. from the Plains eastward, with a
   temporary double center -- one over eastern Upper MI and the other
   near the IL/IN border.

   At the surface, 11Z analysis showed a frontal-wave cyclone over
   western/southern IN, with cold front extending to middle TN, eastern
   AL, the western FL Panhandle, and the central Gulf.  A warm front
   was drawn from extreme southern coastal NC westward over central/
   western SC, becoming diffuse in an area of convection and precip
   over northern GA and eastern TN.  The surface cyclone should deepen
   and occlude as it follows a curved path across eastern Lower MI to
   near the eastern end of Lake Superior by 12Z.  The surface cold
   front is forecast to reach WV, the western Carolinas, central/
   eastern GA and the eastern Gulf by 00Z.  By 12Z, the cold front
   should reach central NY, eastern PA, eastern NC, and the northern/
   western FL peninsula.  The warm front should become more diffuse as
   it shifts rapidly northward through the Carolinas, VA and portions
   of the Mid-Atlantic region through this evening.

   ...Northern FL to Mid-Atlantic region...
   A band of thunderstorms should develop near the surface cold front
   and sweep east-northeastward across the outlook area today, with
   damaging gusts and a few tornadoes possible.  Isolated damaging
   gusts are possible with a more conditional tornado risk accompanying
   convection in an extensive prefrontal corridor.

   A roughly 150-nm-wide plume of prefrontal convection, precip and
   thick cloud cover are apparent at this time, from eastern TN
   southward across much of GA and the central/eastern FL Panhandle to
   the northeastern Gulf.  This plume and associated/downstream
   mid/upper-level cloud cover will substantially impede insolation
   over much of the warm sector today, rendering weak boundary-layer
   lapse rates in many areas.  Still, a combination of subtle/diffuse
   diabatic heating, low-level theta-e advection and weak MLCINH should
   result in adequate buoyancy for surface-based convection, amidst
   northward expansion of rich low-level moisture.  These factors will
   offset modest mid/upper-level lapse rate enough for 1000-1500 J/kg
   MLCAPE over parts of northern FL, 500-1000 J/kg MLCAPE as far north
   as the Carolinas, and 100-500 J/kg possible northward toward parts
   of central/eastern VA and the Delmarva region this evening. 
   Strengthening deep-layer wind fields are expected over the warm
   sector today as the height gradient aloft tightens east of the
   deepening mean trough, and the lower/middle Mississippi Valley
   shortwave trough ejects northeastward.  This will create a
   low-CAPE/high-shear scenario from the Carolinas northward, with
   lesser (but still adequate) shear and larger CAPE over parts of
   southern/eastern GA and northern FL.

   The northern end of the main convective band likely will be manifest
   as a narrow, strongly forced ribbon of low-topped convection with
   little or no lightning -- already evident in non-severe form above a
   relatively stable surface air mass over portions of KY and TN.  This
   band should develop further and accelerate northeastward across the
   central Appalachians today.  Forecast soundings generally show only
   minor variations in lapse rates and low-level theta-e; however,
   those variations occur across neutral stability.  As often is true
   in near-neutral profile scenarios, very minor shifts -- often below
   model-predictability limits and spatially poorly sampled by
   upper-air thermodynamic observations -- may differentiate between
   profiles with unsupportive conditional stability and those with deep
   (albeit low-density) CAPE.  As such, considerable uncertainty exists
   regarding how far north a truly convective wind-damage risk exists
   over the WV/PA/western MD region.  Still, enough progs indicate
   neutral to very weakly unstable lapse rates that some northward
   expansion of the marginal convective-wind risk probably is
   warranted.  Forecast mean-wind and deep-shear vectors are strong,
   but largely parallel to the boundary instead of orthogonal, as would
   represent a more optimal convective wind setup.

   ..Edwards/Dean.. 10/23/2017

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