SPC AC 181234

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0634 AM CST Sat Nov 18 2017

   Valid 181300Z - 191200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS MAINLY THIS
   AFTERNOON/EVENING FOR THE LOWER OH VALLEY AND PARTS OF THE TN
   VALLEY...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   SLIGHT RISK AREA...

   ...SUMMARY...
   Thunderstorms capable of severe wind gusts, marginal hail, or a
   brief tornado will be possible, mainly this afternoon across parts
   of the Ohio and Tennessee Valley regions.

   ...OH/TN Valley regions through late evening...
   A surface cyclone in MO this morning will deepen while moving
   east-northeastward across the OH Valley to the lower Great Lakes by
   tonight, in advance of an amplifying midlevel shortwave trough.  An
   associated surface cold front will surge eastward from MO/AR this
   morning to the Appalachians by early tonight.  Boundary-layer
   dewpoints in the 60s will contribute to a narrow corridor of weak
   buoyancy in the pre-frontal warm sector.  However, clouds will limit
   surface heating and midlevel lapse rates will weaken with eastward
   extent, limiting the degree of warm sector buoyancy.

   Ongoing elevated convection in the zone of stronger forcing for
   ascent (in advance of the surface cyclone) will persist into the
   day, with some threat for isolated large hail given the lingering
   steep lapse rate feed from the west-southwest.  Otherwise, a narrow
   band of low-topped convection should form by midday in the band of
   ascent along the cold front as ascent/moistening weakens the cap,
   and then move quickly eastward through the afternoon/evening across
   the OH/TN Valley regions.  Though buoyancy will remain weak, 50-60
   kt flow just above the surface and strong low-level shear will
   contribute to the threat for damaging winds, and perhaps an embedded
   tornado or two, with the forced band of convection.  The threat for
   damaging winds should diminish by late evening as the cold front
   overturns the remaining weakly unstable warm sector.

   ..Thompson/Peters.. 11/18/2017

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