SPC AC 171246

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0646 AM CST Sat Nov 17 2018

   Valid 171300Z - 181200Z

   ...NO SEVERE THUNDERSTORM AREAS FORECAST...

   ...SUMMARY...
   Severe thunderstorms are not expected today through tonight across
   the contiguous United States.

   ...Synopsis...
   The mid/upper level pattern will feature broadly cyclonic to zonal
   flow over most of the central/eastern CONUS, with high-amplitude
   ridging over the northeast Pacific approaching the West Coast.  In
   between, a positively tilted shortwave trough is located initially
   over the northern Rockies to far northeastern CA.  This feature is
   forecast to move southeastward and elongate somewhat, while
   retaining a decidedly positive tilt.  By 12Z, the trough should
   extend from IA across northern CO to southern NV.  To its south and
   southeast, the westerly to west-southwesterly flow fetch will
   contain mostly very minor perturbations.  One of those, however, is
   fairly readily apparent in moisture-channel imagery over the central
   Gulf of California/Baja area, and should reach the lower/middle TX
   Coast around 06Z. 

   At the surface, the 11Z analysis showed a cold front from Lake Huron
   to eastern IA, through a weak frontal-wave low over western MO near
   SZL, across southeastern KS, northwestern OK, and the northern TX
   Panhandle.  This front should reach southern MO, southwestern OK,
   and the west TX Permian Basin by 00Z.  By 12Z, the front should
   extend from southern ON across northern AR and east-central TX to
   near LRD.  

   ...Coastal deep south TX...
   Isolated prefrontal thunderstorms are possible in the pre-dawn hours
   shortly before 12Z, in the early stage of the regime expected to
   expand across south TX day-2.  By the last 3-5 hours of the period,
   sufficient airmass modification in low levels, and warm-
   advection-related steepening of low/middle-level lapse rates, should
   occur to render the deep-layer environment at least marginally
   favorable for thunderstorms.  Subtle mid/upper-level lift/
   destabilization should occur, related to the passing perturbation
   now near Baja.  Accordingly, time series of forecast soundings
   across available models now yield a strong consensus for convective
   destabilization of the column to equilibrium levels residing between
   250-400 mb pressure levels, all of which will be above the level of
   the -20 deg C isotherm.  MUCAPE values in the 500-800 J/kg range are
   possible.  

   The question then becomes one of coverage of deepest convection
   before 12Z, which will be tied strongly to mesobeta- and
   smaller-scale lift in the warm sector.  The strength of the
   near-coastal confluence/convergence zone may become sufficient for
   deep-convective mass continuity, as indicated by the convection-
   allowing guidance that does extend after 06Z.

   ..Edwards.. 11/17/2018

   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z