SPC AC 180458

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1058 PM CST Wed Nov 17 2021

   Valid 181200Z - 191200Z

   ...NO SEVERE THUNDERSTORM AREAS FORECAST...

   ...SUMMARY...
   The risk of severe thunderstorms appears negligible across the U.S.
   today through tonight.

   ...Synopsis...
   Large-scale mid-level troughing over the interior of North America
   is forecast to progress toward the northern and middle Atlantic
   Seaboard through this period, with a broad embedded lower/mid
   tropospheric cyclone.  The center of the modestly deep and occluding
   surface low appears likely to migrate across the James Bay vicinity
   through much of northern Quebec, with the trailing cold front
   advancing across much of the northern and middle Atlantic Coast
   states by 12Z Friday.  A trailing segment of the front may advance
   more slowly across parts of the south Atlantic coastal plain and
   much of the Gulf coast, but models suggest that it will increasingly
   undercut and cut off the generally modest to weak ongoing moisture
   return off the Gulf of Mexico. 

   A few thunderstorms may be ongoing at 12Z this morning, near or
   ahead of the front across parts of the lower Mississippi Valley and
   Deep South Texas.  While it appears that the risk for thunderstorms
   may be in the process of diminishing across the lower Mississippi
   Valley, guidance suggests that thunderstorm activity may persist
   into midday across the lower Rio Grande Valley, and perhaps include
   an upscale growing cluster of thunderstorms.

   Otherwise, a perturbation emerging from the Caribbean may contribute
   to increasing cloud cover and rain to the north of a weakening
   remnant frontal zone currently south and southwest of the southern
   Florida Peninsula and Keys.  This will impede boundary-layer
   destabilization over the interior southern peninsula during the day,
   but the development of a couple of thunderstorms might still be
   possible.

   Farther north, more appreciable thunderstorm probabilities appear
   likely to be confined to the vicinity of the Gulf Stream, ahead of
   the cold front as it advances offshore tonight.

   ...Deep South Texas...
   A fairly moist boundary layer, coupled with steep lower/mid
   tropospheric lapse rates, may contribute to moderately large
   mixed-layer CAPE (on the order of 1000-2000+ J/kg) along and ahead
   of the southward advancing cold front.  As this occurs coincident
   with weak mid-level troughing progressing across the lower Rio
   Grande Valley, it appears possible that the environment could become
   conducive to the evolution of a substantive cluster of thunderstorms
   with at least some potential to produce strong wind gusts.  However,
   given the weak nature of the lower/mid-tropospheric flow evident in
   the forecast soundings, and the possibility that the convection may
   tend to be undercut by the cold front, the risk for severe wind
   gusts still seems low and probabilities are being maintained at less
   than 5 percent.

   ..Kerr/Lyons.. 11/18/2021

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