SPC AC 191249

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0649 AM CST Fri Jan 19 2018

   Valid 191300Z - 201200Z

   ...NO SEVERE THUNDERSTORM AREAS FORECAST...

   ...SUMMARY...
   A few thunderstorms are possible near portions of the Texas coast. 
   Severe storms are not expected today.

   ...Synopsis...
   In mid/upper levels, a low-amplitude pattern exists across the
   northern 1/2-1/3 of the country, downstream from a high-amplitude,
   positively tilted trough now located just offshore from the West
   Coast.  That trough is forecast to move eastward through the period,
   reaching southern ID, NV, southern CA, and offshore central/northern
   Baja by 12Z.  Meanwhile, a considerably weaker southern-stream
   trough -- now evident in moisture-channel imagery from AR across
   south TX to Sinaloa (MX) -- is expected to move eastward and weaken
   gradually.  By the end of the period, this trough should be located
   from northern AL to deep south TX and northeastern MX.  

   At the surface, stable/low-theta-e conditions will prevail over most
   of the nation in the wake of recent polar frontal passages. 
   However, sufficient low-level moisture, lift and instability above
   the surface will exist to support a thunderstorm risk near the TX
   coast as discussed below.  A north-south elongated, frontal-wave
   cyclone was analyzed at 11Z off the lower TX coast with
   quasistationary front southward parallel to the northeastern MX
   coast and a warm front east-southeastward across the central Gulf. 
   A second front, representing a marine-flux-induced thermal gradient
   in cooler ambient air, arched from the low across the outer shelf
   waters of the TX coast.  The low should become more diffuse this
   evening and overnight as the mid/upper wave moves past, while the
   marine/warm-frontal zones slowly merge and shift northward.

   ...TX coast...
   A plume of convection, with isolated to widely scattered embedded
   thunderstorms, will continue through much of today before 
   diminishing tonight as related lift weakens.  Until then, this
   activity will be supported by a combination of: 
   * Elevated moisture return in an immature but still sufficient
   source-trajectory modification regime;
   * Contributions to large-scale ascent from weak DCVA (ahead of the
   southern-stream trough) and low-level warm advection;
   * Frontogenetic forcing, primarily over water.

   The 12Z CRP sounding yielded about 500 J/kg elevated MUCAPE, which
   should be near the maximum amount likely to occur over land. 
   Buoyancy should be larger and deeper over water and near the low
   where low/middle-level lapse rates are greatest.  Although the bulk
   of thunderstorms will remain over the Gulf, sporadic/isolated
   lightning is possible over the coast and just inland.

   ..Edwards/Broyles.. 01/19/2018

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