SPC AC 181728

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1128 AM CST Wed Jan 18 2017

   Valid 191200Z - 201200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
   OF THE LOWER MISSISSIPPI VALLEY AND CENTRAL GULF COASTAL REGION...

   ...SUMMARY...
   A marginal severe threat will be possible across parts of the Lower
   Mississippi Valley and central Gulf Coast states on Thursday.

   ...Synopsis...
   Substantial progression/evolution of the large-scale upper flow
   field is expected across the U.S. Day 2/Thursday.  An upper trough
   over the West is forecast to move eastward across the Rockies, while
   digging energy on the back side of the trough shifts across the
   eastern Pacific toward the West Coast later in the period.  This
   will result in overall expansion of longwave troughing over the
   western half of the country.

   Farther east, an upper low initially progged to lie over the Central
   Plains is forecast to weaken with time, as it moves east-northeast
   toward/into the Midwest while evolving into an open wave.

   At the surface, a weakening cool front moving eastward across east
   TX/AR/LA early in the period is expected to wash out with time,
   while a weak warm front shifts north across the TN Valley area.  In
   the West, a generally weak surface pattern is forecast, though a
   deep low and associated front over the eastern Pacific -- associated
   with the aforementioned digging upper system -- should begin to
   approach the coast late in the period.

   ...Lower MS Valley and central Gulf Coast region...
   Widespread showers and scattered thunderstorms are expected to be
   ongoing early in the period from parts of east/southeast TX and the
   adjacent western Gulf into the mid and lower MS and TN Valleys, near
   and ahead of a weakening cool front and associated short-wave
   troughing moving east across the region.  Though a high theta-e
   low-level airmass will support modest buoyancy -- allowing
   convection to spread east across the central Gulf Coast region with
   time, widespread clouds and precipitation (driven by
   quasi-geostrophic ascent) should hinder potential for appreciable
   steepening of lapse rates, therefore limiting updraft intensity. 
   Thus, while low-level shear should remain otherwise sufficient, lack
   of stretching potential in low levels suggests only brief/weak
   tornado risk.  

   Some risk for locally gusty/damaging winds will exist, particularly
   with any banded structures (as hinted at by some high-res CAMs). 
   However, with the weakening surface front and thus lack of strong
   linear forcing, a well-organized line is not expected.  Convection
   is expected to spread northeast with time, though generally
   weakening/decreasing in coverage during the second half of the
   period.

   ...Coastal southern CA...
   A short-wave upper trough expected to be moving onshore at the start
   of the period will support showers and a few embedded thunderstorms
   over portions of CA, and later across the CO Valley into portions of
   AZ.  While weak instability across most of the area should hinder
   convective intensity, a few stronger low-topped cells may move
   onshore early in the period across coastal southern CA.  With
   low-level flow not expected to be particularly strong, any risk for
   damaging convective gusts appears too low to warrant introduction of
   a risk area at this time.

   ..Goss.. 01/18/2017

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