SPC AC 180747

   Day 3 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0147 AM CST Wed Jan 18 2017

   Valid 201200Z - 211200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FRIDAY NIGHT
   INTO SATURDAY MORNING FOR PARTS OF THE LOWER MISSISSIPPI VALLEY
   REGION...

   ...SUMMARY...
   Isolated severe storms may occur late Friday night and Saturday
   morning across parts of the lower Mississippi Valley.

   ...Synopsis...
   A broad cyclonic flow regime is expected from the Pacific coast to
   the MS Valley, with the primary jet core likely to extend from near
   the southern CA coast to northern Mexico by the end of the period. 
   A series of embedded shortwave troughs will eject northeastward over
   the central Plains, and from the northwest Gulf coast to the lower
   MS Valley.  This latter wave will interact with a moist low-level
   air mass that is already present across the northwest Gulf coast,
   and support a risk of isolated severe storms late in the period.

   ...Lower MS Valley overnight...
   A relatively quiet period is expected Friday across the lower MS
   Valley, in the wake of the wave forecast to pass on day 2.  A moist
   air mass, with boundary layer dewpoints well into the 60s, should
   remain across the northwest and north central Gulf coast from now
   through Friday, which will contribute to at least weak buoyancy
   within the warm sector.  However, little or no forcing for ascent,
   and related minimal low-level mass response, suggest that
   thunderstorm coverage during the day should be sparse.

   An increase in low-level warm advection and ascent is expected
   Friday night beginning across the Upper TX coast, and continuing
   across LA/MS/AL through Saturday morning.  Strengthening low-level
   southerly flow will result in stronger moisture flux near and just
   above the surface across the lower MS Valley, which will support
   clusters of thunderstorms.  Steeper midlevel lapse rates will also
   spread east-northeastward from south TX above the moist layer,
   resulting in a gradual increase in buoyancy overnight.  On the edge
   of the lapse rate plume, low-level shear will increase overnight,
   supporting the risk for embedded rotating storms.  Given the late
   arrival of the subtle midlevel trough, and the usual mesoscale
   concerns this far in advance, will just introduce low severe
   probabilities, including some hail/wind and perhaps isolated tornado
   risk.

   ..Thompson.. 01/18/2017

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