SPC AC 310825

   Day 3 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0225 AM CST Sat Dec 31 2016

   Valid 021200Z - 031200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS MONDAY ACROSS
   PARTS OF SOUTHEAST TEXAS THROUGH MUCH OF LOUISIANA AND
   MISSISSIPPI...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS
   SURROUNDING PORTIONS OF THE WESTERN INTO CENTRAL GULF COAST STATES
   AND ADJACENT PORTIONS OF THE MID SOUTH...

   ...SUMMARY...
   Severe storms are possible Monday across parts of the western into
   central Gulf Coast states.

   ...Synopsis...
   A blocking mid/upper high, centered near the Gulf of Alaska, appears
   likely to remain prominent through this forecast period.  Downstream
   troughing over the northwestern U.S. may undergo considerable
   deformation, with at least one significant impulse forecast to
   emerge from it, and accelerate east northeastward toward the upper
   Great Lakes region.  

   In lower latitudes, subtropical ridging may elongate east to west,
   with its center of highest heights shifting from the Caribbean into
   southern Mexico.  On the northern periphery of this feature, into
   larger-scale ridging over the southeastern U.S., models generally
   indicate significant weakening of a remnant impulse accelerating
   east northeast of the southern Plains.

   In lower levels, while a significant cold intrusion progresses to
   the lee of the northern Rockies, primary surface low development
   appears likely to remain well north of the Gulf Coast region, from
   parts of the central Plains into portions of the Upper Midwest/Great
   Lakes region.  However, it appears that a secondary surface low
   could accompany the southern mid-level impulse, redeveloping
   eastward/northeastward out of Texas.  

   While a moist return flow off the Gulf of Mexico may support
   substantive boundary layer destabilization inland of northwestern
   and north central Gulf coastal areas by 12Z Monday, guidance is
   suggestive that this air mass may be overturned, and return flow
   cut-off, in the wake of considerable convective development expected
   to spread out of Texas early in the period.  It is possible, though,
   that an appreciable risk for severe weather may continue across the
   central Gulf states through the day Monday.

   ...western into central Gulf Coast states...
   Variability exists among the model output concerning just how fast
   forcing for ascent, associated with the impulse emerging from the
   southern Plains, will become displaced from the stronger boundary
   layer instability and weaken during the day Monday.  However, an
   organized mesoscale convective system may be ongoing at 12Z Monday
   across parts of east/southeast Texas.  Aided by a continuing inflow
   of weak to moderately unstable air (probably characterized by CAPE
   in excess of 1000 J/kg), in the presence of sufficiently strong deep
   layer shear, it appears possible that activity could maintain
   significant strength through much of the day Monday, before
   overturning the better boundary layer instability and cutting of the
   richer return flow.  

   Until convection weakens, it may continue to support the risk for
   potentially damaging surface gusts.  Clockwise curved low-level
   hodographs may also be large enough to support potential for
   tornadoes, particularly with any discrete storms which may form near
   or just ahead of the convective system as it spreads across the
   central Gulf States/lower Mississippi Valley.

   ..Kerr.. 12/31/2016

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