SPC AC 161250

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0650 AM CST Mon Dec 16 2019

   Valid 161300Z - 171200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM CENTRAL
   LOUISIANA TO WESTERN ALABAMA...

   ...SUMMARY...
   Scattered severe thunderstorms may begin midday today over
   Louisiana, peaking during the late afternoon/evening from northeast
   Louisiana into central Mississippi.  Tornadoes and damaging gusts
   are the primary hazards.  Activity should shift into into Alabama
   and the Florida Panhandle this evening and overnight.

   --- Technical Discussion ---

   ...Synopsis...
   The mid/upper-level pattern over the CONUS will be dominated by
   progressive, high-amplitude ridging -- moving inland from the West
   Coast -- and broadly cyclonic flow downstream associated with a
   complex troughing regime.  A northern-stream shortwave trough --
   emanating from the southwestern rim of a cyclone over the Canadian
   Arctic region -- will dig southeastward across central Canada,
   reaching northeastern MB, northwestern ON, and the Boundary Waters
   area of MN by the end of the period.  As this occurs, a separate
   perturbation -- now evident in moisture-channel imagery over
   portions of CO/NM -- will shift eastward across the southern/central
   Plains.  The northern portion will stretch northeastward overnight,
   across the mid Mississippi Valley and Ozarks, in a field of
   confluent 500-mb flow related to the approaching northern-stream
   trough.  The trailing/southwestern portion will phase with an
   initially separate, weaker vorticity banner pivoting south-
   southeastward from the northern Rockies.  

   By 12Z, these processes should result in a well-defined, positively
   tilted synoptic trough from Lake Michigan across MO, OK and central
   NM.  Strong cyclonic flow will exist just southeast of this trough,
   with a 120-130-kt 250-mb jet by 12Z located from west TX
   northeastward up the Ohio Valley, atop a narrow, 85-95-kt 500-mb jet
   core. 

   At the surface, 11Z analysis showed a cold front across portions of
   WV, southeastern KY, middle TN, southeastern AR, east TX, and
   south-central TX.  This front will move southeastward to northern
   GA, southeastern AL,the western FL Panhandle, and the north-central/
   west-central Gulf through the period.  A weak surface low analyzed
   in the SHV-GGG area will propagate northeastward along the front and
   intensify, reaching eastern WV by 12Z.  A warm/marine front was
   drawn from the AL coastline southeastward across the northeastern
   Gulf to south FL and the northern Bahamas.  This boundary will move
   slowly northward through the period, while return flow spread
   favorably higher theta-e inland to the west across LA/MS in support
   of severe potential. 

   ...Lower Mississippi Valley, Southeast...
   Beginning late morning into early afternoon, development and gradual
   strengthening/expansion of surface-based thunderstorms is forecast
   along/ahead of the surface cold front -- initially over western/
   southern parts of the outlook area.  Activity should grow in
   coverage and move across the Delta, Mid-South and MS/AL regions
   through this evening into tonight.  Tornadoes will be possible
   across the entire region, but with greatest potential in a corridor
   from central LA across central/eastern MS where supercells are most
   probable, and may access the most favorable CAPE/shear parameter
   space.  Damaging gusts should be the most common convective event
   area-wide. Severe hail is most probable over southwestern parts of
   the outlook area -- essentially collocated with greatest tornado
   probabilities and for much the same reason (potential for
   supercells).  

   Surface-based buoyancy and low/middle-level lapse rates each should
   be relatively maximized across the LA/MS part of the outlook.  Rich
   boundary-layer moisture (dew points commonly mid/upper 60s F,
   locally low 70s) and diurnal surface heating will destabilize the
   preconvective air mass through the afternoon.  Meanwhile, a section
   of EML air -- emanating from central/northern  MX with 700-mb
   temperatures in the 7-10 deg C range near the upper TX/LA coasts --
   is well-sampled by the 12Z LCH RAOB.  This plume will help to
   maintain steep midlevel lapse rates, along with somewhat stronger
   MLCINH near the immediate coastal areas of LA.  Capping will erode
   through the day, permitting convective growth/intensification in an
   environment over central LA and MS characterized by 1000-2000 J/kg
   MLCAPE, with 45-55-kt effective-shear magnitudes.  Relatively backed
   surface winds, beneath a 40-60-kt southwesterly LLJ, will contribute
   to enlarged hodographs and 200-400 J/kg effective SRH.  

   Given sustained warm-sector supercells with little storm-scale
   interference, such as progged by the sometimes overly aggressive
   HRRR, such an environment can support significant/EF2+ tornadoes. 
   At this time, however, amount and longevity of relatively discrete
   cells in the warm sector are still uncertain, especially given some
   EML-driven capping, and flow aloft being largely parallel to the
   primary corridor of forcing near the front.  A hatched/significant
   area may need to be added in an upcoming outlook if mesoscale
   diagnostic trends and 12Z numerical guidance increase confidence in
   multiple warm-sector supercells.

   Otherwise, regardless of warm-sector convective density, a band of
   thunderstorms should evolve close to the front and shift across the
   remainder of the outlook area through the period.  This activity
   will encounter gradually weaker low-level and midtropospheric lapse
   rates and less inflow-layer moisture content with northeastward and
   eastward extent -- hence the tapering of severe probabilities in
   those directions.  Still, potential for scattered damaging winds and
   a threat of tornadoes is evident.  One relatively minor change made
   to this package is to add more of the FL Panhandle to the outlook
   for overnight.  This is in deference to the potential for convection
   to form over the Gulf or near the coast -- east of the
   aforementioned EML plume in weaker CINH -- and move inland in
   favorable CAPE and vertical wind profiles for supercells.

   ..Edwards/Broyles.. 12/16/2019

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