SPC AC 111249

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0749 AM CDT Sun Mar 11 2018

   Valid 111300Z - 121200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER SOUTHERN
   PORTIONS OF LOUISIANA AND MISSISSIPPI...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ELSEWHERE FROM
   CENTRAL LOUISIANA TO EXTREME SOUTHWESTERN GEORGIA AND NORTHWESTERN
   FLORIDA...

   ...SUMMARY...
   Damaging wind gusts and isolated large hail are possible this
   morning and afternoon across parts of the central Gulf Coast states,
   centered over Louisiana and southern Mississippi.  Isolated severe
   weather is possible across the remainder of the area extending into
   northwestern Florida.

   ...Synopsis...
   The upper-air pattern features split flow over western North
   America, with high-amplitude synoptic ridging from the northern
   Rockies to western Canada, and an "undercutting" fetch of nearly
   zonal flow across the Southwest, southern Plains and Southeast.  A
   strong shortwave trough -- now evident in moisture-channel imagery
   over parts of IA, MO and KS -- is forecast to amplify and move
   southeastward by 00Z to KY, western TN, northern MS, and the
   Arklatex region.  An embedded, 500-mb closed low should develop over
   the Tennessee Valley region as the trough continues digging
   southeastward, reaching GA, the FL Panhandle and the northeastern/
   central Gulf by 12Z.  The re-establishment of mean eastern troughing
   will be aided further by southeastward motion and amplification of a
   strong northern-stream shortwave trough/low, from the western Hudson
   Bay/northwestern ON area to the upper Great Lakes. 

   At the surface, a weak wave low was evident over east-central TX,
   along a cold front drawn from western TN through the Arklatex across
   the Edwards Plateau and far west TX.  A marine frontal zone was
   drawn offshore from the Carolinas and northern FL, wrapping across
   the south-central Peninsula then northwestward over the northeastern
   Gulf, becoming diffuse over the southeastern LA/southern MS region. 
   The frontal-wave low should reach middle/eastern TN by 00Z, with
   cold front southwestward across northern AL, southern MS, and
   southeastern LA.  By 12Z, a new wave cyclone should form along the
   marine front over the Atlantic, offshore from the Carolinas, with
   cold front southwestward across extreme northern FL and the
   central/northeastern Gulf.  The pre-cold-frontal segment of the Gulf
   marine front may reach portions of the west-central/northwestern
   peninsular coast late in the period before being overtaken by the
   synoptic cold front. 

   ...Gulf coastal plain, lower Delta...
   Initial outlook adjustments are based on intensity and timing trends
   with ongoing convection -- primarily a wavy band of thunderstorms
   located from southern portions of AL/MS across central LA to east
   TX.  This activity, which should remain largely prefrontal in
   placement and quasi-linear in mode, is expected to proceed
   southeastward into a gradually destabilizing boundary layer before
   moving obliquely offshore from LA, MS, AL and the FL Panhandle
   through this afternoon.  Damaging gusts will be the main concern,
   and a rogue/brief tornado cannot be ruled out.  The hail potential
   appears to be limited by weak lapse rates and largely unsuitable
   convective mode; therefore, those probabilities have been decreased.

   The antecedent westerly to west-northwesterly flow aloft, combined
   with inertial forcing from storm-top divergence, has spread anvil
   material over much of the outlook area, which will mute (but not
   eliminate) direct diabatic surface heating.  Still, when combined
   with low-level theta-e advection, forecast soundings suggest parcels
   over the LA/MS part of the outlook area will become effectively
   surface-based through midday, prior to the arrival of the convective
   band, amidst roughly 50-kt effective-shear vectors.  A wedge of
   500-1000 J/kg preconvective MLCAPE should develop -- narrowing and
   decreasing in magnitude with eastward extent in tune with the
   observed weakening of low/middle-level lapse rates in morning RAOBs.
   Northeastward progress of the marine front will be slow as well,
   limiting inland penetration of optimally moist air eastward toward
   southeastern AL, southern GA and the central/eastern FL Panhandle,
   where severe potential looks much more marginal and conditional. 
   Strong/isolated severe thunderstorms forming late tonight over the
   Gulf may reach parts of coastal northwestern FL before weakening.

   ..Edwards/Smith.. 03/11/2018

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