SPC AC 141247

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0647 AM CST Tue Feb 14 2017

   Valid 141300Z - 151200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS THIS MORNING
   OVER A SMALL PART OF THE MIDDLE-UPPER TX COASTAL AREA...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS INTO
   AFTERNOON...ELSEWHERE FROM THE TX COASTAL BEND TO EAST TX AND
   WESTERN LA...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS TONIGHT ACROSS
   IMMEDIATE COASTAL AREAS FROM THE MS RIVER MOUTH TO THE WESTERN FL
   PANHANDLE...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE WESTERN
   SLIGHT-RISK AREA ACROSS PORTIONS OF CENTRAL/SOUTHERN MS AND AL...

   ...SUMMARY...
   Severe thunderstorms are expected to shift eastward over southeast
   Texas toward the Sabine River region through early afternoon, with a
   few tornadoes and damaging winds possible.  After a relative lull
   when the risk becomes marginal, the threat of severe weather will
   increase conditionally tonight into Wednesday morning, from the
   mouth of the Mississippi River to the coastal Florida Panhandle.

   ...Synopsis...
   A high-amplitude large-scale pattern will persist over North
   America, characterized by troughing from eastern Canada across the
   Great Lakes to the central Appalachians, and a sharp ridge from
   Yukon and the NW Territories across the northern Rockies and CA. 
   East of that ridge, a strong/southern-stream shortwave trough was
   evident in moisture-channel imagery from western KS southward across
   the southern High Plains to the TX Big Bend region and adjoining
   parts of northern MX.  As a basal vorticity max aloft, now near the
   Big Bend, ejects northeastward, the shortwave trough as a whole will
   deamplify gradually, but still remain strong through most of the
   period.  By 00z, the 500-mb trough should reach eastern OK and
   central TX, with a transient closed low possibly developing over
   eastern OK.  Thereafter the perturbation will enter a broader-scale
   confluent-flow regime, reaching the Mid-South and lower Mississippi
   Valley near the end of the period.

   A complex surface pattern was evident in an 11Z analysis in and near
   the outlook region, with a weak low between CLL-CRS and a wavy cold
   front from there southwestward across the western part of the lower
   Rio Grande Valley region.  A diffuse warm front was drawn from the
   low southeastward across the HOU metro area and over shelf waters
   just off the LA coast.  The surface low pressure should consolidate
   better through this afternoon over southeast and east TX, with a
   fairly well-defined low crossing the Sabine River valley near Toledo
   Bend Reservoir mid-late afternoon.  By 00z, the surface low should
   be located over central or northern LA, with the cold front
   southwestward across southern LA and the northwestern Gulf.  Through
   the overnight hours, the surface low should weaken and accelerate
   east-northeastward toward west-central AL.  By 12Z, the cold front
   should extend from there southwestward over southeastern MS,
   southeastern LA and the west-central Gulf.

   ...South-central/southeast TX to western LA...
   A gradually organizing complex of strong-severe thunderstorms is
   forecast to continue shifting eastward to east-northeastward across
   the outlook area into at least early-mid afternoon, noticeably
   faster than progged by most convection-allowing model solutions from
   the 00Z-09Z initialization cycles.  This activity still will cross
   an area of enhanced low-level SRH favorable for embedded supercells
   and QLCS circulations supporting tornado/wind risk.  By mid-late
   afternoon, we currently anticipate a general decrease in both
   large-scale upper support and inflow-layer instability for most of
   the complex as it proceeds deeper into LA, though isolated severe
   and a persistent supercell or two cannot be ruled out.

   In the meantime, lateral expansion of the MCS is possible both
   northward and southward; however, the extent of that is uncertain
   due to:
   1.  Capping on the south end (evident in the 12Z CRP RAOB) versus
   both frontal forcing and the glancing influence of large-scale
   DCVA/cooling aloft on the inversion layer sampled around 700 mb.
   2.  Northern extent of favorable near-surface destabilization/
   effective-inflow parcels, amidst the rather lax thermal/theta-e
   gradient represented by the warm-frontal zone.
   In between, the area around I-10 and southward to the middle/upper
   TX coast, near the axis of SPC tornado watch 32, represents the most
   favorable severe-weather corridor through early afternoon.

   For additional information and nowcast updates, refer to watch-based
   mesoscale discussions, to be issued as needed through the morning
   hours.

   ...Southeast LA to coastal FL Panhandle...
   A conditional slight risk for tornadoes and marginal
   thunderstorm-wind threat remains apparent over this region
   overnight, as the surface low tracks eastward across MS.  Related
   isallobaric forcing should help to maintain generally southerly
   surface winds beneath an eastward-shifting, 45-55-kt southwesterly
   LLJ.  The net effect will be to strengthen low-level bulk shear and
   hodograph sizes with time in any given spot, while deep shear also
   increase as the shortwave trough crosses the MS River and height
   gradients tighten to its south and southeast.  Forecast hodographs
   accordingly show an eastward spread of effective SRH in the 250-450
   J/kg range and effective-shear magnitude near 50 kt along the coast
   where moisture and instability will be most favorable.

   Thermodynamically, lapse rates aloft will be rather weak for much of
   the period.  However, subtle large-scale ascent from both low-level
   warm advection and weak DCVA should steepen lapse rates overnight
   enough, in tandem with surface dew points mid 60s F near the
   northern rim of the most substantial Gulf return flow, to remove
   CINH and support 500-1000 J/kg MLCAPE.  Buoyancy will diminish
   substantially inland, making the potential even more isolated and
   conditional northward.  The main uncertainty at this time involves
   low-level/pre-frontal forcing for convective-scale lift, and
   resulting coverage/mode of storms in an environment otherwise
   favorable for supercells.  Any sustained cells will have the
   potential to generate cyclic mesocyclones with attendant tornado
   threat and potential for at least isolated nontornadic wind damage.

   ..Edwards/Broyles.. 02/14/2017

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