SPC AC 061248

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0648 AM CST Wed Jan 06 2021

   Valid 061300Z - 071200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   SOUTHEAST TEXAS...

   ...SUMMARY...
   Severe thunderstorm winds and a couple tornadoes are possible today
   into tonight along parts of the western Gulf Coastal Plain.

   ...Synopsis...
   In mid/upper levels, a highly amplified, progressive synoptic
   pattern is dominated by:
   1.  A broad, slow-moving cyclone over the North Atlantic, centered
   south of NS,
   2.  Troughing from the length of MB south-southwestward across the
   Plains states to Chihuahua, and
   3.  A strong shortwave trough offshore from the Pacific Northwest
   and northern CA that will contribute to isolated thunder potential
   near the coast.

   In the southern part of the second synoptic trough, a strong, basal
   vorticity max/PV anomaly and accompanying shortwave trough are
   located initially over eastern NM, based on moisture-channel imagery
   and objective analyses.  This perturbation is forecast to dig
   southeastward to central TX by 00Z, then eastward to northern LA by
   12Z.  This will form a closed 500-mb cyclone in the process, while
   the accompanying 250-300-mb speed maxima blend with a nearly zonal
   subtropical jet preexisting across northern MX and south TX.

   As that occurs, a surface low -- initially analyzed near SPS with
   cold front across the Edwards Plateau -- will move east-
   southeastward over northeast TX and northern LA, while occluding. 
   By 00Z, the cold front should arc from the low near TYR across east
   TX then southwestward over the middle/lower TX Coastal Plain and
   deep south TX.  By 12Z, the cold front should extend from a triple
   point in the SIL/BIX area across the mouth of the Mississippi River
   to the west-central Gulf.  A diffuse/marine warm front will extend
   eastward close to the AL coast then southeastward over the eastern
   Gulf, while a synoptic warm front in cooler/less-modified air should
   extend across the northern/central FL Peninsula. 

   ...Near-coastal TX/LA...
   Scattered thunderstorms should develop close to the front -- as
   early as late morning over central TX -- and build southward with
   time to the middle TX Coastal Plain.  The entire resulting, arching
   MCS should shift eastward and southeastward across the western
   outlook area through this evening, offering damaging to locally
   severe gusts, isolated/marginally severe hail, and some tornado
   potential.  A related but separate area of more conditional,
   marginal severe potential may develop late tonight into early
   tomorrow morning over extreme southeastern LA and vicinity, as the
   convection (which may continue in elevated form across the
   intervening gap) encounters a surge of sufficiently moist, unstable
   boundary-layer air.

   With surface dew points now only in the mid 50s to low 60s F across
   parts of south-central TX, modified RAOBs and RAP soundings already
   indicate minimal MLCINH, related to a combination of cooling air
   aloft and lack of a stronger EML.  This partially modified warm
   sector will shift eastward and destabilize further through the day,
   from a combination of continued theta-e advection and slow diabatic
   surface heating (blunted by cloud cover).  That should support
   around 1000 J/kg MLCAPE over western parts of the outlook area,
   decreasing northeastward and eastward over east-central/southeast TX
   where deep-layer lapse rates will be weaker overall.  Effective-
   shear magnitudes around 40-50 kt and effective SRH in the 100-200
   J/k range will support supercells and line-embedded LEWPs/
   mesovortices, locally boosting the wind/tornado threat.

   As the MCS proceeds eastward through the Golden Triangle region into
   southwestern LA, it will encounter a progressively more-stable
   boundary layer.  This will be related to substantial,
   isallobarically forced easterly component to surface winds that
   emanates from (and helps to maintain) a relatively stable
   continental boundary layer.  With convective inflow accordingly
   becoming elevated, the potential for severe gusts and tornadoes will
   diminish over land, though severe weather may still affect offshore
   interests southeast of GLS and south of the western/central LA
   coastline.  Though the most favorable thermodynamic support will
   remain over the Gulf, non-negligible wind/tornado threat with the
   remains of the MCS may extend northward across parts of southeastern
   LA and coastal MS the last few hours of the period.

   ..Edwards/Leitman.. 01/06/2021

   CLICK TO GET WUUS01 PTSDY1 PRODUCT

   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z