The SPC is forecasting ...Severe thunderstorms expected over parts of the East Texas to lower Mississippi Valley region later today and tonight....   Please read the latest public statement about this event.  View What is a Watch? clip.

   SPC AC 131257

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0757 AM CDT Sat Apr 13 2019

   Valid 131300Z - 141200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   EAST TEXAS...CENTRAL/NORTHERN LOUISIANA...SOUTHEASTERN
   ARKANSAS...AND WESTERN/CENTRAL MISSISSIPPI...

   ...SUMMARY...
   Tornadoes -- some potentially strong to violent (EF2+) -- are
   possible today from east Texas to central Mississippi.  Otherwise,
   numerous severe thunderstorms will pose a risk of large hail,
   damaging wind and tornadoes from central Texas this morning to the
   Tennessee Valley region overnight.

   ...Synopsis...
   In mid/upper levels, a leading/northern-stream perturbation will
   eject northeastward across from its present position over Lake
   Superior and northern Ontario, across Quebec.  To the southwest, a
   deep, high-amplitude trough is evident in moisture channel imagery
   over NM, far west TX, and northern MX.  This trough will pivot
   east-northeastward today, developing a closed 500-mb low by 00Z near
   the DFW Metroplex, and extending southwestward across Coahuila.  By
   12Z, that low should reach eastern MO, with trough southwestward
   over the Arklatex to deep south TX.

   At the surface, the 11Z analysis showed the primary surface low
   along a band of convection south of SJT.  A double structure was
   apparent to baroclinic zones to its east:  the first a
   quasistationary front across central TX to near a SHV-CBM-CHA axis. 
   The southern front was quasistationary just south of UVA and SAT,
   becoming a warm front over southeast TX and south-central LA, then
   quasistationary again over coastal AL and coastal western FL
   Panhandle.

   The primary surface low should deepen and cross the Arklatex region
   late this afternoon into early evening, reaching the lower Ohio
   Valley around EVV by 12Z.  The southern is expected to accelerate
   northward from mid/late morning, to and perhaps past the I-20
   corridor in northeast TX and LA, then over Mississippi, likely
   merging with the northern one.  By that time, a cold front should
   extend from the low south-southwestward across the MS Coast area,
   likely preceded by an organized quasi-linear MCS over the Deep
   South.

   ...TX to Lower Mississippi Valley, Mid-South and TN Valley...
   A regional outbreak of severe thunderstorms, with the main concern
   being damaging tornadoes, still appears possible today in and near
   the categorical moderate-risk area.  Only peripheral adjustments
   were needed for this outlook package, based on convective trends
   this morning and (on the eastern edge) continuity of overnight
   severe potential into the early day-2 period.

   Strong-severe thunderstorms are ongoing across portions of the Hill
   Country southward toward the Rio Grande.  This convection will pose
   a severe threat into central TX through the rest of the morning. 
   Refer to SPC watches 48-49 and associated mesoscale discussions for
   near-term details.

   East of the ongoing activity, the parameter space will become very
   supportive of supercell/tornado potential through the day in the
   warm sector, with the main uncertainties being longevity and
   discreteness of favorable storm mode.  As the southern front moves
   farther inland, a deeply moist boundary layer will spread across the
   region, with surface dew points commonly in the mid/upper 60s F and
   locally near 70, PW 1.5-1.75 inches, and mean mixing ratios
   increasing to 15-16 g/kg, with forecast soundings showing favorably
   low LCL.  Gradual diabatic heating in pockets will weaken already
   meager MLCINH from midmorning onward, enabling storm formation on
   pre-MCS sources for weak lift such as confluence/convergence lines,
   and perhaps persistent/relatively deep horizontal convective rolls. 
   Lapse rates will strengthen with westward extent, supporting
   preconvective MLCAPE ranging from 2000-3000 J/kg over parts of east
   TX to 1000-2000 J/kg over portions of LA and MS.

   Such buoyancy will be more than favorable for all forms of severe,
   amidst strengthening deep shear related to the approach of the
   mid/upper trough.  Furthermore, low-level shear vectors and
   hodograph sizes will be quite large along and south of the warm
   front, with forecast soundings yielding 250-600 J/kg effective SRH. 
   Any sustained supercells in this environment will be capable of
   significant tornadoes.

   With time this evening and overnight, convection should organize
   into a roughly north/south-aligned band over the lower Mississippi
   Valley region, as deep convergence becomes better-focused in the
   mass response ahead of the synoptic wave.  As this occurs, the main
   threat in a bulk sense may become damaging wind tonight.  However,
   given the strong inflow-layer SRH, tornadoes still will be probable
   from both embedded supercells and QLCS mesovortices.

   ...Central/eastern NC and vicinity...
   Widely scattered to scattered thunderstorms are expected to develop
   through this afternoon, offering the potential for damaging gusts --
   perhaps reaching severe limits on an isolated basis -- and isolated
   severe hail.

   Mid/upper-level support will be lacking, beneath a belt of
   southwesterlies aloft.  However, diabatic surface heating amidst
   weak ambient MLCINH should enable only weak lift necessary to
   initiate convection.  The area will straddle a low-level moist axis
   characterized by surface dew points generally in the mid/upper 60s
   F, supporting peak preconvective MLCAPE in the 1500-2000 J/kg range.
   Though low-level flow will be weak, limiting boundary-layer shear
   and hodograph size, the height gradient will remain sufficiently
   tight aloft to support strong storm-relative/ventilating winds in
   upper levels, and effective-shear magnitudes around 40-50 kt.  As
   such, a few organized multicells and sporadic supercell structures
   will be possible.  Convective potential will be strongly tied to the
   diurnal cycle, and should wane precipitously after 00Z.

   ..Edwards/Wendt.. 04/13/2019

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