SPC AC 061251

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0751 AM CDT Fri Apr 06 2018

   Valid 061300Z - 071200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM PARTS OF
   NORTH-CENTRAL/NORTHEAST TEXAS TO WEST-CENTRAL MISSISSIPPI...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ELSEWHERE FROM
   NORTH-CENTRAL/EAST TEXAS TO PARTS OF SOUTHERN ALABAMA...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   SLIGHT RISK...

   ...SUMMARY...
   Severe thunderstorms with damaging winds, occasional hail, and a few
   tornadoes, are possible today from parts of north-central/northeast/
   east Texas to the lower Mississippi Valley and central Gulf Coast
   regions.

   ...Synopsis...
   In mid/upper levels, broadly cyclonic flow is forecast to persist
   over most of the country east of the Rockies, while a high-amplitude
   synoptic-scale ridge extends down the Pacific Coastal states/
   provinces from Yukon.  A well-defined shortwave trough and embedded
   mid/upper cyclone were evident in moisture-channel imagery north-
   northeast of HI and west of CA, between 150-155W.  This feature
   should intensify into a strong cyclone aloft through the period and
   move east-northeastward toward the coastal Pacific Northwest, making
   landfall there on day-2.  Well downstream, a subtle shortwave trough
   was apparent over the central/southern High Plains, in the broader-
   scale cyclonic-flow field.  This feature is forecast to proceed
   east-southeastward, reaching the southernmost Appalachians by 12Z,
   and phasing with weaker/upstream vorticity maxima to form a
   positively tilted 500-mb trough from near the TN/NC border to
   southern LA.

   At the surface, 11Z analysis showed a cold front from Lower MI
   across southern portions of IL/MO to central OK, the TX Panhandle,
   and southern CO, becoming quasistationary in the Rockies across
   central/northern CO, western WY and the ID/MT border region.  A weak
   low was evident along the front over northwest TX/southwest OK, with
   a warm/marine front extending from the northern fringes of the DFW
   Metroplex across the TYR/LFK areas to near BPT and the LA shelf
   waters.  The frontal-wave low is forecast to shift/redevelop
   eastward through the day, likely modulated to some extent by
   convective processes, but nonetheless reaching the central/western
   AL area by the end of the period.  By 00Z, the cold front should
   reach northwestern MS, the Arklatex region, west-central TX and
   northeastern NM.  By 12Z, this front is expected to be located near
   a line from ORF-AVL-BHM-POE-LRD, and will overtake the more slowly
   inland-moving marine boundary from northwest-southeast across
   MS/southern AL this evening and tonight.

   ...North-central/northeast/east Texas to lower Mississippi Valley...

   Widely scattered to scattered thunderstorms are evident from eastern
   OK and parts of northeast TX across AR, in a zone of elevated
   low-level warm advection, moisture transport and isentropic lift to
   varying LFCs, north of the marine/warm front.  Such development
   should continue in a sporadic/episodic manner throughout the day,
   with the approach of the southern High Plains shortwave trough and
   broad, gradually eastward-shifting, 35-45-kt LLJ.  Isolated severe
   hail will be possible north of the warm front.

   The best-organized severe threat is expected to develop as
   convection near and north of the warm front, and near the
   strengthening cold front, shifts southeastward and encounters
   increasingly unstable, surface-based effective-inflow parcels
   representative of the marine sector.  Activity should aggregate
   gradually into a primary QLCS, with elements both forward-
   propagating southeastward across the Arklatex/Delta regions and
   backbuilding over portions of north-central/northeast TX.  Damaging
   wind will be the main concern.  Large hail is possible mainly near
   the west end where deep-later lapse rates are steepest, with
   isolated significant/2+ inch hail possible from any supercells that
   can develop before being undercut my the deepening cold-frontal
   slab.  A few tornadoes may occur either from:
   1.  Mesovortices associated with QLCS LEWP/BOW features and/or
   line-embedded supercells;
   2.  Any warm-sector supercells that can form and mature east of the
   strongest EML over LA or the lower Delta region before being
   overtaken by the QLCS.  This modal potential is more conditional and
   uncertain.

   Forecast soundings ahead of the cold front and south of the
   developing QLCS show lapse rates and MLCINH each increasing
   westward.  Surface dew points 60s F and pockets of favorable surface
   heating should contribute to peak afternoon MLCAPE around 3000 J/kg
   over north-central/northeast TX, decreasing to around 500-800 J/kg
   in central MS, with maximal effective-shear magnitudes generally in
   the 40-50-kt range.

   Severe potential will be limited on the north and east fringes of
   the outlook area by increasing lack of boundary-layer lapse
   rates/buoyancy poleward of the fronts, on the west part by stronger
   EML and cold-frontal undercutting, and on the south by
   nocturnal/diabatic boundary-layer stabilization.

   ..Edwards/Kerr.. 04/06/2018

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