SPC AC 051253

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0653 AM CST Sun Dec 05 2021

   Valid 051300Z - 061200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE ARKLATEX
   REGION TO PARTS OF WESTERN/CENTRAL KENTUCKY...

   ...SUMMARY...
   Severe thunderstorms are possible from the Arklatex region to the
   Mid-South and lower Ohio River Valley.

   ...Synopsis...
   A mid/upper-level split-flow pattern will persist over the western/
   central CONUS through the period, but with amplification of both
   northern and southern streams.  A northern-stream shortwave trough
   -- now evident in moisture-channel imagery from ND across eastern MT
   -- will strengthen and dig southeastward today, extending from
   northern MN to the central High Plains by 00Z.  Associated height
   falls will spread across the mid/upper Mississippi and Ohio Valleys
   overnight, with the trough reaching eastern Upper MI, northern Lake
   Michigan, southern WI, southern IA, and eastern KS by 12Z tomorrow. 

   A positively tilted, southern-stream trough is apparent from the
   northern TX Panhandle across western NM, southern AZ, to north-
   central Baja, then westward for over 1000 nm over the Pacific.  The
   main vorticity lobe -- now over coastal northern Sonora -- should
   move roughly southeastward down the mainland MX coastline toward
   southern Sonora through the period and become a small, cutoff
   cyclone.  By 12Z tomorrow, a weak but lengthy vorticity banner will
   extend from there northeastward across southeast to northeast TX and
   southern AR.

   The 11Z surface analysis showed a primary, already occluded low over
   central ND, with a secondary, weaker low near the frontal triple
   point over west-central/southeastern SD.  A cold front was drawn
   from there over northwestern NE and southeastern WY.  A warm front
   extended from southeastern SD southeastward over southern MO and
   northeastern MS.  The warm front should move diffusely northeastward
   today, with its segment into the Ohio Valley potentially overtaken
   by the northeast tip of a separate marine warm front initially
   analyzed over north TX, southwestern AR and western LA. The main
   surface cyclone should move eastward across northern MN through this
   evening, with another triple-point low forming/deepening overnight
   across lower MI.  The cold front should reach northern IL, northern
   MO, southern KS and southern CO by 00Z.  By 12Z the front should
   reach western parts of OH/KY/TN, central AR, and central to far west
   TX. 

   ...Arklatex and east TX to Ohio Valley...
   Thunderstorms are expected to pose some severe threat in two main
   phases today and tonight:
   1.  Isolated to scattered convection developing in the warm sector
   from midday through afternoon over Arklatex into AR, as the area
   destabilizes from both diurnal heating and boundary-layer theta-e
   advection.  The potential is greater for a discrete supercell in
   this regime of weak MLCINH but also weak low-level forcing.  Any
   sustained cells in this environment will pose a threat for all
   severe modes, amidst 800-1500 J/kg MLCAPE, and favorable hodographs
   with 200-350 J/kg effective SRH.  Coverage is uncertain but likely
   low, especially for surface-based convection once convective
   temperature is attained.  
   2.  Scattered to numerous thunderstorms forming in a quasi-linear
   band near the front tonight, with the band shifting southeastward
   and embedded elements moving eastward to northeastward.  The main
   concern will be damaging to locally severe gusts.  Early-stage,
   marginally severe hail is possible, along with a few line-embedded,
   generally short-duration tornadoes given the favorable low-level
   shear environment into which the front will be impinging.  Given the
   strong component of mean wind parallel to the front, and its motion,
   activity forming along the front may be undercut with time in an
   anafrontal process.  The convective band will maintain some severe
   threat overnight as it moves to as far southeast as eastern KY/TN,
   northern MS, northern LA, and parts of north-central/northeast TX by
   12Z.

   Potential for surface-based effective-inflow parcels ahead of the
   front becomes more uncertain with time and northeastward extent
   tonight over the Ohio Valley region, given the already marginal
   surface theta-e and slow nocturnal/diabatic destabilization.  Still,
   with a narrow corridor of prefrontal/preconvective moisture
   advection expected into the region, modified model soundings suggest
   MLCAPE in the 300-700 J/kg range may extend into portions of
   southern IN and northern KY this evening before being overtaken by
   the convection.  As such, the "marginal" and "slight" probability
   levels have been extended somewhat northeastward for wind potential
   in particular, given the expected quasi-linear convective mode.

   ..Edwards/Goss.. 12/05/2021

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