SPC AC 251256

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0656 AM CST Mon Jan 25 2021

   Valid 251300Z - 261200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS TODAY FROM PARTS
   OF THE ARKLATEX EAST-NORTHEASTWARD INTO PARTS OF KENTUCKY AND
   TENNESSEE...AND TONIGHT OVER PARTS OF THE DEEP SOUTH...

   ...SUMMARY...
   A marginal severe threat is expected today from parts of the
   Arklatex east-northeastward into parts of Kentucky and Tennessee,
   and tonight over parts of the Deep South.

   ...Synopsis...
   In mid/upper levels, mean troughing will be maintained over the
   western CONUS as one perturbation exits, and a trailing one 
   amplifies.  The leading shortwave trough -- evident in moisture-
   channel imagery over the central High Plains to the TX Panhandle --
   will eject east-northeastward and deamplify across portions of
   northeastern KS, southeastern NE, northern MO and IA into this
   evening, reaching the mid/upper Mississippi Valley overnight.  The
   trailing perturbation currently is manifest as a negatively tilted,
   elongated trough extending along much of the West Coast with primary
   vorticity max over central CA.  A closed cyclone should evolve from
   that vorticity lobe today, with its 500-mb center near DAG by 00Z,
   then eastward to near GCN by 12Z tomorrow.

   At the surface, 11Z analysis indicated a low near MLC with cold
   front southwestward across the DFW Metroplex to near DRT.  The warm
   front was drawn over southeastern OK and southern AR, becoming
   diffuse eastward over MS.  The low is progged to move east-
   northeastward to eastern KY through the period, while the warm front
   remains poorly defined to its east and southeast.  The cold front
   should reach central AR, east TX and deep south TX by 00Z, moving
   only slowly eastward from there.  By 12Z, the front should reach
   eastern TN, northern/central MS, and southwestern LA.

   ...Arklatex to parts of Ohio/Tennessee Valleys...
   A near-frontal band of convection, with widely scattered embedded
   thunderstorms, is apparent on the western rim of the outlook area. 
   This activity will continue to shift eastward through the day in an
   environment of favorable deep/speed shear but marginal instability,
   offering the threat for isolated severe wind gusts and marginal
   hail.  A tornado cannot be ruled out.

   In the near term, sporadic hail and strong to marginally severe
   gusts will be possible from the most intense cells embedded in the
   band, as it moves through marginally but sufficiently unstable
   airmass to sustain the convection.  A relative lull in convective
   organization may occur through mid/late morning across the Arklatex
   region eastward to near the Mississippi River, until the activity
   encounters a boundary layer gradually destabilizing from both muted
   diurnal heating and low-level theta-e advection, in and south of the
   ill-defined warm-frontal zone.  Deep shear will be favorably strong,
   but with the 55-70-kt effective-shear vector aligned only slightly
   rightward of the convective band.  As such, the mode should remain
   predominantly quasi-linear, but with embedded LEWPs, small bows and
   perhaps occasional supercells.  The foregoing airmass will be moist,
   with low LCL but also poor lapse rates, keeping MLCAPE generally
   500-1000 J/kg (locally/briefly higher, especially this afternoon).

   ...AL, northwestern GA, eastern MS overnight...
   Widely scattered thunderstorms are possible ahead of the front this
   evening and overnight, in a regime of favorable vertical shear.  Any
   sustained convection may pose a threat of damaging gusts or a
   tornado, but coverage and duration of sufficiently deep convection
   remain too uncertain for more than marginal unconditional
   probabilities.  Still, at least isolated supercell potential exists.

   Forecast soundings indicate a deep, high-RH to saturated boundary
   layer above the surface with weak lapse rates, but also, low LCL and
   surface-based effective-inflow parcels.  Peak MLCAPE may reach the
   500-1000 J/kg range, in a corridor of relatively maximized moisture/
   buoyancy extending southwest/northeast ahead of the cold front. 
   MLCINH will be weak, and a collocated zone of persistent low-level
   warm advection, moisture transport and modest convergence is
   expected.  Substantial upper support will remain northwest of the
   area as the shortwave trough ejects, and gradual veering of surface
   winds with time will keep convergence broad and on the margins for
   initiating/sustaining supercells.  Accordingly, guidance is
   inconsistent with longevity and coverage of strong-severe
   convection.  However, tight mid/upper height gradients will
   contribute to favorable deep shear, with effective-shear magnitudes
   around 50-60 kt possible, and 250-350 J/kg effective SRH even amidst
   veering surface flow.

   ..Edwards/Gleason.. 01/25/2021

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