SPC 1300Z Day 1 Outlook

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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