SPC 1300Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0650 AM CST Sun Jan 24 2021

Valid 241300Z - 251200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
NORTH-CENTRAL/NORTHEAST TEXAS AND SOUTH-CENTRAL/SOUTHEASTERN
OKLAHOMA...

...SUMMARY...
Large hail and isolated wind damage will be possible this evening
and tonight across parts of the southern Plains.

...Synopsis...
In mid/upper levels, a pronounced split-flow pattern exists over the
western CONUS, with the splitting feature being a cyclone over
southern CA and trough extending southwestward from there.  Upstream
troughing -- extending south-southeastward from a cyclone over the
gulf of Alaska -- will dig southeastward toward and across the CONUS
Pacific Coast today and tonight.  As that occurs, the leading
cyclone will pivot eastward and devolve to a strong open shortwave
trough over AZ and Sonora by 00Z.  The trough will deamplify further
-- but remain well-defined -- as it ejects northeastward across NM
and the south-central High Plains by 12Z tomorrow. 

At the surface, 11Z analysis showed a cold front from the upper
Mississippi Valley across southeastern KS, through a weak low near
PNC, then southwestward over southern NM.  A dryline was drawn from
southwestern OK/northwest TX through a weak low near FST, then
southeastward over northern/central Coahuila.  A separate, marine
warm-frontal zone was evident across the northern Gulf shelf waters
westward over deep south TX.  The TX part of this boundary will
diffuse as it shifts northward today, with another warm frontal zone
developing over north-central/northeast TX this evening, on the
southern fringe of a large area of precip and embedded convection. 
The cold front will overtake the dryline from north-south through
the period, as 60s F surface dew points spread northward in the warm
sector.  The southwest TX surface low should ripple northeastward
along the cold front from 00Z onward, approaching the SPS area by
06Z and moving between TUL-FSM around 12Z.  By then, the cold front
should extend from the low across southeastern OK and northeast
through south-central TX. 

...Southern Plains to AR...
Scattered to locally numerous thunderstorms are possible within two
main episodes:  
1.  A broad area of convection forming along and mostly north of the
warm front and surface low this afternoon and evening, spreading/
expanding northeastward across much of north-central/northeast TX,
the Red River region, central/eastern OK, and into portions of AR
and the Ozarks tonight. 
2.  Thunderstorms developing initially over parts of northwest TX
this evening (probably after 00Z) and moving northeastward, while
expanding/backbuilding.  The net effect should be a growing area of 
increasingly organized thunderstorms shifting eastward over north TX
and southern OK overnight, ultimately merging with the northwestern
part of the first regime over eastern OK or AR. 

Large hail will be possible in both regimes, with elevated
supercells possible in the first.  The second may access enough
MUCAPE (up to 1000-1500 J/kg) and greater effective-shear magnitudes
(55-70 kt) to support both supercell and bowing structures.  While a
strong and fairly deep stable layer will persist across most of OK,
sufficient buoyancy and downdraft intensity may develop above that,
in an organized convective complex, to penetrate isolated severe
gusts to the surface.

An antecedent EML is in place over much of central/south TX,
well-sampled by 12Z RAOBs from CRP/BRO/DRT.  The EML will spread
northward and northeastward, weakening north of the surface warm
front, and will remain viable long enough to substantially limit or
even prevent development in the free warm sector (south of the warm
front).  Direct upper-level support from the ejecting trough will be
limited as well, at best glancing the northwest fringes of the warm
sector. The strongest large-scale ascent (and great majority of
substantial convection) should be in a plume of buoyancy elevated
over relatively stable surface conditions farther poleward.

Nonetheless, strong low-level lift will be associated with the
segment of the cold front near and just south of the progressive
surface low, as the boundary impinges on the northern part of the
warm sector late tonight.  This should overcome the basal EML
inversion and encourage convective initiation within about 50 kt
effective-shear magnitudes, and at least some access to relatively
high theta-e in the warm sector, across parts of north-central/
central TX.  The DFW Metroplex already was included in the area of
largest outlook probabilities for its portion of this regime. 
However, more room is being given farther south for potential
backbuilding of thunderstorms, and at least marginal severe
potential therewith.

..Edwards/Gleason.. 01/24/2021

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