Valid 181300Z – 191200Z
…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
WEST-CENTRAL/NORTHWEST TEXAS AND SOUTHWESTERN/CENTRAL OKLAHOMA…
…SUMMARY…
Scattered severe thunderstorms are expected from this afternoon
through tonight across parts of the southern and central Plains.
Severe gusts, large hail, and a few tornadoes are possible.
…Synopsis…
In mid/upper levels, an elongated cyclone is apparent in moisture-
channel imagery with primary vorticity centers near the NV/OR border
and west of northern Baja. The northern segment will pivot back
southward across portions of NV as a closed cyclone through the
period, while the southern ejects northeastward to Sonora/AZ by 00Z,
and southeastern CO/eastern NM by 12Z tomorrow. Minor perturbations
in the preceding southwest-flow field — some convectively induced/
aided, and likely poorly resolved by synoptic models — should cross
northern MX and the southern Plains throughout the period. Deep-
layer mass response to the approaching/ejecting perturbation will
strengthen flow across the southern/central Plains considerably this
evening and tonight, with a broad area of 60-80-kt 500-mb winds and
50 kt at 850 mb.
Surface features over much of the central CONUS are rather muddled
this morning, thanks to areas of convection and precip that have
crossed parts of TX/OK the last couple days and led to diffused
synoptic features. A broad area of low pressure is evident across
parts of WY/northern CO, with lee troughing south-southeastward over
eastern portions of CO/NM. A weak warm frontal zone was evident
over parts of eastern through northwestern KS. Gradual
cyclogenesis/frontogenesis will continue over the central Plains
through the period, with a better-defined surface low likely
developing over eastern CO by 12Z. Warm frontogenesis will extend
roughly up the Ohio Valley overnight.
A rich reservoir of low-level moisture is apparent south of the
outflow boundary, with mid/upper 60s F dew points being common. As
the warm sector over west TX and OK recovers and heats diabatically,
a dryline should become better-defined this afternoon from northern
Coahuila across the Permian Basin/South Plains region and perhaps
into the TX Panhandle. An outflow boundary was analyzed from the
Arklatex area across the southern DFW Metroplex to near SJT. This
feature should move slowly northward and become more diffuse in
baroclinic/wind presentation through late afternoon.
…Southern/central Plains…
Multiple episodes and areas of thunderstorms are possible from
mid/late afternoon onward, generally increasing in coverage through
tonight while moving generally northeastward across the outlook
area. Convection may form in the warm sector, on the dryline, in
higher terrain of northern MX, and in a plume of strong UVV behind
the dryline, becoming better organized into more-favorable low-level
moisture.
As all this activity will be occurring in a broad area of
strengthening deep-layer winds and deep shear this evening and
overnight, at least marginal severe threat will cover a large area.
Total severe potential still appears most concentrated across parts
of west-central TX into southwestern OK, with damaging-wind threat
maximized from upscale-aggregating convection this evening and
tonight. Some of this complex should evolve from relatively
discrete supercells in the western parts of the outlook area, and
perhaps near the outflow boundary. Those supercells will host the
greatest threat for large/damaging hail and a few tornadoes.
Tornadoes also may occur with line-embedded circulations this
evening into tonight, as convection encounters SRH boosted by a
broad, strengthening nocturnal LLJ.
Although only minor changes have been made to the categorical
outlook areas, a couple of primary sources of mesoscale
uncertainty/concern since have arisen, both related to the outflow
boundary.
1. Delay in moistening/destabilization with northward extent from
the boundary’s current location across OK. The enhanced wind
potential is being maintained into OK for now, given the late
arrival of the strongest forcing for ascent, maximizing time for
rapid preconvective airmass recovery from theta-e advection.
2. Potential for a supercell or two to form in the warm sector and
interact with the boundary’s residual vorticity field over parts of
northwest or north-central TX, in a moisture-rich, low-LCL setting.
That would locally enhance its tornado/hail threat, in addition to
that from the other regimes summarized above. This potential is too
conditional and speculative to warrant a categorical upgrade for
now, but is acknowledged in probabilistic shapes.
…Portions of Ohio Valley/KY…
Scattered thunderstorms are expected to move through the outlook
area through the afternoon, with isolated damaging gusts possible,
and a tornado cannot be ruled out.
A field of precip/convection now covering parts of IA/MO/AR is
occurring in a broad swath of large-scale ascent — related both to
low-level warm advection and DCVA preceding a southwest-flow
shortwave trough over eastern KS. Intensification of some of this
activity and/or additional development in the UVV plume may occur as
the foregoing boundary layer gradually destabilizes today. A
45-60-kt southwesterly LLJ preceding the shortwave trough will boost
both low-level shear and moisture content in support of some ramp-up
in convective potential. Though low/middle-level lapse rates will
be weak, forecast soundings suggest the environment may become
surface-based, with MLCAPE reaching 200-500 J/kg, amidst 30-45-kt
effective-shear magnitudes and highly variable (but potentially
supercell/mesocirculation-supporting) effective SRH. The spatial
bound of this threat is more nebulous than a literal line on a map
implies, given its subtly forced character and marginal nature, and
may need adjustment through the day as mesoscale trends warrant.
..Edwards/Goss.. 03/18/2020
$$