Valid 231300Z – 241200Z
…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
THE SOUTH-CENTRAL GREAT PLAINS…
Isolated strong to severe storms are possible across parts of the
south-central Great Plains late this afternoon into early evening.
A progressive split-flow pattern in mid/upper levels will be
maintained across the western/central CONUS through the period. The
split is related to a well-defined cyclone now covering much of the
Great Basin and Southwest Deserts, and centered over the southern
UT/northern AZ region. The cyclone will weaken slightly, but still
maintain its compact and well-defined character, as it shifts
eastward roughly astride the northern borders of AZ, NM and OK
through dawn tomorrow. By then, a trailing, positively tilted
500-mb trough will extend from a low in the PNC/ICT area
southwestward far west TX and central/southern Baja. A separate,
northern-stream perturbation — now evident in moisture-channel
imagery offshore from southwestern BC — will cross the Pacific
Northwest and reach the northern Rockies by 12Z tomorrow.
At the surface, 11Z analysis showed an elongated area of low
pressure along a frontal zone from central CO to southwestern KS.
The boundary extended from there east-northeastward across
northeastern KS as a slow-moving warm front. A surface cyclone is
expected to consolidate across the northern TX Panhandle and central
OK Panhandle through the day as the mid/upper low approaches. By
00Z the surface low should be located over the GUY/PYX area, with
warm front over central KS and Arctic cold front trailing across
northeastern NM. A trough and weak cold front should extend roughly
southward to northern Coahuila. By 12Z, the surface low should move
to near PNC, nearly vertically stacked with its counterpart aloft by
then. The leading cold front should extend from there across parts
of north-central/central TX to near LRD.
…South-central Great Plains…
Isolated to widely scattered thunderstorms are expected to form from
this afternoon through early tonight, at first over the Panhandles
into a small adjoining part of KS, then across western OK. This
activity will offer a marginal hail threat. A tornado cannot be
ruled out given locally strong low-level shear and deep forcing,
though lack of more-robust moisture will be a greatly limiting
factor. Strong gusts also may penetrate a stable layer and reach
the surface though the wind potential still appears too isolated and
conditional for a categorical outlook. The previous outlook has
been reshaped somewhat to account for dry-slot processes disfavoring
severe over parts of the TX Panhandle, and marginal potential for
severe now apparent over more of western OK this evening.
Strongest deep-layer forcing will spread across the region from west
to east, in the form of low-level mass convergence, DCVA and lift
related to the left-exit region of an upper-level jet streak
occupying the southern part of the Southwestern cyclone.
Thunderstorm coverage will be more uncertain amidst greater (but
still weak) moisture with southeastward extent from the northern TX
and OK Panhandles into western OK. Though preconvective surface dew
points are expected to reach only in the 40s northwest to mid 50s
over western OK, time series of forecast soundings reasonably depict
sharp steepening of lapse rates from roughly the LLJ into the
midlevels, as warm advection and strengthening mid/upper-level
ascent collocate. MUCAPE 500-1000 J/kg is expected amidst generally
strengthening deep shear and enlarged low-level hodographs.
Effective SRH 200-400 J/kg should become common across the area as
mass response to the deepening low-level cyclone occurs and enhances
flow near the top of the boundary layer. Effective-inflow parcels
may become surface-based this afternoon over parts of the Panhandles
(where diabatic destabilization is possible behind early-day
clouds/precip), and this evening in western OK, though some capping
near the top of the boundary layer may limit convective coverage
over the main body of OK.