Day 1 Severe Weather Outlook

Valid 141300Z – 151200Z

…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
SOUTHWEST TEXAS…

…SUMMARY…
A few strong to severe thunderstorms are possible this evening over
parts of southwest Texas.

…Synopsis…
In mid/upper levels, a high-latitude omega block over the northeast
Pacific — from the AK coast southward — will maintain a large
fetch of cyclonic flow across the Pacific Coast from BC though
nearly all of Baja. This flow will be anchored by a synoptic
cyclone now centered west of the Columbia River mouth, and forecast
to shift slowly southward off the OR coast through the period.
Downstream, a weak shortwave trough — the feeble remnants of a
former cutoff low — extends from the central High Plains to eastern
NM. This feature will deamplify, move through a mean ridge, and
reach the Chesapeake Bay region by 12Z. A low-amplitude, southern-
stream perturbation was apparent in moisture-channel imagery near
southern Baja, and should eject northeastward toward parts of far
west and southwest TX by this evening.

At the surface, a wavy/quasistationary front was analyzed at 11Z
from southern SC across southern MS to central/southwest/far west
TX. Little substantial movement of the TX part of this boundary is
expected, except for slight northward/warm-frontal movement this
evening and overnight over southwest/far west TX. A dryline should
develop today to its south and mix eastward to northern Coahuila and
west-central TX, retreating westward toward the lower Pecos Valley
overnight.

…Southwest TX…
Thunderstorms are expected to develop this afternoon, initially
isolated over the high country of the Serranias del Burro in
northern Coahuila. The environment from there into the adjoining
Rio Grande Valley near DRT will favor supercells, with accompanying
potential for damaging hail/wind in MX. Steep mid/upper-level lapse
rates will overlie an easterly boundary-layer upslope flow
component, advecting rich Gulf moisture into the region, while
heating of higher terrain minimizes MLCINH. Forecast soundings
suggest peak preconvective MLCAPE near and west of DRT of 2000-3000
J/kg, amidst 45-60-kt effective-shear magnitude. Modest low-level
winds will limit hodograph size, but historic analogs and 2-D hail
models run on such soundings depict significant hail potential over
MX. Initial activity should weaken as it moves eastward or
east-northeastward to near the Rio Grande, though it may briefly
reach TX before dissipating. Considerable uncertainty lingers on
where capping will become too strong to sustain a del Burro-
initiated isolated supercell, especially considering low-predictably
storm-dynamics influences.

Of growing concern: additional thunderstorms may develop either
over the del Burros, or in hotter, drier, more-deeply mixed air over
ranges to the west, late this afternoon and move across the Rio
Grande this evening. Hail and damaging wind would be possible
across northern Coahuila, across the valley toward the southern
Balcones Escarpment in TX. Given the wind profiles expected over
the area, isolated left-moving storms also may occur, capable of
hail. The combined wind/hail threat near DRT and eastward would
increase if upscale aggregation/clustering can add outflow-driven
forced ascent to the convective maintenance/generation process.
Multiple convection-allowing models have suggested such a scenario
since 00Z, though far from unanimously. Favorable factors include
large-scale ascent from the approaching mid/upper perturbation,
strong storm-relative ambient wind component in the boundary layer,
still-favorable effective shear, residual well-mixed layer under a
weak EML inversion, and rich moisture. Enough unconditional
potential now is apparent to introduce a categorical severe area.
The threat initially is set at marginal level due to mesoscale
uncertainties, subject to refinement as observational trends and
later progs further inform uncertainty.

..Edwards/Goss.. 03/14/2020

$$

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