Valid 171300Z – 181200Z
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
NORTHWEST TO SOUTHWEST TEXAS…
…SUMMARY…
Severe hail and damaging gusts are most probable today and this
evening from southwest to northwest Texas. A tornado also is
possible over parts of north-central/northwest Texas.
…Synopsis…
In mid/upper levels, an expansive cyclonic gyre — now centered off
the central CA coast — will change geometry and redevelop inland
through the period. The current main circulation center/vorticity
max should pivot southward, then southeastward, being offshore from
northern Baja around 12Z tomorrow. Meanwhile, a strong vorticity
lobe/shortwave trough initially over the Sierra should pivot
northward past RNO and close off into a second circulation center
across extreme northwestern NV tonight. The net result will be a
meridional dumbbell shape in the 500-mb height fields with this
cyclone by 12Z, before the southern lobe swings inland day-2. As
this occurs, a positively tilted, southern-stream shortwave trough
currently over Baja will eject northeastward, reaching eastern NM,
far west TX, and Chihuahua by 00Z. By 12Z this perturbation will be
fragmented by convective processes, reaching the lower Missouri
Valley and southeast TX.
At the surface, 11Z analysis showed a diffuse, piecemeal warm-
frontal zone aligned west-east across southwest/west-central TX to
southern LA and central MS. The warm frontal zone should shift
northward to north-central/northwest TX through early-mid afternoon,
becoming stationary and exhibiting a much tighter baroclinic
gradient than now, with reinforcement by precip and strong surface
static stability to its north. A convergence zone corresponding to
a lee trough/dryline should become well-defined through the
afternoon from the Big Bend region north-northeastward across the
South Plains, intersecting the warm front over northwest TX, with a
narrow, “bent-back” corridor of surface-based, relatively moist
boundary-layer air extending from there into parts of extreme
eastern NM along and north of the front.
…Southern Plains…
Isolated to scattered thunderstorms should develop near the full
length of the dryline/trough this afternoon, as well as near the
front on either side of the dryline. Frontal convection west of the
dryline may become severe briefly, but with short residence time in
surface-based air, given expected storm motions strongly normal to
the baroclinic zone. Severe gusts and large hail will be possible
with convection accessing the warm/moist sector. A marginal tornado
potential exists near the front, where low-level hodographs/shear/
vorticity will be relatively maximized for any supercell or line
segment encountering it. Storm-mode and mesoscale uncertainties
preclude any tighter refinement of that area for this outlook cycle.
Deep shear generally will increase with northward extent through the
warm sector and especially near the front, where backed surface
winds contribute. Forecast soundings show effective-shear
magnitudes in the 30-45-kt range, indicating a blend of organized
multicells and supercells. Steep midlevel lapse rates atop 60s F
surface dew points, beneath a modest cap, will allow 1500-2000 J/kg
preconvective MLCAPE to build east of the dryline. Aggregation of
some of the convection over west-central TX and northern Coahuila is
possible into the evening, moving east across the Hill Country and
adjoining Rio Grande Valley with a transition to preferential wind
potential. Some additional room was added to the outlook areas on
the east end in central/south TX to account for at least loosely
organized line(s) of convection maintaining severe threat into late
evening/early overnight, before encountering stronger MLCINH.
..Edwards/Goss.. 03/17/2020
$$