Valid 061300Z – 071200Z
…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM EASTERN
ARIZONA TO SOUTHWESTERN OKLAHOMA…
…SUMMARY…
Isolated severe thunderstorms are possible from eastern Arizona to
southwestern Oklahoma, mainly during the late afternoon to
mid-evening.
…Synopsis…
In mid/upper levels, a persistent longwave trough from the Canadian
Arctic to the Mississippi Valley will maintain broadly cyclonic flow
over most of the CONUS east of the Rockies. A small, formerly cut-
off cyclone was apparent in moisture-channel imagery, moving slowly
northeastward across northern Baja and southeastern CA. By 00Z,
this feature should become a positively tilted, open-wave trough
over portions of AZ and western Sonora. By 12Z, the weakening
trough and associated 500-mb vorticity field are forecast to move
east-northeastward across NM. A downstream shortwave ridge will
cross NM today and weaken over west TX tonight. Flow aloft today
will be difluent west of the shortwave ridge, and confluent to its
east, becoming more broadly confluent tonight over the NM/west TX
region as the shortwave breaks down.
At the surface, a wavy and mostly quasistationary frontal zone was
drawn from Carolinas offshore waters across central FL westward to
LA/upper TX shelf waters, then west-northwestward across south-
central TX to a weak low near 6R6, and then very near the MX border
across the Chihuahuan/Sonoran Deserts. The TX segment of this
boundary will shift/redevelop northward as a warm front across
north-central, northwest and west-central TX through the day. A
cold front was analyzed from a low over northwestern IA across
south-central NE to central WY. The cold front will accelerate
southward/southeastward across the central/southern Plains,
overtaking the warm front across parts of northwest TX and the Red
River region late overnight. By 12Z, the cold front should extend
from the lower Ohio Valley across central AR, southeastern OK, to
the Permian Basin of west TX, to south-central NM.
…Southwest to southern Plains…
Areas of precip and convection, with isolated to widely scattered,
embedded thunderstorms, are evident this morning over portions of
central/eastern AZ, as well as southern NM to northwest TX. While
this activity should remain non-severe, it will influence more-
substantial convective potential later in the period. The severe
threat appears best delineated by two areas, nearly (but not
entirely) separated:
1. AZ/southwestern NM:
In the western limb of the risk area, the clouds/precip should eject
out of AZ and across western NM during midday, followed by a few
hours of favorable diabatic heating/destabilization of a moistened
boundary layer. Meanwhile, increasing large-scale ascent/cooling
aloft — related to the approaching shortwave perturbation — will
occur. Steepening low/middle-level lapse rates from both processes
will contribute to the development of weakly inhibited MLCAPE in the
500-1000 J/kg range, amidst 50-60-kt effective-shear magnitudes and
long, nearly straight low-level hodographs. Organized multicells
and a supercell or two are possible, moving across southeastern AZ
and southwestern NM with isolated large hail possible into early
evening.
2. South-central/southeast NM, west TX to southwestern OK:
Farther east, buoyancy will be more limited with northwestward
extent into more-recent stabilization by early clouds/precip.
Meanwhile deep shear will decrease with eastward and southward
extent near the weakening shortwave ridge. Still, thunderstorms
should develop this afternoon and increase in coverage across
south-central/southeastern NM and west/northwest TX, initially
representing a blend of multicell and isolated supercell modes while
discrete. Activity should merge/grow upscale into a swath of
scattered-numerous thunderstorms this evening and tonight from
central/western OK across west TX to the Big Bend region. Isolated
strong-severe gusts and large hail will be possible, with hail more
of a concern in earlier, discrete to semi-discrete stages. Greater
spatiotemporal confidence in timing/duration of favorable mode and
areas of strongest available surface-based instability ultimately
may justify a notch greater probabilities over portions of this
area, but too many mesoscale uncertainties linger at this time. The
overall severe threat should decrease tonight as boundary-layer
instability weakens.
..Edwards/Gleason.. 11/06/2019
$$