Day 1 Severe Weather Outlook

Valid 201200Z – 211200Z

…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
ACROSS PARTS OF THE TEXAS SOUTH PLAINS…

…SUMMARY…
A couple of strong storms are possible across parts of the Texas
South Plains this afternoon, which could pose at least some risk for
large hail or a strong wind gust.

…Synopsis…
Downstream of amplified ridging within the westerlies across the
mid-latitude eastern Pacific into western Canada, flow has split,
with a closed mid-level low now in the process of developing in the
southern stream, near the central into southern California coast.
As a significant upstream short wave approaches the ridge axis, the
northern portion of the ridging is forecast gradually shift across
and east of the Canadian Rockies during this period, with downstream
troughing digging toward Hudson Bay and the northern U.S. Plains.
At the same time, the southern branch low appears likely to shift
inland across southern California, before gradually
weakening/broadening over the southern Great Basin/Colorado Valley
by 12Z Thursday. And the increasingly deformed remnants of a
preceding mid-level closed low are forecast to continue accelerating
northeastward across the southern Rockies through the south central
Plains and mid/lower Missouri Valley.

An influx of mid/high level moisture emanating from the lower
latitude eastern Pacific will precede the lead southern stream
impulse across and northeast of the southern Rockies. Models also
indicate that forcing for ascent will support at least modest
surface cyclogenesis from the lee of the Colorado Rockies into parts
of the Upper Midwest by the end of the period. This will be
accompanied by a strengthening southerly return flow across the
southern Plains toward the Upper Midwest. However, in the wake of
mid-level troughing shifting east of the mid/south Atlantic
Seaboard, boundary layer modification over the western Gulf of
Mexico is only slowly underway, and it appears that boundary layer
moistening inland into the lee surface trough will remain fairly
modest through at least this period.

…Southern Plains northward into Missouri Valley…
Models are suggestive that thunderstorm activity will generally
occur in response to elevated moisture return. Even this may be
hindered by precipitation from preceding higher-level convection,
but initially steep lower/mid tropospheric lapse rates, associated
with elevated mixed-layers, may still allow for at least weak CAPE
and thermodynamic profiles at least marginally sufficient for
lightning.

Even in close proximity to the lee surface trough across the
central/southern high Plains, the extent to which cloud cover,
associated with low-level moistening beneath the capping elevated
mixed-layers, restricts boundary layer destabilization remains
unclear. This will considerably impact any severe weather
potential, as the operation NAM, for example, suggests that storms
will remain rooted above the boundary layer, where weak CAPE and
cloud-bearing layer shear will minimize severe weather potential,
despite the presence of rather strong lower/mid tropospheric wind
fields.

With at least the Rapid Refresh continuing to indicate the possible
development of modest boundary-layer based CAPE across parts of the
Texas South Plains, low severe weather probabilities will be
maintained at least for now. However, even this risk may be limited
to an isolated storm or two, which could pose potential for severe
hail or a strong surface gust, as the southern stream impulse
overspreads the central/southern High Plains during the mid to late
afternoon.

…Arizona…
As the exit region of a 70-90 kt 500 mb jet overspreads Phoenix and
the surrounding lower deserts, associated mid-level cooling, forcing
for ascent, and strengthening deep layer shear could potentially
contribute to an environment conducive to an isolated supercell or
two late this afternoon. However, model forecast soundings continue
to raise considerable uncertainty concerning the evolving
thermodynamic profiles, which currently seem likely to only become
very marginally favorable at best. As a result, severe weather
probabilities are being maintained at less than 5 percent.

..Kerr/Cook.. 11/20/2019

$$

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