Day 1 Severe Weather Outlook

Valid 191200Z – 201200Z


Strong thunderstorms may impact portions of southern Arizona late
this afternoon and evening, perhaps accompanied by at least some
risk for severe hail and wind.

Within the mid-latitude westerlies across the eastern Pacific,
models suggest that amplifying ridging will build northeastward
through British Columbia, the Yukon and portions of the Northwest
Territories, taking on more of a positive tilt during this period.
As it does, the southern portion of a splitting downstream short
wave trough, and a trailing perturbation, are forecast to dig near
and west of the Sierra Nevada, and eventually contribute to the
evolution of a deepening mid-level low near the southern California
coast by 12Z Wednesday.

In response to these developments, an initially cut-off mid-level
closed low, currently still west of Baja, likely will be forced
northeastward. A slow northeastward acceleration toward northern
Baja may be underway by 12Z this morning. As it progresses more
rapidly northeastward, it appears that the low will transition to an
open wave and perhaps undergo considerable further deformation while
overspreading southern Arizona and Sonora late this evening into the
overnight hours.

Farther downstream, beneath building larger-scale ridging across the
southern and central Plains into the lower and middle Mississippi
Valley, a southerly return flow may gradually commence across the
lower Rio Grande Valley into portions of the southern Plains, in the
wake of large-scale troughing beginning to shift east of the
Atlantic Seaboard.

While convective potential is expected to remain negligible across
most areas east of the Rockies, an influx of moisture from the lower
latitude eastern Pacific, and through the Gulf of California,
probably will contribute to destabilization supportive of scattered
thunderstorm activity across the Southwest. This may include
portions of the Colorado Plateau and southern Rockies into southwest
Texas, in addition to much of central and southern Arizona.

…Central and southern Arizona…
On the leading edge of the moisture return, increasing forcing for
ascent, perhaps aided by a mid-level perturbation pivoting around
the approaching closed low, may support considerable convection and
embedded thunderstorms across parts of northern Sonora into southern
Arizona by 18-21Z. This activity is expected to be generally weak
with negligible severe risk, and may tend to diminish while
spreading toward the Mogollon Rim.

In its wake, boundary layer moistening and breaks in overcast may
allow for the development of weak to moderate CAPE across Sonora
into at least the southern Arizona border vicinity. This may become
sufficient to support vigorous thunderstorm development,
particularly near the leading edge of the northeastward advancing
mid-level dry slot, where steepening mid-level lapse rates may
coincide with sufficient strengthening of deep-layer shear to
support isolated supercells. While strongest cells may initiate
across Sonora, activity may spread across the international border
into Arizona with at least some continuing risk for severe hail and
locally strong surface gusts, mainly during the 20/00-03Z time

..Kerr/Cook.. 11/19/2019


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