Valid 191300Z – 201200Z
…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
SOUTHERN ARIZONA AND THE LOWER COLORADO RIVER VALLEY REGION…
…SUMMARY…
Isolated severe hail and/or gusts are possible over parts of
southern Arizona and the lower Colorado River Valley region.
…Synopsis…
In mid/upper levels, a progressive, rather highly amplified pattern
will take shape over the CONUS this period. As a series of
shortwaves traverse associated cyclonic flow, a synoptic-scale
trough will move eastward across the eastern U.S., to the Carolinas
and Mid-Atlantic by 12Z. Upstream synoptic ridging will shift
eastward from the Rockies across the Great Plains and mid/lower
Missouri Valley.
Farther upstream, a northern-stream shortwave trough, initially over
the Pacific Northwest, will eject eastward across the northern
Rockies and assume more-positive tilt. A couple of shortwave
troughs closely behind it — now evident in moisture-channel imagery
over the Pacific, northwest of Vancouver Island and west of OR —
will dig southeastward and phase, then amplify greatly. The
resulting closed cyclone should be located over central/southern CA
and the Sierra by 12Z. However, progs differ considerably on the
cyclone’s 500-mb center/low position, largely depending on which
Pacific perturbation’s vorticity field becomes more dominant within
the broader deepening vortex. Regardless, amidst the
broader/synoptic height-fall field, the initially cut-off cyclone
now west of Baja is expected to eject northeastward while weakening,
cross northern Baja between 03-06Z, then devolve to an open wave.
By 12Z, the trough should cross much of Sonora and southern AZ,
while continuing to weaken.
At the surface, generally low-theta-e and convectively unsupportive
boundary-layer air will prevail east of the Rockies. Lee troughing
over the High Plains and resultant return-flow/warm-advection regime
will become well-defined, however, ultimately supporting convective
potential beyond day-1. In the meantime, elevated moisture/
instability will foster an eastward spread of general-thunder
potential tonight across parts of south-central/southeastern NM and
west TX.
…Southern AZ…
Large-scale ascent — first in the form of warm advection (with
attendant moist advection from the Gulf of California), then
increasing DCVA in proximity to the mid/upper trough — will spread
northeastward across Sonora and south-central/southeastern AZ
through this evening. This will support two probable rounds of
convection — the first a swath/arc of scattered showers and likely
non-severe thunderstorms moving northeastward out of Sonora during
midday and early afternoon. This activity should encounter a
relatively dry and weakly destabilized low-level air mass across
south-central/southeastern AZ and the NM bootheel, with its own
cloud cover also limiting heating and low-level lapse rates
downshear.
Leftover moisture, moist advection, warm advection, and
diabatic/afternoon heating behind the initial convection, overlain
by steepening midlevel lapse rates and rapidly weakening MLCINH,
will support additional thunderstorm development near the
international border this afternoon into early evening. This
convection should be more isolated to widely scattered in coverage,
in greater buoyancy and shear than the earlier activity. Peak
MLCAPE should reach the 500-1500 J/kg range, amidst 40-50-kt
effective-shear magnitudes. Low-level hodograph size will be
limited by weak near-surface flow over most areas, but deep shear
will support isolated supercell potential, along with organized
multicells.
…Southeastern CA/lower Colorado River region…
Deep-layer vertical shear will weaken with westward extent beneath a
more-difluent mid/upper flow regime north-northeast to north of the
ejecting/weakening cyclone. On the other hand, the period of
favorable surface heating will last longer, with still-favorable
steepening of low/middle-level lapse rates occurring atop boundary-
layer moist advection from the southeast. As a result, widely
scattered predominantly multicell thunderstorms are expected to
form, preferentially/initially over higher terrain of northern Baja
by late afternoon, then spread northeastward with additional
development possible over southeastern CA. A window of a few hours
of strong/isolated gusts and marginal hail potential is possible,
amidst MLCAPE reaching into the 1000-2000 J/kg range.
..Edwards/Mosier.. 11/19/2019
$$