SPC AC 261252
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0752 AM CDT Sun Sep 26 2021
Valid 261300Z - 271200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS SOUTHERN
PARTS OF ARIZONA AND NEW MEXICO...
Isolated severe hail and/or gusts are possible today across southern
parts of Arizona and New Mexico.
A progressive synoptic-scale pattern will be noted through the
period, as the eastern trough deamplifies and shifts east, and so
will the ridging now over the Intermountain West and Rockies.
Upstream, a strong cyclone was evident in moisture-channel imagery
over the northeastern Pacific, centered near 49N142W, with troughing
extending approximately 1,000 nm southward. This system will shift
eastward through the period and remain offshore, while amplifying
southward, with the parent cyclone west of the BC coastline by 12Z
tomorrow. Still, a broad area of associated cyclonic flow and
height falls aloft will spread inland tonight ahead of the trough,
and across northern CA and the Pacific Northwest. As that occurs, a
longstanding cut-off now -- now over northern Baja -- will eject
northeastward over southwestern through east-central AZ.
At the surface, 11Z analysis showed a cold front offshore most of
the Atlantic Coast except central FL, where it became
quasistationary and extended westward over the Gulf. A field of
Pacific moisture was manifest at the surface over southern AZ, with
dew points commonly in the mid 50s to low 60s F (with slight
decrease due to mixing expected today). PW commonly around 1.25-1.5
inches was noted at 12Z over southern AZ, based on the TUS sounding,
limited GPS PW sampling, and objectively analyzed RAP fields.
Convective coverage is expected to increase from midday through the
afternoon over the region as the boundary layer destabilizes and
MLCINH becomes negligible. By midafternoon, widely scattered to
scattered thunderstorms are expected across much of southern AZ and
far southwestern NM, offering isolated gusts near severe limits and
marginally severe hail. Convection subsequently should spread in a
fan-shaped pattern (strongly resembling the difluence of mean-wind
and mid/upper flow vectors) northward and northeastward toward both
the high country north of the Mogollon Rim, and toward south-central
NM. The threat for isolated severe may be maintained as far as near
the southern NM I-25/Rio Grande corridor before evening weakening
takes place, and the outlook has been somewhat enlarged eastward
accordingly. Intensity and coverage should diminish this evening
and overnight as activity encounters a more-stable boundary layer,
both from outflows and nocturnal diabatic cooling.
Forecast soundings suggest that the preconvective environment will
be characterized by:
1. Somewhat greater low-level moisture content and steeper midlevel
lapse rates westward contributing to 1000-1500 J/kg peak MLCAPE, but
weaker low/middle-level flow/shear, and
2. Steeper low-level lapse rates with larger DCAPE and stronger
mid/upper winds eastward. These factors only overlap diffusely, but
will contribute to potential for at least brief/localized
organization with attendant marginally severe potential.
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