Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0730 AM CDT Wed Oct 21 2026

Valid 211300Z - 221200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
ARIZONA...

...SUMMARY...
Severe thunderstorms capable of producing large hail, a few
tornadoes, and damaging winds are expected today and tonight across
parts of Arizona and vicinity. Isolated severe gusts and perhaps a
tornado may also occur along the central/southern California Coast.

...Southwest...
Ongoing thunderstorms across central/northern AZ early this morning
are being aided by large-scale ascent associated with a strong
southwesterly mid/upper-level jet. This activity should continue to
pose a threat for mainly isolated severe hail in the short term,
with 40-50 kt of effective bulk shear aiding updraft organization
and supercell potential in an otherwise weakly unstable environment
(MUCAPE generally 500-1000 J/kg at the moment). Current expectations
are for these thunderstorms to continue advancing northward into an
even less unstable environment, but regenerative convection may
persist for much of the morning as additional cells move over the
Mogollon Rim.

In the wake of this morning convection, an already unseasonably
moist low-level airmass present across southern/central AZ is
forecast to slowly spread northward across the lower CO River Valley
and Mogollon Rim vicinity through the afternoon. Even with
widespread cloudiness, filtered daytime heating will support gradual
airmass destabilization and the development of up to 1000-1500 J/kg
of MLCAPE across this region (locally stronger in central AZ), as
convective inhibition becomes minimal by 18-20Z. Additional
thunderstorms will likely develop across central/northern AZ in this
time frame, as persistent forcing for ascent attendant to the
mid/upper-level southwesterly jet remains over the Southwest.

Strong deep-layer shear (40-55+ kt) will easily support supercells,
with an associated threat for large hail. Greater instability is
forecast with southward/westward extent in central AZ and southern
NV, but lower thunderstorm coverage may exist in these areas. Still,
a conditional risk for 2+ inch diameter hail remains apparent with
splitting supercells given the rather favorable overlap of moderate
instability and elongated/nearly straight hodographs at mid/upper
levels. Occasional severe/damaging winds may also occur, but the
tendency for convection to remain mostly discrete may limit
clustering potential to some extent.

The tornado threat with these supercells remains somewhat uncertain,
as temperatures will remain fairly cool across northern AZ today.
Even so, the risk for a few tornadoes may focus across parts of
northwest/north-central AZ this afternoon and early evening, where
greater low-level shear is forecast to reside amid generally
east-southeasterly low-level winds. It should also be noted that
multiple rounds of strong to severe thunderstorms appear likely
today into tonight. Primary changes with this update were to extend
the Slight Risk westward across northwest AZ/southeast NV/southwest
UT mainly for large hail potential with supercells this
afternoon/evening, and to expand the Marginal Risk in southern NV
and western NM.

...Coastal Central/Southern California...
A large, closed upper low over the eastern Pacific will approach the
central CA Coast through the period, with a belt of strong southerly
flow at mid levels expected to overspread coastal central/southern
CA through the day. A low-topped line of convection may develop in
response and impact this region during the afternoon/evening.
Instability is forecast to remain minimal/shallow, and this activity
may struggle to reach sufficient height for charge separation and
lightning production. Still, given modestly enhanced flow and
low-level shear, an isolated risk for severe gusts and perhaps a
tornado may exist as this convection moves onshore.

..Gleason/Kerr.. 10/21/2026