Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0253 PM CDT Wed Oct 21 2026
Valid 212000Z - 221200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
CENTRAL ARIZONA...
...SUMMARY...
Severe thunderstorms capable of producing large hail, a few
tornadoes, and damaging winds remain possible 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.
...20z Update...
No changes were made to the existing outlook with this update as the
forecast remains generally on track. Convective activity/severe
potential is generally expected to peak this afternoon/early evening
coincident with peak diurnal heating/destabilization. However,
additional rounds of thunderstorm development and the potential for
supercells capable of primarily large hail are likely to continue
into the late evening/overnight hours, especially across
central/northern Arizona, amid a continuing southerly, warm-air
advection regime. See MCDs #2387 and #2388 along with the discussion
below for additional short-term details.
..Chalmers.. 10/21/2026
.PREV DISCUSSION... /ISSUED 1122 AM CDT Wed Oct 21 2026/
...Southwest...
Recent satellite imagery continues to show deep southwesterly flow
aloft from the central Baja Peninsula through AZ and the Four
Corners region. Warm-air advection associated with this
southwesterly flow aloft has contributed to showers and
thunderstorms across central/northern AZ this morning. The low-level
airmass here is still relatively cool and stable, but strong
vertical shear has still resulted in several supercells. As
mentioned in MCD #2385, additional/isolated thunderstorms are
possible within this low-midlevel moisture plume, potentially aided
by embedded speed maxima moving north-northeastward. Strong vertical
shear will persist as well, suggesting several of these storms could
become supercellular. Large hail will remain the primary risk until
the low levels destabilize.
Deep cyclone off the central/southern CA coast is forecast to
gradually shift east-southeastward today. As it does, southwesterly
flow aloft within its eastern periphery will strengthen, with
deep-layer vertical shear across AZ also strengthening in response.
Low-level moisture will advect northward as well. Even with
widespread cloudiness, this increased moisture combined with
filtered daytime heating will support gradual airmass
destabilization and the development of up to 1000-1500 J/kg of
MLCAPE across this region. Strongest buoyancy is anticipated in
south-central AZ.
Forcing for ascent will increase in tandem with airmass
destabilization and the general expectation is for increasing
thunderstorm coverage along and just south/west of the Mogollon Rim
this afternoon. Supercell wind profiles are in place, and large to
isolated very large hail (i.e. 2"+ in diameter) is expected to be
the primary hazard. However, storm motion will take these storms
north-northeastward, potentially interacting with the more
southeasterly low-level flow along/near the Rim. Resulting increase
in low-level shear will support an increased tornado potential. This
potential will be maximized from northwest into central AZ this
afternoon and early evening, but low-probability tornado potential
will extend to the southern AZ/NM border. Strong wind gusts are
possible as well, particularly from central into southeast AZ where
relatively stronger heating/mixing is possible.
Tornado threat is expected to diminish gradually with the loss of
daytime heating, but thunderstorms will likely persist throughout
the evening and overnight. Large hail will remain possible with this
overnight activity.
...Coastal Central/Southern California...
The large cyclone 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.
Limited instability is forecast, and updrafts may struggle to reach
sufficient depth for charge separation and lightning production.
Even so, 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.
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