SPC Oct 21, 2026 1630 UTC Day 1 Convective Outlook

LIVE MAP (ABOVE) ... SPC 1630Z Day 1 Outlook


Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1122 AM CDT Wed Oct 21 2026

Valid 211630Z - 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 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...
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.

..Mosier/Thompson.. 10/21/2026


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SPC Oct 21, 2026 1300 UTC Day 1 Convective Outlook

LIVE MAP (ABOVE) ... SPC 1300Z Day 1 Outlook


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


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SPC Oct 21, 2026 0600 UTC Day 1 Convective Outlook

LIVE MAP (ABOVE) ... SPC 1200Z Day 1 Outlook


Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1251 AM CDT Wed Oct 21 2026

Valid 211200Z - 221200Z

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

...SUMMARY...
Severe thunderstorms with a potential for large hail, severe wind
gusts and a couple tornadoes are expected today along the Mogollon
Rim across parts of Arizona. Marginally severe wind gusts and an
isolated tornado threat will also be possible along the southern and
central California Coast.

...DISCUSSION...
An upper-level low will approach the California Coast today, as
southwesterly divergent flow aloft remains in place over much of the
Southwest. Ahead of the system, low-level moisture will advect
northward into the Desert Southwest today. An axis with surface
dewpoints in the 60s F will set up to the southwest of the Mogollon
Rim across southern, central and western Arizona. By afternoon,
MLCAPE is forecast to peak in the 1500 to 2000 J/kg range along and
near the moist axis. Over the top of the moist and unstable airmass,
strong mid-level southwesterly flow will be in place. Moderate
instability and strong deep-layer shear will support a substantial
severe threat with storms that develop along the Mogollon Rim from
this afternoon into the evening, near the northern edge of the
stronger instability.

As the upper-level system approaches, a 50 to 70 knot mid-level jet
will overspread the Desert Southwest today. RAP forecast soundings
within the exit region of the jet over north-central Arizona in the
mid to late afternoon have gradually veering winds with height and
50 to 60 knots of 0-6 km shear. In addition, steep mid-level lapse
rates and long hodographs are evident which will result in a
potential for supercells with large hail. The strongest cells could
produce hailstones greater than 2 inches in diameter. Forecast
soundings also have 0-3 km storm-relative helicity in the 175 to 250
m2/s2 range late this afternoon along parts of the Mogollon Rim
suggesting that the stronger supercells will be capable of
tornadoes. The severe threat is expected to persist from mid
afternoon into the evening, with a potential for two rounds of
severe storms impacting parts of north-central Arizona.

...Coast of Southern and Central California...
A large upper-level low pressure system will move toward the
California Coast today. The left exit region of a mid-level jet will
approach the southern and central California Coast. A wave of
precipitation associated with this feature will move to the southern
and central California Coast this afternoon. Instability along the
coast will be sufficient for thunderstorm development. RAP forecast
soundings along the coast this afternoon have strong deep-layer
shear, which will support the development of low-topped supercells.
Some model forecasts have 0-3 km storm-relative helicity around 200
m2/s2 near the coast suggesting that an isolated tornado threat will
be possible. Any cells that rotate and move onshore could also be
associated with isolated severe wind gusts as well.

..Broyles/Leitman.. 10/21/2026


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SPC Oct 21, 2026 0100 UTC Day 1 Convective Outlook

LIVE MAP (ABOVE) ... SPC 0100Z Day 1 Outlook


Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0751 PM CDT Tue Oct 20 2026

Valid 210100Z - 211200Z

...NO SEVERE THUNDERSTORM AREAS FORECAST...

...SUMMARY...
A severe threat is not forecast across the U.S. through tonight.

...DISCUSSION...
An upper-level low will move toward the coast of California this
evening as south-southwesterly flow aloft remains in place over the
Desert Southwest. Ahead of this system, instability will be
sufficient for thunderstorm development this evening into tonight
across parts of the Desert Southwest and Intermountain West.
Thunderstorms will also be possible tonight along and near an axis
of low-level moisture over the southern and central Great Plains. No
severe threat is forecast across the continental U.S. through
daybreak Wednesday.

..Broyles.. 10/21/2026


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