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Categorical Day2 1730Z Outlook
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Images courtesy of the NWS Storm Prediction Center
Open the discussion at the Storm Prediction Center
Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1254 PM CDT Tue Oct 20 2026
Valid 211200Z - 221200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
OF ARIZONA...
...SUMMARY...
Severe thunderstorms including large hail and tornado risks are
expected on Wednesday especially across Arizona and nearby parts of
the Four Corners area and Lower Colorado River Valley. A localized
severe risk may also exist along the coast of central/southern
California.
...Arizona/Lower Colorado River Valley/Four Corners...
Severe thunderstorms are expected on Wednesday, including the
potential for a full-period/multi-round of severe weather for some
areas. A climatologically rare and a concerning/well-above-typical
risk for severe storms is expected, potentially including all severe
hazards with large hail, damaging winds and tornadoes all possible.
Ahead of a prominent/deep upper trough approaching the California
coast, this scenario is attributable to a notable strengthening of
deep-layer winds (southwesterly above the boundary layer), along
with relatively rich low/mid-level moisture. The severe risk should
tend to peak in most areas during the afternoon and evening, but it
should be noted that the severe risk, particularly for parts of
central/northern Arizona, may persist well through the late-night
hours of Wednesday night into early Thursday.
Broadly speaking, there is relatively high confidence that wind
profiles, with elongated/semi-straight hodographs, combined with
expected destabilization (500-1000 J/kg MLCAPE) will support
sustained/splitting supercells with viable large hail potential
particularly with rotating storms, including both right-moving and
left-moving (generally moving northward) supercell-splitting
components. Hail in excess of 2 inches in diameter may locally
occur.
Forecast soundings imply that storms will be a bit higher based with
south-southeastward extent across the desert floor, but lower
temperature-dewpoint spreads farther north closer to/in the rim and
more forested areas of central/northern Arizona, which also
coincides where more of a southeasterly boundary layer wind
component may exist, boosting low-level hodograph curvature/SRH and
contributing to tornado potential particularly across interior
central Arizona and a broader part of northern Arizona, and possibly
even into nearby southeast Nevada and/or far southern Utah.
...Coastal southern/central California...
In closer proximity to the approaching upper low, the core of an 80
kt 500 mb jet will overspread central and southern California during
the later half of the period (00-12Z late Wednesday and Thursday),
resulting in relatively straight but highly elongated hodographs.
During this time frame, a moist marine airmass will advect into
coastal areas, while being overspread by 6.5-7 C/km mid-level lapse
rates, yielding 250-500 J/kg of short and thin MLCAPE. Given the
scant buoyancy, convection should be low-topped, with short line
segments and brief/transient supercell modes possible. A few
damaging wind gusts are the main threat with any stronger storm that
can reach the shore, but near-coastal waterspout/brief tornado
potential may also exist.
..Guyer.. 10/20/2026
