SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0246 PM CDT Fri Aug 23 2024
Valid 232000Z - 241200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS PORTIONS OF
WESTERN MONTANA...
...SUMMARY...
Strong to severe storms with severe wind gusts and some hail are
possible across Montana, the Intermountain West and central Rockies,
southeast Arizona to extreme southwestern New Mexico, and parts of
the High Plains.
...20z Update...
The primary adjustment for this update was an expansion of the
Slight risk/15% wind probability southward to include more of
western MT. Slowly intensifying convection across eastern ID is
expected to remain relatively benign for the near-term (see MCD
#1976 for additional details regarding short-term trends), but may
pose a more robust severe wind threat later this afternoon/evening
as convection spreads north into a deeply mixed environment across
western MT. The last several HRRR solutions have captured recent
convective and environmental trends well and consistently suggest
the best potential for severe wind will be associated with this
activity as it spreads north. The adjustment to the Slight risk area
should better reflect this potential.
Elsewhere, the forecast remains on track per the previous discussion
(below). See MCD #1977 for the latest details regarding the severe
threat across southern Arizona.
..Moore.. 08/23/2024
.PREV DISCUSSION... /ISSUED 1127 AM CDT Fri Aug 23 2024/
...Intermountain West to northern Rockies...
The lead portion of an upper trough centered along the coastal
Pacific Northwest will shift north-northeastward over the northern
Intermountain region in tandem with the exit region of a
strengthening mid/upper-level jet. Scattered thunderstorms are
expected in this regime preceding the Pacific Coast trough, with
initial development over the higher terrain ahead of the surface
cold front. Isolated severe gusts and marginally severe hail will be
possible, with the potential for a more focused corridor of stronger
wind gusts across northwest/north-central Montana late this
afternoon through early evening.
Additional thunderstorms should form this afternoon across
east/northeast Oregon, northern/eastern Idaho, to western Wyoming,
and southeastward into Utah/western Colorado, as eastern fringes of
large-scale ascent preceding the Pacific Northwest trough and
embedded shortwaves overlie destabilizing, elevated terrain.
Throughout the corridor, sufficient deep-layer shear (30-45 kt)
should exist for organized multicells and small
bowing/outflow-driven clusters, and perhaps a few supercells, with
some hail and severe-caliber wind gusts possible.
...High Plains...
Isolated to widely scattered thunderstorms are expected to form over
the central/northern Rockies, as well as near the lee trough over
eastern Montana this afternoon, with another favored corridor across
the central High Plains including northeast Colorado/western
Nebraska. Activity should move east-northeastward across the
adjoining High Plains. This convection will encounter a diurnally
destabilizing, well-mixed boundary layer with steep low-level lapse
rates, supporting isolated severe wind gusts. Isolated large hail
also will be possible.
Strongly difluent mid/upper-level flow will exist over most of this
corridor, between the Pacific Northwest cyclone and the southern
Plains anticyclone, with speeds generally increasing northwestward
and low-level moisture greater southward across the central High
Plains. Still, sufficient moisture should persist near the Montana
lee trough, along with enough veering with height over the entire
area to yield enough deep shear (30+ kt effective-shear magnitudes)
for organized convection.
...Southeast Arizona/far southwest New Mexico...
Widely scattered to scattered thunderstorms are expected to develop
this afternoon with a relatively moist air mass and ample
insolation. Increasing mid-level southerlies around the western rim
of the anticyclone should enable 20-30 kt effective-shear
magnitudes, and enough northward storm motion to augment already
strong downbursts from the most intense cells locally. An instance
or two of marginally severe hail could also occur.
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