SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0752 AM CDT Fri Aug 23 2024
Valid 231300Z - 241200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS
MONTANA...THE INTERMOUNTAIN WEST AND CENTRAL ROCKIES...SOUTHEAST
ARIZONA TO EXTREME SOUTHWESTERN NEW MEXICO...AND THE CENTRAL AND
NORTHERN HIGH PLAINS...
...SUMMARY...
Strong to severe storms with severe gusts and possibly some hail are
possible across Montana, the Intermountain West and central Rockies,
southeast Arizona to extreme southwestern New Mexico, and the
central and northern High Plains.
...Synopsis...
A highly amplified but somewhat progressive mid/upper-level pattern
is in place over the CONUS, led by a cyclone departing New England
for the Canadian Maritimes. Weak mean troughing will persist in its
wake over the Atlantic Coast States. A persistent anticyclone --
initially centered over southwestern OK -- should expand somewhat
across the southern/central Plains while its 500-mb high drifts
northeastward over OK through the period.
West Coast troughing is anchored by a strong cyclone -- initially
centered just off the OR Coast. This feature should shift
eastward/inland through tonight, while breaking up into a couple
strong shortwave troughs as internal vorticity maxima pivot over the
Northwest and northern CA. A broad fetch of downstream southwest
flow will extend across the Great Basin, Intermountain West, and
northern/central Rockies. Moisture-channel imagery indicates
embedded shortwave troughs over southeastern ID -- forecast to move
northeastward over the northern Rockies today -- and over the Four
Corners region. The latter perturbation should cross much of
western/northern CO and southern WY today, reaching to WY/NE border
vicinity by 00Z, then northeastward to eastern SD by 12Z.
11Z surface analysis showed a longstanding, quasistationary to warm
frontal zone, from northern FL across the northeastern Gulf shelf
waters to southern LA, north-central TX, southwestern OK, to a low
over the northern TX Panhandle. This boundary still defines the
northern rim of the richest Gulf moisture/theta-e, and should drift
northward over the southern Plains through the period while moving
little elsewhere. A low was drawn over eastern WA, with cold front
across northeastern OR, central NV to south-central CA. This low
should occlude and move northward into southern BC today, while the
cold front moves slowly eastward across ID and northern NV, and
loses definition farther south. A secondary low should form by 00Z
over northwestern MT and move northeastward to the southern AB/SK
border region by 12Z. Meanwhile, a lee trough will strengthen and
extend from the secondary low southeastward over central/eastern MT,
then over eastern arts of WY/CO. The trough should move
northeastward over more of eastern MT, and into the western Dakotas,
overnight.
...Intermountain West to northern Rockies...
Scattered thunderstorms are expected in the field of southwesterly
mid/upper flow, preceding the Pacific Coast trough, and mostly ahead
of the surface cold front. Within that plume, isolated severe gusts
and marginally severe hail will be possible.
Severe potential may begin as early as midday to early afternoon --
and continue through the afternoon -- across parts of the eastern
Great Basin/western Colorado Plateau region. This will occur as
clearing behind the shortwave trough permits sustained heating of
higher terrain from the Wasatch Range southward and southeastward.
Additional thunderstorms should form this afternoon from western WY
and eastern ID across western MT, as eastern fringes of large-scale
ascent preceding the Pacific Northwest trough and embedded
shortwaves overlie destabilizing, elevated terrain.
A monsoonal plume and associated parallel moisture-transport vectors
will ensure sufficient boundary-layer moisture supply, even through
the diurnal heating/mixing process, with somewhat less moisture and
deeper inverted-v thermodynamic profiles northward. MLCAPE should
range from around 1000-2000 J/kg in southern UT, in richest
moisture, to around 200-800 J/kg over the northern Rockies, in
stronger flow aloft. Throughout the corridor, sufficient deep shear
(30-45 kt) should exist for organized multicells, small bowing/
outflow-driven clusters, and perhaps a few supercells. Despite the
weak moisture, enough is apparent to suggest isolated severe-gust
potential into parts of eastern WA and northeastern OR, including
areas of dry-thunder potential in the fire-weather outlook.
...Northern/central 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 MT this afternoon. Activity should move northeastward to
eastward across the adjoining High Plains. Along the way, this
convection will encounter a diurnally destabilizing, well-mixed
boundary layer with steep low-level lapse rates, supporting isolated
severe gusts. Isolated large hail also will be possible.
Strongly difluent mid/upper-level flow will exist over most of this
corridor, between the Northwest cyclone and the southern Plains
anticyclone, with speeds generally increasing northwestward and
low-level moisture greater southward down the central High Plains.
Still, sufficient moisture should persist near the MT 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. Interestingly, most progs show a relative minimum in
convective coverage in the area of greatest midlevel DCVA/ascent
head of the ejecting shortwave trough, compared to farther north and
south. For now, a long 5%/Marginal outlook will be maintained, with
the understanding that conditional/localized concentrations of
severe-gust potential in particular may exist where upscale
aggregation can occur. Potential for activity to persist later into
the evening is most apparent over the southern part of the outlook
area, which has been expanded somewhat eastward accordingly.
...Southeastern AZ, extreme southwestern NM...
Widely scattered to scattered thunderstorms are expected to develop
this afternoon in a plume of monsoonal moisture and favorable
surface heating today from western Chihuahua across southeastern AZ
and southwestern NM, north-northeastward over eastern AZ and western
NM. Increasing midlevel 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. Isolated, marginally severe
hail also may occur in some cores. Forecast soundings reasonably
suggest that lower-elevation dewpoints, remaining in the 50s to near
60 F through most of the heating/mixing cycle, contribute to MLCAPE
in the 1000-2000 J/kg range.
..Edwards/Bentley.. 08/23/2024
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