SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0755 AM CDT Sun Jul 24 2022
Valid 241300Z - 251200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER A CORRIDOR
EAST OF THE LOWER GREAT LAKES IN PENNSYLVANIA AND NEW YORK...
...SUMMARY...
Severe thunderstorms are expected to develop across the lower Great
Lakes vicinity into St. Lawrence Valley by late this afternoon, with
a threat for damaging gusts and a couple tornadoes.
...Synopsis...
A belt of cyclonic flow will persist across the Upper Midwest, Great
Lakes and Northeast, south of a persistent and large cyclone
wobbling erratically around the James Bay/southern Hudson Bay area.
An embedded, convectively enhanced shortwave trough -- initially
located over Lower MI -- is expected to eject east-northeastward
across southern ON to southern QC through the period. That process
will be encouraged by the progression of a much stronger shortwave
trough -- apparent in moisture-channel imagery over parts of far
northwestern ON and adjoining central/southern MB. That trough
should pivot southeastward over Lake Superior, Upper MI and northern
WI around 00Z, then eastward across Lake Huron and nearby sections
of ON.
Upstream, a shortwave trough and intermittently closed/small cyclone
are evident over the northern Rockies. This feature should move
slowly eastward over western/southern MT and adjoining parts of
ID/WY through the period, while gradually weakening. Meanwhile,
farther south -- in the face of repeated incursions of vorticity
lobes from the lower-latitude easterlies turning northwestward over
the Desert Southwest, the persistent anticyclone previously anchored
over the Four Corners States should erode eastward. Ridging will
remain over the southern Rockies, southern Plains and eastward past
the Tennessee Valley.
The 11Z surface analysis showed a cold front from near the east end
of Upper MI southwestward across southern WI, southern IA, to a low
over southwestern KS, to northeastern NM. By 00Z, this front should
advance to southeastern Lower MI, central IL, south-central MO and
south-central KS, becoming a stationary to warm front westward to a
hybrid frontal-wave/lee-side low over southeastern CO. By 12Z, that
low should migrate eastward along the front to near ICY, with a
nearly stationary boundary segment eastward over the MO Ozarks to
extreme southern IL. The cold front should resume from there across
southern OH, northern PA and northern NY.
...Lower Great Lakes and vicinity...
Scattered thunderstorms are expected today into this evening,
along/ahead of the cold front from the Lower Great Lakes region to
the Ohio Valley. Convection in the northern/northeastern parts may
include a few supercells, and will pose a risk of scattered severe
gusts and a few tornadoes. The "enhanced" area may represent an
aggregate coverage of multiple strong-severe convective episodes
crossing the region, as much as for the intensity of any one of
them. The convective environment will be supported by the
glancing/southern influence of the ejecting MI shortwave trough --
both in terms of large-scale DCVA/ascent, and enhancement of deep
shear through tightening of the height gradient aloft. Modest
low/middle-level lapse rates will offset favorable low-level
moisture enough to keep MLCAPE maximized at 1000-1500 J/kg in a
narrow corridor east of the Lower Great Lakes, but with 40-50-kt
effective-shear magnitudes and 150-300 J/kg effective SRH in support
of mixed supercell/multicell modes and possible some bowing
segments.
In the higher-moisture, more strongly heated, but weaker shear
environment farther southwest across IN/IL, clustered multicells
(with a few small MCSs possible) will be the dominant modes, with
sporadic damaging to severe wind possible. Forecast soundings
suggest a deep troposphere supporting 2000-2500 J/kg MLCAPE, but
with only about 25-35 kt effective-shear magnitudes. Overall
frontal lift and deep shear each will weaken further with
southwestward extent from there, though isolated pulse/multicell
severe may be noted.
...Central/northern Rockies and adjoining High Plains...
Isolated to scattered thunderstorms are forecast to develop this
afternoon, along a lengthy expanse of the interface between the
eastern Rockies front and the High Plains. Activity should move
roughly eastward onto the Plains, with more longevity/persistence
likely in middle and southern segments of the outlook area (hence,
its broadening southward). The main concern will be at least
isolated severe gusts.
Diurnal heating will steepen lower/middle-tropospheric lapse rates
in the presence of sufficient/residual low-level moisture advecting
into the region in the post-frontal boundary layer. This should
lead to a well-mixed low-level thermodynamic profile, beneath
minimal MLCINH, with a plume of 500-1000 J/kg peak/preconvective
MLCAPE that generally narrows northward. Though low-level moisture
and theta-e should be somewhat lower on the northwest end, large-
scale ascent ahead of the northern Rockies perturbation should
overlie surface heating to contribute to very steep low/middle-level
lapse rates. Continued easterly flow component north of the
front/low should contribute to both favorable storm-relative
boundary-layer winds for activity moving off the higher terrain, and
strong veering of flow with height. Several loosely organized bands
or clusters of convection may aggregate cold pools for a few hours
in support of gust potential. Depending on subsequent mesoscale
trends and convective guidance, some subset of this area may gain
enough spatial certainty to warrant greater probabilities.
...AZ...
The onset of diurnal convective potential today may be delayed
somewhat by passage/dissipation of a band of morning clouds/
convection and a few embedded thunderstorms now crossing parts of
southern AZ. Still, strong heating is expected, in the clearing
evident in satellite imagery behind the early activity, especially
of higher terrain in the east and south. Loftier areas from the
Mogollon Rim to the Chiricahuas and westward across higher terrain
south through east of TUS should support initial, widely scattered
to scattered thunderstorms this afternoon. This activity should
evolve into clusters and move roughly westward to west-northwestward
over the deserts, with damaging/isolated severe gusts possible into
the evening as still more convection develops atop assorted outflows
emanating from the afternoon activity.
Low-level moisture will remain favorable with PW around 1.5-inches,
with lower/desert-elevation surface dewpoints in the 50s to low 60s
F, supporting around 1000 J/kg MLCAPE. A deep layer of easterly to
southeasterly flow is expected to persist in the middle/upper
troposphere, maintained by perturbations now over parts of Chihuahua
and northern Sonora, despite some weakening of the ridging to the
north. This will occur atop a well-heated/mixed boundary layer,
encouraging locally strong-severe convective gusts and discrete
westward propagation of convection and successive outflows.
..Edwards/Gleason.. 07/24/2022
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