SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0754 AM CDT Tue Aug 27 2024
Valid 271300Z - 281200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE GREAT
LAKES REGION AND PORTIONS OF THE MIDWEST...
...SUMMARY...
Clusters of damaging thunderstorm winds and occasional large hail
are possible across the Great Lakes region and portions of the
Midwest.
...Synopsis...
The mid/upper-level pattern temporarily will become less amplified
over the CONUS, as an anticyclone now over the mid Mississippi
Valley region weakens, and a nearly zonal northern stream takes
shape across most of the northern tier of states. This will occur
downstream from a strong synoptic-scale cyclone -- evident in
moisture-channel imagery over southwestern/near-coastal BC. The
500-mb low should move east-southeastward to the northern Rockies by
12Z tomorrow, with trough and cyclonic flow southward to the
northern Great Basin. Downstream, a strong shortwave trough was
apparent over the western Dakotas to the NE Panhandle, with embedded
circulation center near DIK. This perturbation is forecast to
deamplify gradually, but still remain a well-defined shortwave
trough, as it moves eastward to the Lake Superior/northern Lake
Michigan region by the end of the period.
At the surface, 11Z analysis showed a low near the end of the
Keweenaw Peninsula in Upper MI, with cold front extending diffusely
southwestward through post-outflow air across northern WI, southern
MN, central NE, to parts of western KS and eastern CO. The Great
Lakes low should migrate eastward along the front to ME by 12Z
tomorrow. The front is expected to reach Lake Huron, central Lower
MI, southeastern WI, northern MO, central KS, and southwestern CO by
00Z, preceded by several outflow boundaries. By 12Z, the front
should extend from the ME low across central/western NY, Lake
Ontario, northern IN, and central MO, becoming quasistationary from
there to central KS, and a warm front over eastern CO. The latter
will occur in response to return flow setting up ahead of the
Northwest cyclone aloft, with strengthening lee troughing between
the front and a low over eastern MT. That low will be attached to
another, stronger cold front emerging from the Rockies, and moving
eastward over central/southwestern MT by 12Z.
...Great Lakes to central Plains...
Widely scattered to locally numerous thunderstorms (in episodic
clusters) are expected to cross the outlook area through the period,
offering damaging to severe gusts, occasional large hail, and a
marginal tornado threat. Within the broader 15%/"slight risk" area,
one or more corridors of greater severe-wind threat may develop in
concert with the best-organized portions of any cold-pool-driven,
forward-propagational complex(es) that may develop. However,
specific tracks of such axial maxima are still too uncertain at this
time, due to dependence on iterative mesoscale factors.
A residual arc of thunderstorms over Lake Huron and southeastern
Lower MI has left an outflow boundary across southern Lower MI to
south-central WI to near the southeast tip of MN, where it is being
intercepted from the west by fresh outflow from another complex
moving northeastward over western WI. The relatively stable airmass
north of the first MCS' outflow boundary is shallow and subject to
destabilization from both warm advection and (later this morning)
diurnal heating, lending some potential for expansion or
intensification of any part of the WI complex that still is present
and extends into the increasingly favorable theta-e. Meanwhile, the
outflow in/from the second ongoing complex may:
1. Produce isolated severe gusts through the shallow stable outflow
layer over WI in the near term,
2. Modify favorably across parts of WI/IL/lower MI today (the
latter assuming it reaches Lower MI to reinforce the first outflow
pool) and
3. Focus additional development an uncertain distance farther south
-- possibly including upscale growth into more complex(es), today
into this evening.
Overall, the environment along and south of the outflow boundaries
should be favorable thermodynamically, with strong diabatic heating
and rich moisture (70s F surface dewpoints being common). In
satellite imagery, a plume of moisture with origins in the Southwest
monsoon -- and still connected to MCS activity in western MX --
extends near an ELP-CAO-GLD-FSD axis. A residual EML to the east of
this, and around the northwestern quadrant of the anticyclone,
results in a plume of warmer 700-mb temperatures (per last night's
00Z charts and available soundings so far this morning), as well as
somewhat steeper midlevel lapse rates and slightly stronger capping
atop the moist boundary layer. This contributes to the potential
for areas of 3000-4500 J/kg MLCAPE along and south of the
boundary(s) from parts of WI and Lower MI southwestward into
northern MO. While such strong buoyancy will support upscale
growth, longevity of clusters also is uncertain due to lack of a
supportive LLJ.
This far southeast of the main mid/upper-level shortwave trough,
flow aloft and deep shear will be rather weak, so steering flow will
be less of an influence on convective motion than internal cold
pools and foregoing mesoscale boundary alignment. Farther
southwest, nearer to the cold front over the lower Missouri Valley
into KS, shear and CAPE both will be weaker, but still with enough
instability to support isolated to scattered afternoon development.
A few organized multicells are possible atop a strongly heated
boundary layer, with pulse gusts and hail near severe limits
possible.
..Edwards/Bentley.. 08/27/2024
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