SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0749 AM CDT Thu Aug 29 2024
Valid 291300Z - 301200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
THE UPPER MISSISSIPPI VALLEY REGION...
...SUMMARY...
Severe thunderstorms are most probable this afternoon and evening
over portions of the upper Mississippi Valley region, with a threat
for tornadoes, damaging to severe gusts, and sporadic large hail.
...Synopsis...
The mid/upper-level pattern will be characterized by a progressive
belt of westerlies across the northern tier of states, and messy
area of ridging across the south-central/southeastern CONUS,
suffused with numerous small vorticity maxima and a few slow-moving
MCVs. In that northern stream, a strong shortwave trough -- with
embedded closed cyclone now located over extreme southern SK -- will
move eastward to southern MB and the Dakotas by 00Z. The trough
should deamplify overnight and eject northeastward to
northwesternmost ON by 12Z, while a positively tilted trailing
portion of the trough lingers across parts of WI/IA/NE/KS. Farther
east, a weak perturbation -- evident in moisture-channel imagery
from just north of Lake Huron in ON to astern Lake Superior -- will
weaken gradually and move southeastward across the remainder of ON
to NY and New England by the end of the period. An even weaker
shortwave trough -- initially extending from central OH across
southern IN -- should move slowly southeastward into the central
Appalachians.
At the surface, 11Z analysis showed a triple-point low over
northeastern SD, with a warm front southeastward over southwestern
MN and northern IA, becoming quasistationary to a weak cold front
over central IL, southern IN, central OH, central PA , and NJ. The
eastern part of the front should drift southward over the Mid-
Atlantic through the period, while the western part proceeds
northeastward across MN/WI as a warm front. A cold front was drawn
from the low southwestward over west-central NE and east-central CO.
By 00Z, the cold front should reach northern/central MN,
northwestern IA, eastern NE, north-central/southwestern KS, and
parts of southeastern CO/northeastern NM. By 12Z, the front should
extend across parts of Lake Superior, WI, northern/western MO,
southeastern KS, northern/western OK, northwest TX, and southeastern
NM.
...Upper Mississippi Valley to central Plains...
Scattered thunderstorms should develop along/ahead of the surface
cold front -- earliest (around midday) where deep-layer lift is
strongest over northern/central MN, then later where stronger
capping, weaker upper support, but stronger heating will be present
from around the Missouri Valley southwestward. All severe hazards
will be possible with greatest vertical shear supporting a threat of
supercells in MN and perhaps northern IA for the first few hours of
the convective cycle. If supercells can remain discrete/mature long
enough before the quasi-linear evolution occurs, tornado potential
could be relatively maximized over central MN where the overlap of
large buoyancy, low LCL and greatest SRH is forecast. Activity
overall should evolve into a line of numerous thunderstorms by late
afternoon and evening, with severe gusts and marginal hail becoming
the dominant concerns. Convection should weaken over eastern and
southern parts of the "marginal area tonight, while moving into a
less-unstable, larger-CINH airmass from WI to southern KS.
Although the main mid/upper cyclone/trough will be pivoting
northeastward over Canada, the southeastern rim of associated height
falls and mass response will pass across the "slight risk" outlook
area. This should back the near-surface flow ahead of the front
enough to enlarge low-level hodographs and shear vectors, and to
strengthen effective-shear magnitudes into the 30-40-kt range
(decreasing southward over IA). A deep troposphere, rich boundary-
layer moisture with dewpoints commonly in the upper 60s to low 70s
F, and steep low-level lapse rates, should support MLCAPE commonly
in the 2500-3500 J/kg range, and locally nearing 5000 J/kg, within
the northward-narrowing warm sector, over IA/MN part of the outlook.
Farther southwest, buoyancy will weaken due to somewhat less
moisture content and greater boundary-layer mixing.
...VA/NC to southern PA...
Scattered multicell thunderstorms are expected to form this
afternoon along and south of the front over the outlook area,
offering isolated gusts near severe limits and marginally severe
hail. This activity will develop in a plume of rich low-level
moisture, with lower-elevation surface dewpoints in the upper 60s to
low 70s F. That, along with diurnal heating offsetting modest
midlevel lapse rates, should support MLCAPE in the 2000-3000 J/kg
range. Weak deep-layer flow/shear should keep activity relatively
disorganized, with pulse severe possible.
..Edwards/Bentley.. 08/29/2024
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