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ACUS03 KWNS 010729
SWODY3
SPC AC 010727
Day 3 Convective Outlook
NWS Storm Prediction Center Norman OK
0227 AM CDT Mon Oct 01 2018
Valid 031200Z - 041200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS WEDNESDAY
AFTERNOON AND EVENING ACROSS PARTS OF NORTHEASTERN IOWA AND
SOUTHEASTERN MINNESOTA INTO NORTH CENTRAL WISCONSIN...
...SUMMARY...
Severe thunderstorms are possible Wednesday afternoon and evening
across parts of the Upper Midwest. It appears that this will
include a risk for tornadoes, in addition to damaging wind gusts.
Strong storms may also impact parts of the Southwest, accompanied by
at least some risk for severe weather.
...Synopsis...
Models continue to indicate that a significant short wave trough
will pivot southeastward, then eastward, across and east of the
Canadian Prairies during this period. As it does, it still appears
that there will be increased phasing with a vigorous short wave
impulse turning eastward/northeastward along the international
border into northern Ontario. Associated forcing for ascent appears
likely to support rapid deepening of an initially deep surface
cyclone migrating northeastward out of the eastern Dakotas, across
James Bay into northwestern Quebec by late Wednesday night. As it
does, a trailing cold front is forecast to surge through the upper
Great Lakes region, upper half of the Mississippi Valley and
northern through central Plains.
At the same time, subtropical ridging centered over the Gulf Coast
region is expected to maintain a prominent influence over much of
the Southeast. However, a pair of significant short wave troughs,
within separate belts of westerlies emanating from the Pacific, will
begin to migrate inland. The lead impulse, initially a closed low
off the central/southern California coast, is forecast to accelerate
inland across the southern Sierra Nevada and lower Colorado Valley
by the end of the period.
...Upper Midwest into portions of the Missouri Valley...
Guidance continues to generally indicate that the sub 1000 mb
cyclone center will migrate northeastward out of eastern South
Dakota through northern Minnesota by mid afternoon Wednesday. It
appears that the onset of more rapid deepening may commence as early
as late afternoon near/west through north of Lake Superior, before
continuing northeastward toward James Bay. This likely will be
accompanied by strengthening deep layer mean flow across much of the
Upper Midwest, including 40-50 kt southwesterly flow at 850 mb,
across northeastern Iowa through Wisconsin and the upper Great Lakes
region by early evening.
The northward advection of seasonably high moisture content
(including surface dew points in the upper 60s to near 70f) along
the low-level jet, coupled with a narrow zone of stronger
pre-frontal surface heating, is forecast to contribute to a corridor
of moderate boundary layer destabilization (CAPE of 1000-2000 J/kg)
by late afternoon. This environment is expected to support the
initiation of thunderstorm development. The southern fringe of
stronger mid-level forcing for ascent may spread across central
Minnesota through northern Wisconsin by early evening, but
scattered, initially discrete storm development seems probable at
least as far south as northeastern Iowa, near/shortly after peak
heating, when supercells accompanied by a risk for tornadoes appear
most likely. Some upscale convective growth, accompanied by a risk
for damaging wind gusts, is possible into the evening hours,
spreading eastward and southeastward ahead of the front across the
Upper Midwest, with scattered strong to severe storms possible along
the instability axis as far southwest as the mid/lower Missouri
Valley.
...Southwest...
A belt of strong cyclonic mid/upper flow, to the south and east of
the inland migrating upper trough, is expected to overspread the
lower Colorado Valley vicinity by late Wednesday afternoon,
contributing to strong deep layer shear. In the presence of a
residually moist boundary layer, it appears that this may coincide
with sufficient daytime heating to contribute to an environment
supportive of at least isolated severe storms. This may include the
risk for a supercell or two, before convection wanes Wednesday
evening.
..Kerr.. 10/01/2018
$$
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