Valid 292000Z – 301200Z
…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM FAR
SOUTHERN NEBRASKA INTO MUCH OF NORTHERN KANSAS…
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER MUCH OF
KANSAS…SOUTHERN NEBRASKA…AND NORTHERN MISSOURI…AS WELL EXTREME
SOUTHEAST WISCONSIN INTO NORTHERN ILLINOIS…
…SUMMARY…
Scattered severe thunderstorms are expected this afternoon into
tonight from far southern Nebraska across much of northern Kansas
with damaging winds and hail. A few severe storms are also possible
over southeast Wisconsin and far northern Illinois.
…Southern NE into northeast KS…
The air mass continues to destabilize across the region, with ample
low-level moisture beneath steep lapse rates aloft. Visible imagery
shows towering CU near the boundary from eastern CO into northwest
KS where temperatures continued to rise into the lower 90s F. Rapid
storm development is expected once the cap is completely breached,
with additional development likely to follow along the primary
east-west boundary over southern NE. Supercells are expected
initially with very large hail, with an increasing damaging wind
threat as storms bow toward the east/southeast. A more pronounced
turn to the southeast is expected later tonight as activity develops
into eastern KS, with a continued wind threat. For more information,
see mesoscale discussion 1886.
…WI and IL…
Outflow from earlier activity has created a relatively stable area
from southeast IA into western IL and northeast MO. Given the
progressing cold front just west of this area, it is unlikely that
much severe activity will occur there. The best chance of a few
severe storms, including a supercell or brief tornado, will be over
southeast Wisconsin where the best combination of instability and
shear exist. For more information see mesoscale discussion 1885.
..Jewell.. 08/29/2019
.PREV DISCUSSION… /ISSUED 1129 AM CDT Thu Aug 29 2019/
…WI/Lower MI this afternoon/evening…
Within a belt of broadly cyclonic flow aloft, an embedded shortwave
trough/speed max will move over WI/Upper MI this afternoon, driving
a surface cold front southeastward across WI/MI. A band of elevated
thunderstorms immediately precedes the front across WI as of late
morning, and some form of this convection is likely to persist into
the afternoon into southeast WI and Lower MI. The degree of surface
destabilization prior to frontal passage is the primary concern
across WI/Lower MI this afternoon/evening, as the stronger forcing
for ascent appears likely to overspread northeast WI/northern Lower
MI, just northeast of the more substantial destabilization. Vertical
shear profiles will favor supercells if sufficient destabilization
can occur prior to frontal passage, though storm coverage is also in
doubt northeast of the IA cluster and south of the stronger ascent.
Farther southwest, a persistent cluster of thunderstorms will likely
be maintained into early afternoon from southeast IA to northeast
MO, in a zone of low-level warm advection. Additional storm
development is expected west of this cluster late this
afternoon/evening as the surface cold front reaches southern
IA/northern MO. The background environment with large buoyancy and
marginal deep-layer vertical shear appears to favor multicell
clusters and some embedded supercells initially, though
back-building as clusters/line segments is most probable along the
front this evening into tonight. Damaging gusts and large hail will
be the main threats.
…Southern NE/northern KS this afternoon through tonight…
An embedded speed max over southern WY will translate
east-southeastward to the NE/KS border region by early tonight. At
the surface, a weak lee cyclone near the southwest NE/northwest KS
border this morning will develop southward across western KS by this
evening, as a cold front likewise moves southward in the wake of the
upper Great Lakes shortwave trough. Strong surface heating is
expected beneath steep midlevel lapse rates, and convective
inhibition will be largely removed once surface temperatures reach
the lower 90s F. Boundary-layer dewpoints of 65-72 F from the cold
front/dryline triple point eastward along the front will contribute
to strong buoyancy this afternoon (MLCAPE 3000-4000 J/kg), in an
environment with effective bulk shear around 35 kt. Thus, a mix of
supercells and multicell clusters appears probable, with an
attendant threat for large hail and damaging winds. Steep lapse
rates will favor a threat for both isolated very large hail with
initial supercells, and isolated significant gusts with either
supercells or cell mergers this evening into early tonight. Some
upscale growth into one or more MCSs appears likely tonight from
northwest MO into northern KS with merging clusters along the cold
front.
$$