Valid 291630Z – 301200Z
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
INTO TONIGHT FROM SOUTHEAST WI TO NORTHERN KS…
…SUMMARY…
Scattered severe thunderstorms are possible this afternoon into
tonight from northern Kansas to southeast Wisconsin. Large hail,
damaging winds, and a tornado or two will be possible.
…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.
..Thompson/Cook.. 08/29/2019
$$