Valid 031300Z – 041200Z
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS PARTS OF THE GREAT
LAKES AND MIDWEST…
…SUMMARY…
Scattered severe thunderstorms are expected mainly during the late
afternoon to mid-evening across parts of the Great Lakes and Midwest
regions. A few tornadoes, large hail, and damaging winds are all
possible.
…Great Lakes and Midwest…
Expansive stratiform and convective swaths are ongoing from northern
Lower MI to eastern WI and northern IL. The leading convection is
associated with an eastward-moving MCV, while upstream activity is
being forced by robust low-level warm advection ahead of a vigorous
shortwave trough over northern MN. The cluster over far northern IL
may pose an isolated severe threat through midday as it spreads
towards northern IN.
Latest guidance is largely underestimating the extent of ongoing
convection (especially the 06Z NAM). The 06Z ESRL-HRRR and GFS
appear closer to reality compared with the 00Z HREF members/ECMWF
and 06Z HRRR. This yields continued uncertainty in how the
convective scenario will play out later this afternoon and evening.
As such, while potential exists for a significant tornado, an
upgrade to a category 3-Enhanced Slight Risk has been deferred.
Seasonably rich low-level moisture is prevalent upstream of the
warm-advection driven convection across IA/southern MN/southwest WI,
characterized by near 70 surface dew points. This rich moisture will
advect eastward in the wake of this morning activity beneath an
elevated mixed layer. Boundary-layer heating will probably be of
more limited duration with northeast extent across Lower MI, but
will be greater with southwest extent into IL. A strongly unstable
air mass should develop near and southwest of Lake Michigan with
greater uncertainty in the degree of buoyancy farther northeast.
At least isolated, mid to late afternoon convective redevelopment
will probably occur with approach of a cold front from western Lower
MI into northern IL. Deep-layer shear will be strongest northeast
compared to more veered flow and less enlargement to hodographs with
southwest extent. With some indications of veer-back-veer wind
profiles, convective mode may tend to evolve into a few clusters
with embedded supercell structures. The severe threat should wane
overnight as the primary shortwave trough becomes increasingly
separated from the lingering buoyancy plume over the Midwest.
…East-central/northeast FL Coast…
Hurricane Dorian is forecast to move northward and remain offshore.
This will place most of the FL Atlantic Coast in the unfavorable
northwest quadrant for tornadoes. Nevertheless, some enlargement to
the low-level hodograph combined with weak instability might support
a couple rotating cells capable of a brief tornado through tonight.
..Grams/Smith.. 09/03/2019
$$