Day 1 Severe Weather Outlook

Valid 031630Z – 041200Z


Scattered severe thunderstorms are expected mainly during the late
afternoon to evening hours across parts of the Great Lakes and
Midwest. A few tornadoes, large hail, and damaging winds are all

…Midwest/Great Lakes…
An upgrade to Enhanced Risk could still be warranted for portions of
the region later today pending convective/observational trends.
However, as mentioned in the prior (13Z) Outlook discussion,
short-term guidance continues to generally sub-optimally depict
and/or remains slow to adjust to the influence of ongoing
convection, which casts some uncertainty regarding the convective
scenario later today. This is principally with respect to the
thermodynamic environment and related likelihood and
north-northeastward extent of appreciable moistening and
destabilization later today, coincident with a notable deep-layer
wind field. Morning observations including 12Z sounding and WSR-88D
VWP data sample deep-layer winds that are very strong by late-summer
standards, particularly at a latitude around southern Lake Michigan
and northward including Wisconsin/Michigan.

Beneath a respectable elevated mixed layer, seasonably rich
low-level moisture (with around 70F surface dewpoints) is prevalent
at late morning across eastern Iowa, southern Wisconsin, western
Illinois near and ahead of an eastward-advancing cold front. This
moisture will steadily advect eastward today, with the morning
convection/residual cloud cover potentially effectively
reinforcing/delaying an eastward-shifting warm front toward and
across Lower Michigan and northern Indiana. Boundary-layer heating
will probably be of more limited duration with north/northeast
extent across Lower Michigan, but will be greater with southwest
extent into Illinois. A strongly unstable air mass should develop
initially near Lake Michigan southwestward across Illinois, with
greater uncertainty in the degree of buoyancy farther north across
Lower Michigan.

At least isolated, mid-to-late afternoon deep convective
redevelopment will probably occur with the approach of a cold front
from western Lower Michigan into northern Illinois, with deep-layer
winds well-supportive (40-50 kt effective) of initial supercells.
Hail/damaging wind potential aside, low-level shear/SRH will be
maximized near the effective warm front and within the increasingly
narrow warm sector with northward extent across Lower Michigan.

Although short-term forecast uncertainty lingers as previously
discussed, on presumed/probable air mass recovery a respectable
potential does exist for a couple of tornadoes, including some
possibility of a strong tornado, aside from an increasing risk for
damaging winds this evening. Severe risks currently appear most
probable to occur across northern Indiana and southern Lower
Michigan, and possibly into northern/western Ohio late tonight. 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 Florida Coast…
Hurricane Dorian is forecast to move northward and remain offshore;
reference NHC for the latest details. This will maintain most of the
Florida Atlantic Coast in the tornado-unfavorable western periphery
of Dorian. Nevertheless, some enlargement to the low-level hodograph
combined with weak instability might support a couple of rotating
cells capable of a brief tornado through tonight.

…Deep South Texas…
Reference NHC for the latest details regarding Tropical Depression
Seven, which may gradually strengthen/organize over the far western
Gulf of Mexico. A modest low-level strengthening of easterly winds
on its northern periphery could influence low-topped supercell
potential and possibly some tornado risk into Wednesday.

..Guyer/Nauslar.. 09/03/2019


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