Day 1 Severe Weather Outlook

Valid 201300Z – 211200Z


The greatest severe-storm threat will be destructive winds from an
ongoing complex of thunderstorms, which is forecast to continue
southeastward at least across portions of Missouri, Illinois, and
Indiana, as well as remaining parts of Iowa.

In mid/upper levels, a belt of seasonally active northern-stream
westerlies will continue across the northern 1/2-2/3 of the CONUS,
while a high shifts/erodes westward from the southern Plains toward
the southern Rockies. An elongated cyclone across northern portions
of QC/ON/MB and adjoining parts of Hudson Bay will split, with the
western portion digging southeastward across northern ON. The
trailing strong shortwave trough will approach northern MN and Lake
Superior late in the period. Meanwhile, a pre-existing shortwave
perturbation over the western IA/southern MN region — with
reinforcing/convectively generated vorticity to its southeast —
will move southeastward to Indiana by 00Z then across the Ohio
Valley into parts of the central Appalachians by 12Z.

At the surface, 11Z analysis depicted a cold front offshore from
most of New England, transitioning to a wavy/quasistationary front
from northern MD across northern WV, southern OH, southern IN,
becoming a warm front over central IL and a small part of
southeastern IA prior to being overtaken by an ongoing MCS. The
Ohio Valley States part of this front should move little before
being overtaken by the MCS. Meanwhile, a separate cold front —
initially analyzed from northwestern MN and southeastern ND across
southwestern SD and northeastern WY — will move southward across
much of the central Plains and Upper Midwest this period. By 12Z,
the front should be positioned from Lake Michigan across northern
MO, northern KS and east-central CO.

…Corn Belt…mid Mississippi Valley…
An expansive, severe wind-producing MCS will continue to organize
and offer damaging, possibly significant-severe wind across the mid
Mississippi Valley and toward the lower Ohio Valley through the
early-mid afternoon. Please refer to SPC watch 602 and related
mesoscale discussions for the latest near-term guidance.

With a favorably moist and diurnally destabilizing air mass
projected downshear, within and south of the warm-frontal zone, the
distance this MCS will travel before weakening is uncertain. It
will depend to a great extent on internal cold-pool characteristics
such as rear-inflow jet organization and lifting related to
eventually decreasing depth of the density current along and shortly
behind the gust front. In the meantime, continued lateral expansion
of the cold pool and related forced ascent will encourage additional
convective development along the margins of the MCS, especially in
any bookend vortices, while the greatest severe-wind potential
should continue to be in the bowing portion either side of its apex.
Diabatic heating and rich low-level moisture, beneath favorable
midlevel lapse rates, will help to boost preconvective MLCAPE to the
3000-4500 J/kg range ahead of the complex.

…Central Plains…
Widely scattered thunderstorms are expected to develop late this
afternoon onto evening, initially over higher terrain in the
post-frontal moist/upslope-flow regime. This activity should
proceed southeastward across parts of western/central NE this
evening and tonight, offering large hail and severe gusts. Some of
the hail may become especially destructive, exceeding two inches in
diameter. Additional convection may form tonight ahead of the
downshear progression of early storms, as well. Uncertainties exist
regarding convective coverage, especially after about 03Z and over
eastern parts of the outlook area; however, any sustained convection
in this environment will be capable of an organized severe threat.

Although some weaknesses will exist in the midlevel wind profiles,
strong veering with height will contribute to favorable deep shear,
with effective-shear magnitudes in the 35-45-kt range. This will
favor supercell characteristics. An axis of strong buoyancy will
set up across western through central/southeaster NE, with the width
of the favorably unstable air mass increasing eastward. Rich
low-level moisture — with surface dew points commonly in the mid
60s to mid 70s F — will underlie steep lapse rates in support of
MLCAPE in the 3000-4500 J/kg range.

..Edwards/Marsh.. 08/20/2019


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