Day 1 Severe Weather Outlook

Valid 181300Z – 191200Z


The main severe-thunderstorm threat today will be damaging winds,
isolated hail and a few tornadoes are possible.

In mid/upper levels, a belt of zonal to gently cyclonic mean flow
will be observed through the period across the northern half of the
CONUS. As a cyclone moves slowly northeastward across northern MB,
the trailing shortwave trough should pivot from the Dakotas across
MN and Lake Superior to northeastern ON. As that occurs,
convectively aided/induced vorticity lobes should move eastward
across the upper Great Lakes. This includes a shortwave
perturbation associated with a pronounced MCV now located over
southern WI, and extending south-southwestward behind a band of
convection across southern WI and northwestern IL.

The 11Z surface chart showed a low near EAU, related to the MCV,
with wavy warm front extending across western/southern WI and
southern Lower MI. This front should drift northward before being
overtaken by an ongoing MCS and additional convection developing
farther east. A cold front was drawn from southeastern MN across
southeastern NE and northwestern KS. By 00Z this front should reach
eastern Upper MI, central WI, southern IA, central/southwestern KS,
and southeastern CO. By 12Z this front should extend from northern
Lower MI to northern IL, central/western MO, northwestern OK,
becoming quasistationary northwestward into southeastern CO.

…Great Lakes vicinity…
An intermittently organized band of convection, continuing eastward
from overnight activity across IA, has persisted across the
Mississippi River the past couple hours. Some of this activity may
strengthen across the remainder of the northern IL/southern WI/Lake
Michigan region eastward over northern IN and southern lower MI, as
the foregoing/inflow-layer air mass destabilizes from a combination
of warm advection and diabatic surface heating. Recovery behind
newer convection over northeastern IL is uncertain in magnitude and
northern extent. See SPC severe thunderstorm watch 597 and related
mesoscale discussions for near-term details.

In concert with seasonally steep low/middle-level lapse rates and
rich low-level moisture (60s to near 70 F surface dew points), areas
of 2000-3000 J/kg preconvective MLCAPE are expected to develop.
This will occur amidst difluent but generally westerly mid/upper
winds, enhanced on the mesoscale by height falls related to the MCV,
and a plume of large-scale DCVA/lift preceding the shortwave trough.
As the heating continues, MLCINH effectively will vanish over
northeastern IL and eastward through midday, potentially fostering
additional convective development and severe potential in and near
the watch area, regardless of the morphology of the initial MCS.
Subsequent aggregation and upscale growth should shift the primary
threat of damaging winds across lower MI and northern IN. In
addition, areas of relatively maximized effective SRH may support
localized tornado potential in QLCS circulations.

…Ozarks to portions of OK…
Isolated to scattered thunderstorms, with highly variable coverage
in space and time, are possible through the overnight period across
this region. Activity should be concentrated mainly near outflow/
differential-heating boundaries area-wide and near a weak
convergence zone over the southwestern OK/northwest TX region this
afternoon. Isolated severe gusts and hail may occur, with well-
mixed air below LFC developing this afternoon, and persisting into
tonight above a slowly stabilizing near-surface layer, aiding in
maintenance of hail/gusts to surface.

A relative concentration in severe potential also may become
apparent this afternoon over portions of this region, especially in
the Ozarks/eastern OK/southeastern KS area. Beneath steep midlevel
lapse rates, pockets of relatively convectively undisturbed
boundary-layer moisture and strong heating may support MLCAPE around
4000 J/kg. Uncertainties regarding location, orientation, and
strength of supportive mesoscale boundaries preclude a more-
concentrated area of unconditional probabilities at this time.

..Edwards/Marsh.. 08/18/2019


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