Day 1 Severe Weather Outlook

Valid 111300Z – 121200Z



The greatest severe-storm risk is today into this evening over parts
of the central High Plains, where large, damaging hail and severe
gusts are expected, and a few tornadoes are possible.

A progressive westerly wave train will characterize the general
upper-air pattern over the northern 1/2-2/3 of the CONUS this
period. the main feature aloft aiding severe potential will be a
synoptic-scale trough — initially evident in moisture-channel
imagery over western MT, ID, eastern NV, and the lower Colorado
River Valley. This trough will move to an axis near GTF-JAC-PGA by
00Z, and GGW-CDR-ALS by 12Z, with an intermittently closed 500-mb
low shifting eastward across MT. To its east, an MCV is noted in
midlevel satellite imagery and composited radar loops over eastern
MN, with shortwave trough southward over southern MN. This
perturbation should proceed eastward across WI, Lake Michigan, and
part of northern Lower Michigan through this evening.

At the surface, the 11Z analyses showed a wavy, mostly stationary
front from Lake Ontario across southern Lower MI and southern WI, to
a low near LSE, then west-southwestward across southeastern SD and
northwestern NE, behind some convective outflow. The outflow
boundary extended across northeastern/central IA and south-central
NE near I-80. An elongated low-pressure area was located from
southeastern WY across eastern CO. The low-pressure area should
consolidate through the day, then shift roughly northeastward across
NE overnight, reaching the FSD vicinity by 12Z. A cold front will
sweep southward/southeastward across the central Great Plains behind
that low, reaching east-central NE, north-central/southwestern KS
and the Raton Mesa area by 12Z.

The eastern frontal segment should settle somewhat southward across
the mid-Atlantic region, but remain quasistationary from there to
near the Mississippi River, modulated on the mesoscale by MCS
outflow over the southern Upper Great Lakes region. The combined
front/outflow boundary also should move slowly northward as a warm
front ahead of that low tonight over eastern NE/IA.

…Central Plains…
Scattered thunderstorms, including a few supercells, are forecast to
form over higher terrain of eastern WY as early as midday to early
afternoon, in the presence of strengthening deep-layer forcing that
precedes the mid/upper trough. This activity should move generally
eastward across the NE Panhandle/Sandhills regions, and perhaps
adjoining parts of southern SD, offering the threat for all forms of

The most supportive parameter space may develop over portions of
west-central NE, where rich low-level moisture should advect into
the region. This will be most pronounced along and south of the
northward-moving outflow boundary merging with the synoptic front —
in the eastern sector of the low-level cyclogenesis and associated
mass response to the approaching synoptic trough. Favorable mid/
upper-level lapse rates and a deeply buoyant troposphere overlying
that moisture, in combination with diabatic surface heating, will
contribute to MLCAPE in the 2000-3500 J/kg range. Veering low-level
winds with height will contribute to favorable deep shear (e.g., 40-
50-kt effective-shear magnitudes), though lack of stronger speeds in
low levels may temper hodograph size somewhat away from mesobeta- to
local-scale boundary influences. Still, with the boundary and
supercell-favorable bulk shear, a tornado risk does exist, along
with the potential for very large/damaging hail of 2 inches or
larger in diameter.

Another upscale-growth process is forecast to evolve from the early
supercell and multicell activity, amidst general height falls and
increasing large-scale forcing for ascent that precedes the
progressive mid/upper trough. This should result in an evening/
overnight MCS sweeping into a recovering air mass located from
central/northeastern NE into the Siouxland area. Damaging and
severe gusts will be the main concern once the MCS develops, though
brief/embedded QLCS mesovortices may develop for an isolated threat
of tornadoes.

…Upper Great Lakes region…
Isolated strong-severe gusts cannot be ruled out the next few hours
from the MCS crossing the upper Mississippi Valley, particularly in
the warm sector and very near the front on the north side, with
increasing static stability in the boundary layer reducing that
potential northward across western WI and Lake Michigan. MCS
outflow has stabilized much of the boundary layer in the original
outlook area(s) across IA, rendering unconditional severe potential
marginal in nature and skewed toward late in the period, as the
outflow/frontal zone moves back northward.

A continuation of the MCS and/or additional convective development
near the front/outflow boundary will pose a greater threat of severe
from midday through at least early evening, shifting eastward to
east-southeastward across the eastern “slight risk” area. Damaging
gusts will be the main concern, though supercell-favorable kinematic
fields will exist near the boundary, compelling marginal tornado/
hail lines for part of the region. The low-level air mass in the
baroclinic zone and south of the surface boundary will destabilize
today from a combination of thetae advection and pockets of
sustained diabatic surface heating. This will boost boundary-layer
lapse rates and overall buoyancy amidst favorable moisture. Surface
dew points in the upper 60s to lower 70s F and PW around 1.75 inch
should be common along and south of the preconvective frontal zone
today, contributing to 1500-2500 J/kg MLCAPE. 30-40-kt effective-
shear magnitudes and somewhat enlarged low-level hodographs (with
200-300 J/kg effective SRH near the boundary) are possible, despite
the area’s being under the upper ridge.

..Edwards/Dial.. 09/11/2019


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