Day 1 Severe Weather Outlook

Valid 271300Z – 281200Z


Large hail and isolated damaging winds are expected this afternoon
and evening from parts of the central Plains to the mid Mississippi
Valley/southern Upper Great Lakes.

The upper-air pattern this period will exhibit height falls across
much of the West, as a small cyclone initially located over central
BC digs south-southeastward over the Pacific Northwest and
amplifies. By 12Z, the 500-mb low should be located over central/
southern WA, with troughing southward across CA to offshore northern
Baja. As this occurs, a former cut-off low — now evident in
moisture-channel imagery as an open-wave trough over AZ and the
northern Gulf of CA – will deamplify and eject east-northeastward to
eastern NM by the end of the period. Downstream from these
features, a belt of westerly to southwesterly flow — with embedded/
low-amplitude shortwaves — will extend from the Rockies across the
Great Lakes to southeastern Canada.

At the surface, 11Z analysis showed a weak low northwest of Lake
Superior with a cold front southwestward across northern SD then
westward over southern MT, This boundary will merge with another,
slower-moving front analyzed initially across central MN, through a
weak low in the SUX area, to another weak low near HLC, then west-
southwestward over eastern CO. A separate cold front was drawn
across Atlantic waters south of New England to eastern NC, becoming
wavy/quasistationary over northern GA, northern AL, central MS, and
central LA, then a warm front from the Arklatex area northward to
the low near SUX.

The eastern front will lose definition gradually from west to east,
amidst the broad/low-level fetch of return flow and related warm
advection preceding the western front(s). By 00Z, the largely
combined latter front should extend from eastern Lake Superior
across eastern/southern WI, through a weak frontal-wave low over the
eastern IA, northwest IL area, then across northern MO, south-
central KS, and the Oklahoma Panhandle, to another weak low over
southeastern CO. By 12Z, the front should reach southern ON,
southern Lower MI, west-central IL, eastern KS, northwestern OK, and
the TX Panhandle.

..Central Plains to southern Upper Great Lakes…
Scattered to widely scattered thunderstorms have developed and are
moving northeastward across portions of northeastern MO,
southeastern IA, and western IL. This activity is supported by warm
advection, strengthening low-level moisture above the boundary
layer, favorable midlevel lapse rates, and isentropic lift to LFC.
Nearly unidirectional flow from effective inflow layer to LFC is
strong, but effective shear is not (only 20-30 kt based on modified
RAOBs and model soundings) thanks to lack of greater speed shear.
As such, isolated hail is possible as the convection shifts east-
northeastward across northern IL the remainder of this morning,
before weakening, but the severe threat appears marginal and poorly

Behind this activity, the air mass in and south of the surface
frontal zone will destabilize diabatically, amidst continuing
low-level moisture advection/transport. By mid/late afternoon,
warm-sector surface dew points in the mid 60s to near 70 deg F
should be common from western IL southwestward, with 60s eastward
toward the northern IN. The rich moisture, overlain by steep
midlevel lapse rates related to an EML, will lead to peak MLCAPE
values in the 2500-4000 J/kg range. However, the same EML
responsible for the favorable lapse rates also will bring strong
capping via a basal inversion layer, beneath neutral to slightly
rising height tendencies aloft. This will tend to suppress
surface-based convection for much of the period.

As such, the bulk of activity is expected to occur behind the
surface front, where a combination of isentropic and frontal forcing
boosts otherwise favorably unstable/moist parcels to LFC. In such a
scenario, the dominant threat is hail, especially amidst deep shear
suitable for both supercells and organized multicells. Convective
coverage should increase after dark, with erratic backbuilding
possible in the frontal zone as far southwest as east-central/
northeastern KS.

East of the frontal-wave low and behind the morning convection,
mainly across northwestern/west-central IL, the environment will
become conditionally very favorable for severe, with aforementioned
moderate-strong buoyancy overlapping 40-50 kt effective-shear
magnitudes, and large hodographs with effective SRH 300-500 J/kg.
At this time, forecast forcing appears too weak to support
development in this setting, which would become supercellular and
possibly tornadic if such convection were to form. While the
scenario will be monitored for more-favorable mesoscale trends,
confidence remains too low in surface-based initiation off the front
to boost unconditional probabilities yet.

…Southern High Plains…
Widely scattered to scattered thunderstorms may develop this evening
and overnight, offering mainly isolated large hail. While strong
gusts also may occur, poor boundary-layer lapse rates with nocturnal
cooling will mitigate that potential to a considerable extent.
Convection should develop over southern NM and far west TX this
evening, then move northeastward and strengthen tonight, as large-
scale forcing for ascent related to the approaching AZ shortwave
trough impinges upon strengthening low-level moisture near a
retreating dryline. Increasing deep shear related to the
approaching trough also will contribute to potential storm
organization. At this time, unconditional probabilities are held at
5%/marginal given concerns over coverage of severe cells.

..Edwards/Broyles.. 09/27/2019


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