Day 1 Severe Weather Outlook

Valid 241300Z – 251200Z


The greatest severe-storm threat today covers parts of eastern
Nebraska, Iowa, Minnesota and Wisconsin, with severe hail, damaging
gusts and a few tornadoes possible.

In mid/upper levels, a progressive and fairly high-amplitude
northern-stream pattern will regulate most of the severe/convective
potential this period, except for a temporarily cut-off low
initially located over southwestern AZ. The latter cyclone has
detached from the midlatitude westerlies and will meander generally
southward to the northern Gulf of California through the period. To
its northeast, embedded in a synoptic-scale height weakness and
deformation zone, a small shortwave trough is evident in moisture-
channel imagery over the southern CO/northern NM area. This feature
will migrate eastward across KS, potentially reinforced by
convectively generated vorticity tonight.

Meanwhile, an extensive area of height falls aloft is predicted over
the northern Plains, Upper Midwest and upper Great Lakes. This will
occur in advance of an amplifying, synoptic-scale trough extending
southward from a cyclone moving eastward over the northern border of
MB. The associated surface cold front — analyzed at 11Z across the
central Dakotas to east-central WY — is forecast to strengthen and
sweep eastward/southeastward over the northern/central Plains and
much of the Upper Midwest through the period. By 00Z, the front
should extend across northern through southwestern MN, northwestern
IA, eastern NE, west-central KS, and east-central/southeastern CO.
A dryline should intersect the front over western KS and extend
south-southwestward across the TX Panhandle and extreme southeastern

…Upper Midwest to KS…
Scattered thunderstorms are expected to form near the cold front,
perhaps as early as mid/late this afternoon, but more confidently
late afternoon into evening. A short window of potential for
discrete or semi-discrete supercells will offer the risk of
large/damaging hail, sporadic damaging gusts, and a few tornadoes,
before activity aggregates upscale and evolves toward more of a wind
with marginal hail and slight risk of brief/QLCS tornadoes.

The scenario generally will be characterized by stronger large-
scale/deep-layer forcing and deep shear northward from the Missouri
Valley across the upper Mississippi Valley, but with greater lapse
rates, low-level moisture and buoyancy southwestward through KS.
The overlap of favorable parameters generally defines the 15%/slight
and 30%/enhanced-hail areas, with greatest confidence in relatively
dense coverage of severe convection in and near the enhanced
outlook. Moist advection and diabatic surface heating are expected
to destabilize the warm sector diurnally, with surface dew points
commonly in the 60s F. This will underlie favorable midlevel lapse
rates to support a corridor of 2000-3500 J/kg MLCAPE from central KS
to southern MN, amidst effective-shear magnitudes ranging from
around 30-35 kt over the KS subset of the outlook area to 40-50 kt
across parts of MN/WI. Evolution from mixed multicell/supercell
mode to quasi-linear and clustered should occur through the evening,
with embedded bowing segments and mesocirculations offering locally
maximized severe potential, before activity weakens later tonight.

…Portions of northern MN and northern Plains…
Widely scattered, somewhat low-topped thunderstorms are expected to
develop and move southeastward to eastward this afternoon, offering
isolated severe hail. This threat has become apparent in a mostly
post-frontal regime characterized by:
* Favorable synoptic-scale forcing/cooling associated with the
deepening mid/upper trough (and a related series of reinforcing
small vorticity maxima);
* Surface diabatic heating in the relatively clear post-frontal air,
especially behind a band of ongoing clouds/precip moving across/out
of eastern ND, though elevated convection may produce hail in and
east of that band later today as well;
* Residual moisture (e.g., surface dew points in upper 40s to mid
the 50s F behind the front) enabling surface-based inflow parcels
with areas of 300-800 J/kg MLCAPE, locally/briefly higher;
* Robust deep/cloud-layer speed shear in support of cell

Unconditional confidence in severe gusts is lower. However, given
the likely presence of a cool but well-mixed boundary layer under
some of this activity, locally strong gusts cannot be ruled out as
well. For now, 5% hail probabilities are added, and only a little
more certainty regarding wind would be needed for categorical-level
probabilities as well.

…Southern AZ, extreme southwestern NM…
Widely scattered to scattered thunderstorms across the area will
offer the risk of marginal hail and isolated gusts near severe
limits through parts of this afternoon. The duration and amount of
destabilization in low levels are in question because of extensive
cloud cover related to ongoing convection this morning, and the
related spread of outflow air. Still, pockets of diabatic surface
heating, beneath large-scale ascent/cooling east of the mid/upper
cyclone, should provide at least modest deep-layer destabilization
supporting convective potential. While all ingredients generally
should be weaker than yesterday, isolated severe potential may
linger through parts of this afternoon.

..Edwards/Leitman.. 09/24/2019


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