Day 1 Severe Weather Outlook

Valid 101300Z – 111200Z


An enhanced threat for severe hail and wind should develop this
afternoon and evening across parts of the north-central Plains,
mostly near the Nebraska/South Dakota line. A tornado threat exists
in that area as well.

A strong shortwave trough located about 1000 nm west of Vancouver
Island is forecast to deepen into an expansive, progressive 500-mb
cyclone today. As this occurs, downstream ridging across eastern
Canada and a closed cyclone over the Pacific Northwest each will
begin to break down somewhat, thereby eroding the Rex configuration
now in place over northwestern North America. A substantial
synoptic-scale trough extends from the Pacific Northwest cyclone
south-southeastward across coastal southern CA. As the cyclone
weakens/opens slightly, the trough will move eastward, reaching to
near an axis from MSO-TWF-LAS-YUM by 12Z. Associated height falls
will spread across the central Rockies and High Plains during the
latter half of the period, supporting a broad, hybrid frontal-wave/
lee cyclone at the surface over the central High Plains.

A weak mid/upper-level shortwave trough — now evident in moisture-
channel imagery from the southern MB/ON border across northwestern
MN — will move eastward across the width of northern ON, reaching
western QC by 12Z. As this occurs, the associated surface frontal-
wave cyclone — analyzed at 11Z over northern MN — should ripple
eastward across the length of Lake Superior and much of adjoining
northeastern ON toward QC this evening. The trailing cold front was
drawn initially from the MN low to north-central NE, becoming nearly
stationary from north-central NE west-southwestward across northern
CO. A trough was drawn from a weak low over east- central CO
north-northeastward to the front in western NE. By 00Z, the cold
front should reach eastern Upper MI and south-central WI, becoming
quasistationary from there across extreme northern IA, then a warm
front across northern NE near the SD line and into east-central/
southeastern WY. By 12Z, the front should extend across central
Lower MI, south-central WI, north-central/northwestern IA and
northern NE near SD, with much of the NE part likely having been
altered by convective outflow.

…North-central Plains…
Thunderstorms are expected to develop this afternoon in two
principal regimes: convergence along/near the surface front over
northwestern NE and perhaps nearby southwestern SD, and upslope
postfrontal lift across parts of eastern WY into the Black Hills
region. During the first few hours of both regimes, supercells will
be possible with all severe hazards (tornado, wind, hail), with some
significant/damaging hail supported by forecast soundings.

The most unstable air mass will exist along and south of the warm
front, where surface dew points in the 60s F and steep low/middle-
level lapse rates will contribute to peak preconvective MLCAPE
commonly reaching the 3000-3500 J/kg range, transitioning to 1500-
2500 J/kg westward across eastern WY in the upslope regime.
Northwest and south of the front, long low-level hodographs and
favorable deep shear will prevail, while backed flow along and just
north of the front will enlarge low-level shear and storm-relative
boundary-layer winds. Deep shear should increase area-wide from
late afternoon through evening, with forecast soundings reasonably
depicting 45-55-kt effective-shear vectors. Between the latter
factor and focused convergence, the frontal zone should serve as the
focus for upscale convective growth through evening.

One or two clusters of convection should evolve from the
supercellular activity and shift eastward across the outlook area,
offering a growing threat for severe gusts — some of which also may
reach significant levels (hurricane force) on a localized basis. A
roughly eastward forward-propagational MCS appears possible astride
the NE/SD line through the evening and into parts of the overnight
hours, with a continuing wind-damage threat toward the Siouxland
area of northwestern IA and southeastern SD before the activity
wanes tonight.

…Southern Lower MI…
Scattered thunderstorms are ongoing over eastern WI and portions of
Lake Michigan, in a regime of strong low-level warm advection and
moisture transport. While expected to remain below severe limits,
the downshear spread of clouds and precip across Lower MI should
foster a prefrontal outflow/differential-heating boundary with
relatively maximized low-level lift, over portions of southern Lower
MI. While the specific location of the boundary remains somewhat
uncertain, confidence in its development and in some related
afternoon genesis of at least isolated strong/severe convection has
increased enough to warrant introducing marginal unconditional
probabilities. The main concerns with any sustained convection near
the boundary will be damaging gusts and marginally severe hail.

Forecast soundings in the nearby warm sector suggest that strong
surface heating and dew points generally in the 60s F will support
peak mid/late-afternoon MLCAPE near 2000 J/kg. A well-mixed/
inverted-v thermodynamic profile should develop in the boundary
layer, amidst minimal MLCINH. Very little directional shear is
expected, limiting hodograph sizes, though deep/cloud-layer shear
may be sufficient to support organized multicells. Convective
potential also exists farther north nearer to the cold front, but
this is very conditional on destabilization within/behind the
morning clouds/convection, and too conditional for any probabilities
at this time.

…Portions of Intermountain/northern Great Basin…
Widely scattered to scattered thunderstorms should form through much
of the afternoon and into the early evening over an irregularly
shaped swath from northeastern NV to extreme southern MT, northern
UT, western WY and northwestern CO. Isolated severe gusts/hail will
be possible. Cooling aloft related to the approaching mid/upper-
level trough, overlying diurnal surface heating, will result in
minimal MLCINH and steep deep-layer lapse rates (e.g, near 8 deg
c/km from surface-300 mb in some forecast soundings), amidst
sufficient low-level moisture to support sustained/deep convection.
Cooling aloft will offset diabatic low-level cooling enough to
maintain an isolated strong-severe threat a few more hours into the
evening/nighttime hours than less synoptically forced scenarios.

..Edwards/Dial.. 09/10/2019


Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.