Valid 261300Z – 271200Z
…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PORTIONS
OF MISSOURI…SOUTHEASTERN KANSAS…NORTHEASTERN OKLAHOMA…AND
EXTREME NORTHWESTERN ARKANSAS…
…SUMMARY…
The greatest severe-weather threats today will involve thunderstorm
winds and hail, in a corridor from western/central Missouri to
northeastern Oklahoma.
…Synopsis…
A progressive and amplified pattern will continue across roughly the
northern 1/2 of the CONUS through the period, featuring a synoptic
trough across the northern/central Plains and Upper Midwest.
Numerous small shortwaves will traverse the associated cyclonic
flow, including MCVs initially evident over the central Plains and
mid Mississippi Valley. One of the larger shortwave troughs — now
evident in moisture-channel imagery from the southern MB/SK border
southward over the western Dakotas — should pivot generally
eastward to western/central MN by 00Z. Through the rest of the
period, it will eject northeastward, lose amplitude, and become
absorbed in the southeastern fringes of a developing closed cyclone
centered over southern Lake Winnipeg in MB. Meanwhile, an upstream
perturbation — currently over northwestern AB and northeastern BC
— will dig southeastward across southern SK and eastern MT,
reaching the western Dakotas by 12Z. By that time, cyclonic flow
related to the MB vortex will cover the northern/central Rockies,
northern/central Plains, mid/upper Mississippi Valley, and most of
the Great Lakes.
At the surface, 11Z analysis showed a cold front from south-
central/southeastern MB across eastern portions of ND/SD, central
NE, and east-central CO. This front is forecast to move eastward/
southeastward to a 00Z position over western MN and central IA, to a
surface low at or near a triple-point junction with a separate
boundary over west-central MO, then southwestward across
southeastern KS, northwestern OK, the southern TX Panhandle, and
northeastern NM. By 12Z, this front should extend across eastern
Lake Superior, Lake Michigan or western Lower MI, western IL,
eastern/southern OK, and the Llano Estacado of west TX and eastern
NM.
…Mid Mississippi Valley to OK…
A complex scenario is underway and will continue to unfold across
the region through tonight, but with the greatest severe threat
still related to convective wind in and near the “enhanced” risk
area. A loosely organized, ongoing complex of strong/locally severe
thunderstorms over southeastern NE and northeastern KS is expected
to continue moving southeastward across northeastern KS toward
northwestern MO, offering the potential for sporadic severe
hail/gusts through the remainder of the morning. This activity is
expected to follow an instability/theta-e gradient related to the
western limb of a mesoscale baroclinic boundary that extends into
MO.
This afternoon into evening, scattered to numerous thunderstorms are
expected to form in a northeast/southwest corridor close to the
front. Development may be somewhat earlier over northern parts of
the combined “slight” and “enhanced” outlook areas, where EML-
related MLCINH will be somewhat weaker and deep-layer forcing more
robust, followed by thunderstorms in the stronger capping (but also
stronger diurnal heating) of southeastern KS and northeastern to
north-central OK. All severe hazards will be possible, with the
hail and tornado concerns being greatest in the first few hours of
the convective cyclone with any relatively discrete/sustained storms
that can become supercells. Damaging wind will be likely, both from
those supercells and any upscale evolution into bows and/or mergers
of cold pools.
A northern bound to the more-robust severe threat appears to be
represented by the combined effects of two processes affecting the
prefrontal sector over western/central MO and extreme eastern KS:
1. An MCV will continue to move northeastward from western IL
through today. A low-level boundary produced by related convection
over western IL and eastern MO arches from near STL across central
and west-central MO to northeastern KS, merging with the pr-existing
frontal zone. This boundary will retreat northward across
western/central MO, but with some modulation by the next factor…
2. The influence of cloud cover and precip from aforementioned/
ongoing convection over the southeastern NE area. This activity is
projected to shift southeastward toward northwestern MO, and some
uncertain distance into/atop the relatively stable layer north of
the boundary, before it dissipates. This precip and cloud over
should help the air mass north of the boundary to remain relatively
stable. As such, the outlook areas have been tightened across MO,
and trimmed from IA, which will reside between the stronger
mid/upper forcing to the north and optimally unstable boundary layer
to the south.
The afternoon air mass from the prefrontal boundary southwestward
into northern/central OK will become strongly unstable, thanks to
the overlap of steep low/middle-level lapse rates with 70s surface
dew points and strong surface heating. A broad area of 4000-5000
J/kg peak preconvective MLCAPE is expected across northern OK,
southern KS and western/southwestern MO, locally even higher.
Buoyancy should diminish eastward across the Ozarks with weaker
midlevel lapse rates, but with MLCAPE exceeding 3000 J/kg as far
east as southern IL and central/eastern MO. Low-level shear and
hodograph size should be relatively maximized near the prefrontal
boundary, with 200-300 J/kg effective SRH possible, gradually
diminishing southwestward. These conditions will support a mixture
of supercells and organized/deep multicells, with modes getting
messy and merged over time. The convective band may maintain a
severe threat overnight into parts of central/southeastern OK,
western/northern AR, and eastern MO/western IL.
…Upper Mississippi Valley…
A band of thunderstorms is expected to develop near the front today
and move eastward from roughly the I-35 MN corridor across
western/northwestern WI during its most intense stage, with a
marginal threat for severe wind/hail. Strong deep-layer lift,
including large-scale ascent/cooling from an approaching shortwave
trough, and the low-level mass response including frontal
convergence, will contribute to the convective environment.
Boundary-layer heating/lapse rates will be tempered by cloud cover
and an antecedent air mass characterized by weak theta-e relative to
areas farther south over MO. However, cooling aloft should steepen
midlevel lapse rates enough, in combination with surface dew points
generally in the mid 50s to mid 60s F, to support a corridor of
500-1000 J/kg MLCAPE, with around 30-35-kt effective-shear
magnitudes. The already marginal severe potential should diminish
eastward into a more-stable air mass across central/north-central WI
this evening.
..Edwards/Gleason.. 08/26/2019
$$