Day 1 Severe Weather Outlook

Valid 021300Z – 031200Z


A few marginally severe thunderstorms are possible today from
portions of the southern Plains into the mid Mississippi Valley.

The large-scale pattern of eastern ridging and western troughing
will persist through the period, but with changes on the synoptic to
subsynoptic scales. A strong, persistent synoptic cyclone —
initially centered over western MT — is forecast to move
northeastward to southwestern SK and weaken considerably.
Meanwhile, a strong, basal shortwave trough — now evident in
moisture-channel imagery over southeastern ID — will pivot rapidly
across the northern Rockies and phase with an initially weaker
perturbation now over northern WY and eastern MT. The net result
should be the development of a new 500-mb closed cyclone over
eastern ND by 06Z, reaching northern MN within 6 hours thereafter.

At the surface, 11Z analysis showed a wavy, mostly stationary
frontal zone across western NY, southern Lower MI, northern IL,
through a weak low near MLI, then southwestward across eastern/
southern KS to another low near RTN. As the mid/upper cyclone
emerges/redevelops over the northern Plains, a frontal-wave low
should develop over the TOP/MKC area during the afternoon, rippling
east-northeastward to the northeastern IL/northwestern IN vicinity
by 12Z. In its wake, the frontal zone will accelerate as a cold
front, reaching southern IL, central AR, southern OK, and the TX
South Plains region by the end of the period.

…Southern Plains to mid Mississippi Valley…
Scattered thunderstorms are forecast to develop along/ahead of the
front from mid-afternoon through this evening, offering isolated
severe gusts. A tornado cannot be ruled out, but the threat appears
very isolated/conditional, given anticipated brevity of discrete
supercell modes, lack of larger progged low-level hodographs, and
messy storm character in general.

Lapse rates will be modest above the boundary layer, amidst a plume
of residual 700-mb moisture and relatively warm 500-mb air emanating
from the vicinity of now-dissipated eastern Pacific T.D. Narda.
Related weak lapse rates aloft are evident in morning RAOBs from
MAF, DDC, OUN, SGF, and TOP. This tropical/subtropical midlevel air
will temper buoyancy, relative to what would be expected with
similar low-level theta-e under an EML, and should mitigate severe-
hail potential to a great extent. Breaks in cloud cover —
especially from eastern KS into parts of western MO — should permit
sufficient insolation by mid/late afternoon to remove MLCINH. Peak
preconvective MLCAPE may reach 1500-2500 J/kg over southeastern KS
and western MO amidst aforementioned heating and rich moisture
(e.g., upper 60s to low 70s F surface dew points) despite those
modest midlevel lapse rates. Buoyancy should diminish southwestward
from northern OK across the southern High Plains amidst still-weaker
deep-layer lapse rates.

Only the weakest, southeasternmost fringes of height changes and
tightened gradients aloft, related to the rapidly evolving northern
Rockies/northern Plains cyclone(s), will affect the eastern KS/
northern MO part of the outlook area. Mainly their impact will be
to enhance deep shear subtly over the frontal zone, with substantial
DCVA/cooling aloft staying well northwest of the region. Forecast
soundings show effective-shear magnitudes in the 40-50-kt range over
eastern KS and MO, decreasing to around 30-35 kt over northwestern
OK. This will support a mix of multicellular and supercellular
storm characteristics. Convection should ramp down in coverage and
intensity after about 03Z, as supportive boundary-layer lapse rates

..Edwards/Broyles.. 10/02/2019


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