Valid 161300Z – 171200Z
…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS THIS EVENING
AND TONIGHT OVER PARTS OF KANSAS AND MISSOURI…
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
CENTRAL HIGH PLAINS…
…SUMMARY…
The greatest threat for severe thunderstorms appears to be this
evening and overnight, for hail and wind over portions of
northeastern Kansas and western/central Missouri.
…Synopsis…
In mid/upper levels, a set of cyclones across western and northern
Canada, along with assorted shortwaves to their south, is
contributing to a zonal to slightly cyclonic pattern across the
northern half of the CONUS. A leading shortwave trough — initially
over New England, will eject away from the region. Meanwhile
another perturbation — apparent in moisture-channel imagery over
parts of WI and northern IL — should eject northeastward across
Upper MI and eastern Lake Superior to adjoining parts of
northeastern ON. A weak shortwave trough currently over OH will
move east-northeastward across parts of PA and NY, weakening further
through the period. Upstream, another perturbation is evident over
southeastern MT and eastern WY, and should shift east-southeastward
to near the OMA/FSD corridor by 00Z, then reaching Lower MI by 12Z.
At the surface, 11Z analysis showed a weak surface low over eastern
PA, with quasistationary frontal zone southwestward roughly down the
Blue Ridge to north-central GA, then arching northwestward into an
area of outflow over southeastern MO. That part of this front east
of the Mississippi River should move little through the period.
Another frontal zone — weakly cold to stationary — was drawn from
central IA across central/southwestern KS. This boundary should
move eastward across portions of WI and northern IL by 00Z, just
ahead of the mid/upper trough, then eastward over Lower MI and much
of IL by the end of the period, while the remainder of the front
remains across parts of KS/MO.
…KS/MO and vicinity…
Scattered to numerous thunderstorms in clusters should form this
evening and overnight in and near the enhanced-risk area, offering
large hail — some significant/2 inches being possible from any
supercells. Damaging gusts also cannot be ruled out, though this
threat is more conditional on location/extent of upscale cold-pool
aggregation and depth/strength of near-surface static stability in
the nocturnal preconvective environment.
An extensive area of convective outflow is present at outlook time
over much of MO and eastern KS, reinforced by multiple rounds of
convection during the prior overnight period. Considerable
uncertainty exists in the northeastward extent and timing of
favorable air mass recovery by late afternoon and evening — both
surface and aloft over western/northern MO. This is related not
just to the mesoscale and at least partly elevated nature of the
putative low-level forcing, but also both synoptic and convection-
allowing models’ poor performance with duration/extent of ongoing
convection from northeastern OK across the KS/MO border region.
Still, the concern is more of location and timing of overnight
severe rather than density/intensity. With strong low-level warm
advection and moisture transport accompanying a 50-60-kt
southwesterly LLJ tonight, expect one or two primary clusters to
move eastward to east-southeastward over the region with severe
potential lingering through much of the overnight part of the
period.
…Central High Plains…
Widely scattered to scattered thunderstorms will form this afternoon
over some of the higher terrain just east of the eastern mountains
in CO/WY, including the Palmer Divide and Cheyenne Ridge, as strong
surface heating preferentially removes CINH on the elevated terrain.
Additional, isolated to widely scattered convection may form
elsewhere across the western parts of the outlook area. This should
occur primarily driven by boundary-layer forcing, both
thermodynamically and kinematically (weak convergence and
upslope-lift areas north of the front), since mid/upper-level
support is meager. Severe hail and gusts will be the main concerns,
with localized severe concentrations in both coverage and magnitude
where sustained supercells and/or bow echoes can develop.
Moisture and buoyancy will increase eastward, as will MLCINH with
lowering altitude, amidst steep midlevel lapse rates. An axis of
1500-3000 J/kg MLCAPE should develop from extreme eastern WY
southeastward to extreme eastern CO, amidst 40-45 kt effective-shear
magnitudes. The threat generally should diminish through mid/late
evening as the foregoing boundary layer stabilizes, though isolated
severe activity may persist to near 06Z.
…Northeast…
Widely scattered thunderstorms should form from midday through this
afternoon, in zones of weak boundary-layer convergence related to a
surface trough and remnant frontal zone. Isolated gusts to severe
limits may occur from the most intense cells, and subsevere gusts
still may produce minor damage.
Areas of sustained surface heating are expected today amidst patchy
low and high clouds, and behind the initial shortwave trough. This,
combined with favorable surface moisture (e.g., dewpoints increasing
through the 60s to near 70 F with southward extent), will offset
modest mid/upper-level lapse rates to eradicate MLCINH and produce
favorable buoyancy for afternoon convective development. Weak lift
ahead of the subtle shortwave trough also may contribute to a
favorable convective environment. Forecast soundings suggest MLCAPE
generally in the 500-1200 J/kg range. Meanwhile, mid/upper-level
winds and deep shear will increase northward, leading to
effective-shear magnitudes in the 30-40-kt range area-wide.
Organized multicells and brief/marginal supercell behavior are
possible.
..Edwards/Marsh.. 08/16/2019
$$