SPC AC 281259
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0759 AM CDT Wed Jun 28 2017
Valid 281300Z - 291200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM NEAR THE
NEBRASKA/IOWA/MISSOURI BORDER AREA TO SOUTHWESTERN WISCONSIN...
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ELSEWHERE FROM
EASTERN WISCONSIN AND NORTHERN ILLINOIS TO NORTHEASTERN COLORADO...
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK AND EXTENDING TO THE YELLOWSTONE REGION...
Severe thunderstorms with large hail, wind damage and a couple of
tornadoes will be possible from the Mid Missouri Valley
northeastward into the mid to upper Mississippi Valley. Other
severe storms may occur in the central Plains with isolated,
marginally severe storms in the central and northern High Plains.
In mid/upper levels, the intense ridge that has been dominant over
much of the Southwest has been shunted somewhat southward, with a
high just offshore from northern Baja, and a northward-branching
ridge arching across the northeastern Pacific to the AK Panhandle
and Yukon. Downstream, part of a closed cyclone over north-central
AB will merge with another perturbation over northern SK, while the
rest digs southward over ID and western MT as a strong shortwave
trough. This will contribute to the eastward ejection of a
lower-amplitude perturbation now over southeastern OR and northern
NV, reaching the western Dakotas and southeastern WY by 12Z.
Farther east, a well-defined, nearly neutral-tilt shortwave trough
-- augmented by several areas of convective vorticity generation --
is evident in moisture-channel imagery over the Dakotas, NE and KS.
This perturbation is forecast to move east-northeastward through the
day and weaken somewhat, reaching MN and northern/eastern IA by 00Z.
The trough should continue to weaken overnight, with the
primary/residual vorticity max reaching the western Lake Superior
area by the end of the period. Meanwhile, the longstanding
northeastern-U.S. trough, now over New England and just offshore
from the Mid Atlantic, will continue to eject away from the region
and over the Canadian Maritimes.
At the surface, the 11Z analysis showed a low over the eastern part
of the SD/ND border, with cold front southwestward across the Pine
Ridge region to northeastern WY. The low should move roughly
eastward over the upper Mississippi Valley by 00Z, with trailing
front southwestward across western IA and southeastern NE to
north-central/western KS and west-central CO. Another low should
develop around that time over southeastern CO, at or just south of
the intersection of the front with a lee trough extending southward
across eastern NM. That low should move over western KS overnight,
as the front stalls across northern KS and southeastern NE. The
northern-stream surface cyclone, meanwhile, is progged to shift
eastward and become zonally elongated across the Lake Superior
...Central Plains to Upper Great Lakes...
Though exhibiting overall weakening in the past few hours, a large
area of precip and embedded thunderstorms continues in a
ragged/broken arc from northern MN to southwestern WI, northwestern
MO, and eastern/southern IA, and northwestern MO, with a narrower
area of elevated convection from central/northeastern KS to
northwestern OK. The KS/OK activity is progged to weaken as it
moves eastward throughout the remainder of the morning. The bulk of
the northern convective area also should weaken, while moving over
WI/MI and Lake Michigan.
Behind the morning convection, prefrontal air-mass moistening and
destabilization are forecast across the outlook area today, from
southwest to northeast, related to a combination of low-level
theta-e advection and diabatic surface heating. The eastward extent
of recovery suitable for surface-based effective-inflow parcels is
nebulous and uncertain, so a gradual gradient in severe
probabilities will be maintained from the mid/upper Mississippi
Valley to MI. The most concentrated potential still appears to be
along/ahead of the IA and perhaps southeastern NE frontal segment,
where scattered thunderstorms should form from midafternoon through
evening and move eastward to southeastward over parts of IL/WI. The
main concerns will be damaging gusts and large hail. Additional,
more isolated to widely scattered thunderstorms may develop over
northern KS atop a hotter, more well-mixed boundary layer, with aid
of frontal lift, and move northeastward across northern KS and
perhaps southeastern NE this evening. Scattered thunderstorms also
are possible in an arc corresponding to the zone of stronger
large-scale ascent ahead of the main mid/upper vorticity lobe,
across the upper MS Valley and WI, with gusts and large hail
Surface dew points in the immediately preconvective warm sector
should be in the mid/upper 60s F near a moist axis running from
eastern KS to southeastern NE/northwestern MO and southern/central
IA, trending to low/mid 60s east of the MS River and mid/upper 50s
in west-central KS. Peak MLCAPE 2000-3000 J/kg are possible with
those higher dew points, amidst 35-40-kt effective-shear magnitudes.
Although SRH and deep shear will be restricted somewhat by
southwesterly surface flow (nearly parallel to that part of the
front), forecast hodographs still show enough low-level and deep
shear for supercells and the risk of a couple tornadoes before storm
modes get messier.
...Central/northern High Plains/Rockies...
Although absolute and elevation-adjusted moisture will be lacking
relative to farther east, enough will remain to enable widely
scattered convection this afternoon into evening, initiated in a
regime of strong diabatic heating of higher terrain. Activity
should move eastward with some upscale expansion possible, as deep
and well-mixed subcloud layers support outflow-dominant storm
behavior. Sufficient deep shear will exist to support an assortment
of supercellular, multicellular, and forward-propagational bow/arc
structures. Though low/midlevel lapse rates will be steep across
the entire area, buoyancy should generally weaken with northward
extent due to weaker low-level theta-e.
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