- Day Three
acus01 kwns 281300
Storm Prediction Center ac 281259
Day 1 convective outlook
National Weather Service 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 Alaska 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 Idaho and western Montana 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 Wyoming 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 Kansas.
This perturbation is forecast to move east-northeastward through the
day and weaken somewhat, reaching Minnesota and northern/eastern Iowa 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 South Dakota/ND border, with cold front southwestward across the Pine
Ridge region to northeastern Wyoming. The low should move roughly
eastward over the upper Mississippi Valley by 00z, with trailing
front southwestward across western Iowa and southeastern NE to
north-central/western Kansas and west-central Colorado. 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 Kansas overnight,
as the front stalls across northern Kansas 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 Minnesota to southwestern WI, northwestern
MO, and eastern/southern IA, and northwestern MO, with a narrower
area of elevated convection from central/northeastern Kansas to
northwestern OK. The Kansas/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/Michigan 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 Michigan. The most concentrated potential still appears to be
along/ahead of the Iowa 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 Kansas atop a hotter, more well-mixed boundary layer, with aid
of frontal lift, and move northeastward across northern Kansas 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 Kansas 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 Kansas. 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.
acus11 kwns 280434
sels mesoscale discussion
Storm Prediction Center mesoscale discussion 280433
Mesoscale discussion 1167
National Weather Service Storm Prediction Center Norman OK
1133 PM CDT Tue Jun 27 2017
Areas affected...eastern NE and western Iowa
Concerning...severe potential...watch unlikely
Valid 280433z - 280530z
Probability of watch issuance...20 percent
Summary...a local watch extension-in-area could be utilized to
address some remaining risk for severe gusts in the eastern
NE/western Iowa region but the overall severe risk appears to be
decreasing. A new Severe Thunderstorm Watch will likely not be
issued to the east of Severe Thunderstorm Watch 374.
Discussion...radar mosaic at 0430z shows a mature squall line from
Antelope County south-southwestward into Kearney County with east
movement of the line of 35-40 kt. Grand Island recently
observed/measured a gust of 54 kt at 0421z. The 00z oax raob showed
a 7.9 degrees c/km 700-500 mb lapse rate and the koax VAD shows 50+
kt southerly flow in the 925-850 mb layer as of 0430,z with a strong
low level jet. However, despite the eastward migration of a shortwave trough
into the middle MO valley late tonight, the 00z oax raob exhibited
large mlcinh with over -200 j/kg noted. While strong/localized
severe gusts may continue east of the eastern edge of the Severe
Thunderstorm Watch, the severe risk may lessen as the squall line
moves into an environment less supportive for intense downdrafts to
reach the surface.
..please see www.Spc.NOAA.Gov for graphic product...
Latitude...Lon 42229664 42239528 40409519 40489685 42229664