U.S. Severe Weather Forecast (Convective Outlook)

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. 

.Edwards/picca.. 06/28/2017 


Mesoscale Discussion

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. 

.Smith/Hart.. 06/28/2017 

..please see www.Spc.NOAA.Gov for graphic product... 


Latitude...Lon 42229664 42239528 40409519 40489685 42229664