U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 291259 
Storm Prediction Center ac 291258 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0758 am CDT Thu Jun 29 2017 

Valid 291300z - 301200z 

..there is a slight risk of severe thunderstorms from parts of the 
Central High plains to the corn belt... 

..there is a slight risk of severe thunderstorms from northeastern 
Nebraska to northen Iowa... 

..there is a marginal risk of severe thunderstorms elsewhere from 
parts of Wyoming to the lower Great Lakes... 

Severe thunderstorms, with damaging gusts and large hail, are 
expected this afternoon into tonight within a corridor from the 
Central High plains across the lower Missouri Valley region and corn 
belt. Isolated severe storms are also possible over parts of the 
central rockies and lower Great Lakes region. 

In mid/upper levels, a zonal to gently cyclonic mean flow pattern is 
expected over the northern 2/3 of the conterminous U.S. Through this 
period. The best-defined at this time are (from east to west): 
1. A perturbation over WI, western Upper Michigan and Lake Superior is 
forecast to move eastward across eastern upper mi, northern lower 
mi, and Lake Huron today, while weakening amidst gradual, 
broader-scale height rises. 
2. A weak, convectively reinforced perturbation -- currently over 
southern NE and northern Kansas -- forecast to reach the southwestern 
Iowa/southeastern NE/northwestern MO area around 00z, then northern in 
and central Illinois by 12z. 
3. A shortwave trough -- now evident in moisture-channel imagery 
near the eastern borders of Wyoming and Montana -- should shift eastward 
across the Dakotas today, reaching Minnesota overnight. 
4. A strong, positively tilted shortwave trough -- presently 
located over western Mt, northern Idaho and northeastern or -- will dig 
southeastward across the northern rockies. By 12z, this 
perturbation should extend from the Black Hills/badlands region 
across Wyoming to northeastern Utah. 
Other than those, numerous, minor shortwaves and vorticity lobes -- 
some convectively induced -- will move eastward through that belt 
between the northern rockies and New England. 

At the surface, an ill-defined synoptic front extended southwestward 
from a low near the Keweenaw peninsula, intermingled with 
convectively processed air from western WI across western IA, 
east-central/south-central NE and northwest KS, then analyzed to a 
surface low between spd-eha. A secondary baroclinic zone was 
evident over southern SD, western NE and northeastern Colorado. The 
northern low should move eastward to Eastern Lake Superior by 00z, 
with the front southwestward to central Kansas and northeastern nm. The 
secondary boundary will sag southward across eastern Colorado and western 
Kansas. Both will be preceded by mesoscale convective outflows across 
the central/eastern Kansas and central MO areas. 

..Central Plains to corn belt... 
A large area of outflow air is apparent in the boundary layer across 
northern/eastern KS, southern NE, central/northern MO and southern 
IA, and still is being reinforced by scattered to numerous ongoing 
convection from north-central Kansas to parts of northern/central MO. 
Synoptic models (i.E., NAM, spectral, arw-kf) and 
high-res/convection-allowing guidance alike (e.G., Hrrr, until being 
forced to by radar-assimilating hot starts) consistently have failed 
to depict the breadth and magnitude of convection in this region 
overnight. Resulting unfavorable effects on the mass fields and 
thermodynamic characteristics north of the boundary likely have been 
underplayed by most progs as well. 

Some convective/precip reinforcement of this air mass is expected to 
persist for a few more hours as the supportive low level jet -- with 
associated isentropic lift to level of free convection occurring atop the outflow pool -- 
only gradually weakens through the remainder of the morning. This 
casts more uncertainty on air-mass recovery (especially for 
surface-based effective-inflow parcels), and thus severe threat, 
with northward extent from the boundary. Slight northward retreat 
of the boundary is possible across eastern Kansas and central MO once 
the reinforcing precip shifts away and/or dissipates, and favorable 
diabatic heating of the boundary layer is expected not only along 
and south of the boundary, but somewhat to its north as well. 
However, this is south of the stronger flow aloft, and deeper into 
the eml air mass above the boundary layer, than reflected in earlier 
outlooks. Given these developments and uncertainties, unconditional 
severe probabilities appear reduced overall, but still may be 
relatively concentrated near the boundary this afternoon and 
evening. More boundary-focused re-introduction of greater 
probabilities may be required as mesoscale uncertainties become more 
clarified during the day. 

Additional/afternoon and evening thunderstorm development near the 
outflow boundary may aggregate into one or two southward- to 
southeastward-propagating and outflow-driven convective complexes 
tonight, potentially affecting parts of the Ozarks and/or 
northeastern Oklahoma. 

..Central High plains/rockies... 
Afternoon convective potential will be tied largely to a combination 
of upslope flow/lift and diabatic heating of elevated terrain. The 
greatest juxtaposition of low-level lift, buoyancy, moisture and 
vertical shear may be for convection moving eastward to 
southeastward off the Palmer Divide genesis zone in east-central co, 
and into a low-level convergence zone downshear. Forecast soundings 
suggest MLCAPE may exceed 1000 j/kg atop a deep/well-mixed boundary 
layer, yet still with 50s f surface dew points. Directional shear 
(veering with height) in low levels and progged 35-40 kt 500-mb flow 
will be more than sufficient for some supercell potential, with 
effective-shear magnitudes around 50 kt. Activity may evolve into a 
small forward-propagational complex with time and eastward extent 
into KS, given the high local and potential for outflow-dominant 

Farther north from north-central Colorado and the Cheyenne Ridge Onward, 
moisture/Theta-E and buoyancy each will be more limited, though 
steep low/middle-level lapse rates and weak mlcinh still can support 
isolated severe thunderstorms developing mainly over higher terrain 
and moving eastward onto the plains. This potential may linger into 
the evening/overnight period as well, with the approach of the 
northern rockies perturbation (number 4) and related large-scale 
lift/destabilization aloft. 

..NE/Iowa slight-risk area... 
A separate area of widely scattered to scattered thunderstorms may 
develop this afternoon near the synoptic front, ahead of 
perturbation number 3, and move eastward into a pocket of relatively 
high-moisture boundary-layer air not as intensely overturned by 
earlier convection. Large hail and damaging winds are possible, and 
a tornado cannot be ruled out. While overall Theta-E should not be 
as large as south of the outflow boundaries, colder air aloft and 
stronger deep shear may support some supercellular organization, as 
well as the potential for a small forward-propagational cluster to 
evolve. The overall threat should diminish into the evening as 
activity moves over more stable low-level conditions. 

.Edwards/picca.. 06/29/2017 


Mesoscale Discussion

acus11 kwns 290952 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 290952 

Mesoscale discussion 1182 
National Weather Service Storm Prediction Center Norman OK 
0452 am CDT Thu Jun 29 2017 

Areas affected...portions of Kansas 

Concerning...Severe Thunderstorm Watch 379... 

Valid 290952z - 291045z 

The severe weather threat for Severe Thunderstorm Watch 379 

Summary...the potential for isolated instances of large hail and 
strong, gusty winds continues across watch 379, but may become more 
limited over the next few hours. 

Discussion...regional radar data depict a pair of ongoing clusters 
of strong/severe thunderstorms across the watch early this morning. 
One is a generally sub-severe southeastward-moving system over 
northeast Kansas, with upshear development aided by ascent over an 
outflow boundary related to the low-level jet (best observed in 
upstream kddc vwp data). These storms will depart the ongoing watch 
over the next hour, and further upscale growth may favor occasional 
strong winds; however, their direction of movement will take them 
away from the maximum in ascent related to the aforementioned 
low-level jet. Therefore, downstream watch issuance/extension is not 
currently anticipated. 

Farther west, a small bowing segment has evolved from a 
warm-advection Wing of thunderstorms advancing east/northeast across 
north-central Kansas. As these storms approach the aforementioned 
zone of outflow-related ascent, convective activity may increase in 
spatial extent, yielding a local/brief uptick in the potential for 
severe hail. However, these cells are pushing farther into an 
environment characterized by earlier convective overturning. In 
turn, while isolated large hail and gusty surface winds remain 
possible, the severe threat may become more limited with these cells 
as well. 

.Picca.. 06/29/2017 

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


Latitude...Lon 38669766 39419966 39789945 40059869 40069707 39629567 
39179517 38579501 38199555 38469705 38669766