U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 281959 
Storm Prediction Center ac 281958 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0258 PM CDT Wed Jun 28 2017 

Valid 282000z - 291200z 

..there is an enhanced risk of severe thunderstorms from far 
southeast NE into northern Illinois/southern WI... 

..there is a slight risk of severe thunderstorms from the Central 
High plains to the middle/upper MS valley... 

..there is a marginal risk of severe thunderstorms from the 
northern rockies to the Great Lakes... 

Severe thunderstorms with large hail, wind damage and a few 
tornadoes are expected across the middle Missouri Valley 
northeastward into the middle and upper Mississippi Valley. Other 
more isolated severe storms may occur in the Central Plains and 
northern rockies. 

..severe-weather threat areas... 
No changes were needed with the severe-probabilities/categorical 
severe risk areas, as previous forecast reasoning remains valid with 
this outlook issuance. 

..general thunderstorm areas in the Dakotas and eastern New York... 
Based on convective/lightning trends in eastern Mt, the general 
thunderstorm line in western ND was shifted a little to the east to 
indicate some potential for thunderstorms to reach far western ND 
late this afternoon to early evening. A little more of central South Dakota 
was added to general thunderstorm potential for later tonight. 
Meanwhile, much of eastern South Dakota/ND into far northern Minnesota was removed 
from the general thunderstorm area, given the decrease in 
thunderstorm potential within an area of midlevel subsidence and/or 
weaker instability. 

Farther east, convective trends suggest thunderstorm potential 
should continue to shift east/northeast away from portions of 
eastern New York. 

.Peters.. 06/28/2017 

Previous discussion... /issued 1136 am CDT Wed Jun 28 2017/ 

..middle MO River Valley to middle/upper MS River Valley... 
Related to last night's MCS, scattered showers/thunderstorms and 
related prevalent cloud cover continue to progress eastward across 
eastern portions of Minnesota/Iowa into WI/northern Illinois late this morning. 
This is just ahead of an eastward-moving shortwave trough over Minnesota, 
with a few other convectively related disturbances/mcvs noted 
farther south, embedded within a belt of moderately strong mid-level 
westerlies. A surface low analyzed at 15z near the Minnesota/SD/ND border 
vicinity will continue to spread eastward across central Minnesota toward 
northern WI and the Western Upper Peninsula of Michigan through this 
evening, while a cold front spreads southeastward across MN/IA/WI. 

Near and ahead of this cold front, low-level moistening and general 
air mass recovery will steadily occur this afternoon in the wake of 
the lingering early-day precipitation and cloud cover, particularly 
given relatively steep mid-level lapse rates and ample low-level 
moisture sampled in source-region 12z observed soundings from Omaha 
NE and Topeka Kansas. Increasingly prevalent mid/upper 60s f surface 
dewpoints will support upwards of 2000-3000 j/kg MLCAPE particularly 
from southeast NE/northern Kansas into IA, with somewhat more modest 
(and uncertain) degree of pre-frontal destabilization farther north 
into WI/eastern Minnesota owing to early-day cloud cover and some 
regenerative precipitation this morning across northern Iowa. 

While the aforementioned cloud cover/precipitation lingering into 
midday still casts some uncertainty, it seems most likely that 
surface-based storms will redevelop just ahead of the front 
semi-focused along outflow and zones of differential heating across 
southern/eastern Iowa and nearby far northern MO and northern Illinois. 
Pending adequate destabilization, other potentially severe 
development may occur by late afternoon in closer proximity to the 
surface low/front from far eastern Minnesota into western/central WI. 
Around 40-45 kt of effective shear will support some initial 
supercells capable of large hail, with a few tornadoes possible as 
well given the strength of 2-3 km above ground level southwesterly winds and 
related 150-250 m2/s2 0-1 km srh (with prospective surface-based 
convection). Eastward-moving clusters should evolve by evening with 
a related increase in damaging wind potential particularly across 
southern Iowa/northern MO into northern Illinois. 

..co/Wyoming Front Range to northern Kansas/southern NE... 
Within a modestly moist environment, widely scattered thunderstorms 
should develop and spread eastward across the region by late 
afternoon into evening, including some potential for at least 
small-scale mesoscale convective system development this evening. Modest buoyancy and ample 
vertical shear will support the possibility of sporadic large hail 
and locally damaging winds into this evening. 

..northern rockies... 
Ahead of an amplifying shortwave trough over the northern 
intermountain region, modest moisture and a strengthening belt of 
west-southwesterly mid-level winds should contribute to the 
possibility of isolated strong to severe storms spanning parts of 
Wyoming/southern Montana and eastern Idaho/northern Utah. 


Mesoscale Discussion

acus11 kwns 282230 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 282229 

Mesoscale discussion 1174 
National Weather Service Storm Prediction Center Norman OK 
0529 PM CDT Wed Jun 28 2017 

Areas affected...southeast NE...IA...northern MO 

Concerning...Tornado Watch 375... 

Valid 282229z - 290000z 

The severe weather threat for Tornado Watch 375 continues. 

Summary...severe threat continues across ww375. 

Discussion...an elongated corridor of severe thunderstorms has 
evolved within the base of a strong short-wave trough over the upper 
MS valley. Numerous supercells are noted from extreme southeast NE, 
through central Iowa into southwest WI, though storm mergers are 
beginning to take place which may lead to an mesoscale convective system-like cluster if 
trends continue. Shear profiles remain favorable for storm rotation 
and potential tornadoes along with very large hail. With time 
convection should spread across the remainder of ww375 focusing 
along the Iowa/MO border and Iowa/Illinois border later this evening. 

.Darrow.. 06/28/2017 

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


Latitude...Lon 40169646 42949391 42948977 40179248 40169646