U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus02 kwns 210541 
Storm Prediction Center ac 210540 

Day 2 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1240 am CDT Thu Sep 21 2017 

Valid 221200z - 231200z 

..there is a slight risk of severe thunderstorms from eastern South 
Dakota into northern Minnesota... 

..there is a marginal risk of severe thunderstorms from the 
southern High Plains northward to the eastern Dakotas and northern 

Scattered thunderstorms are expected to pose a severe risk from the 
eastern Dakotas to Minnesota Friday morning through early Saturday, 
with a more isolated severe threat southward into the southern High 
Plains. Hail and gusty winds are the primary threats, although some 
tornado potential may exist from South Dakota into Minnesota. 

A longwave mid/upper trough will remain nearly stationary throughout 
the day across the west, while Stout ridging remains anchored across 
the mid-Mississippi Valley and Great Lakes region. Between these 
two systems, strong, meridionally oriented mid-level flow will 
extend from the southern rockies northward to the Dakotas and 

At the surface, a nearly stationary boundary will extend from 
northwestern Minnesota south-southwestward to western Nebraska, and 
a weak low will migrate northeastward along this surface boundary, 
reaching western Minnesota during the evening. Farther south, 
broadly confluent low-level flow will exist from eastern Colorado 
into southeastern New Mexico along a weak surface trough. 
Ahead/east of these surface boundaries, a modified maritime airmass 
maintained by southerly low-level flow will remain in place. 
Farther east, high pressure will dominate most areas from New 
England southward to the lower Mississippi Valley, while tc Jose 
lingers just southeast of the New England coast. 

..eastern Dakotas into Minnesota... 
Convection should be ongoing in the early morning hours within a 
warm-advection regime as 30-40 kt southerly low-level flow continues 
to interact with the surface boundary across the region. 
Thermodynamic and kinematic profiles suggest potential for isolated 
instances of 1"+ hail with the strongest activity, and some of these 
elevated storms will extend as far west as central North Dakota. 
This hail threat should persist into the afternoon, although the 
bulk of this activity should migrate northeastward into Canada 
during that time frame. 

Despite weak upper forcing/subtle height rises across the region, 
models suggest that surface-based convection will develop during the 
afternoon along the surface boundary and just ahead of the western 
Minnesota surface low. Near 70f dewpoints and steep lapse rates 
will Foster moderate to strong destabilization within the 
pre-convective airmass, and vertical wind shear will be supportive 
of organized convection with updraft rotation. Large hail, gusty 
winds, and a few tornadoes will be possible with this activity 
(primarily within the slight risk area). This severe threat will 
persist into the overnight hours, although storms should gradually 
become more elevated above a cooling boundary layer with time. 

..Nebraska south-southwestward to eastern New Mexico... 
Isolated to scattered storms are expected across this region 
during/after peak heating hours. Although mid-level lapse rates may 
be a bit less steep compared to areas farther northeast, modest 
destabilization will occur due to surface warming and boundary layer 
mixing, and veering vertical wind profiles may Foster updraft 
rotation. Isolated damaging wind gusts are the primary threat, 
although a few instances of large hail are also possible. 

Overnight, convergence on the nose of an increasing low-level jet in 
western Kansas will Foster an area of elevated convection in 
portions of western and central Nebraska from 03z Onward. An 
isolated hail threat may exist with this convection. Low (5%) 
severe probabilities were expanded northward/westward to address 
this threat. 

.Cook.. 09/21/2017