U.S. Severe Weather Forecast (Convective Outlook)

Day Three

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Storm Prediction Center ac 181112 

Day 3 convective outlook corr 1 
National Weather Service Storm Prediction Center Norman OK 
0612 am CDT Tue Sep 18 2018 

Valid 201200z - 211200z 

..there is a slight risk of severe thunderstorms from the Central 
Plains and Midwest to the upper Great Lakes... 

Corrected thunder line 

Severe storms will be possible Thursday from portions of the Central 
Plains northeastward to the upper Great Lakes region. 

Stout upper ridging is progged to persist Wednesday over the eastern 
states, which will result in the advancing trough being shunted to 
the northeast with time as it exits The Rockies and crosses the 
north-central states. By the end of the period, the deamplifying 
trough should be centered over the upper Great Lakes region, while 
the dominant ridge remains centered over the southeast. 

At the surface, a low initially progged to be crossing the 
mid-Missouri Valley/upper Midwest should deepen as it shifts across 
the upper Great Lakes toward southern Ontario/western Quebec. A 
trailing/advancing cold front will focus a zone of thunderstorms -- 
and some severe risk -- from the Central Plains to the upper Great 
Lakes region. 

..the mid-Missouri and upper Mississippi valleys, and upper Great 
diurnal heating of the pre-frontal boundary layer is expected, as 
the low -- and warm front -- shift northward into/across the upper 
Great Lakes. Resulting destabilization, combined with large-scale 
ascent focused near the surface cold front will support afternoon 
storm development -- from Kansas northeast to the upper Great Lakes. 

With strong southwesterly flow aloft spreading atop the frontal zone 
-- particularly from roughly Iowa northward, a few stronger storms 
appear likely to evolve through the afternoon, possibly growing 
upscale with time into a line/band of locally severe convection. 
Large hail and locally damaging winds appear to be the primary risk 
with stronger updrafts, with risk likely diminishing gradually after 
dark into the Midwest and toward the lower Great Lakes. 

.Goss.. 09/18/2018