U.S. Severe Weather Forecast (Convective Outlook)

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Day Three

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Storm Prediction Center ac 210646 


Day 3 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0146 am CDT Thu Sep 21 2017 


Valid 231200z - 241200z 


..no severe thunderstorm areas forecast... 


... 
a very low severe risk will exist across portions of the southern 
and Central High plains northeastward to central Minnesota. 


... 
A longwave trough will continue to persist across the west while 
only making slow eastward progress. Meanwhile, ridging will remain 
in place across the Ohio Valley and Great Lakes region, and strong, 
meridionally oriented flow will remain in place across the High 
Plains and upper Mississippi Valley. 


At the surface, a nearly stationary boundary will extend from 
northwestern Minnesota south-southwestward to a surface low over 
eastern Colorado. A surface trough will extend southward from the 
low across eastern New Mexico. To the east of these features, 
southerly low-level flow will maintain a modified maritime tropical 
airmass across much of the plains and upper Mississippi River 
valley. An anticyclone will remain nearly stationary across the 
east, while surface ridging slowly builds into portions of the 
northern plains. 


Sustained convergence along the surface boundary and Lee trough 
should Foster ongoing deep convection at the beginning of the 
period, and these storms should increase in coverage and intensity 
throughout the day in response to surface heating and attendant 
destabilization. Although shear profiles in much of the plains 
would support updraft organization (and perhaps a few instances of 
rotation), marginal mid-level lapse rates will act to limit the 
overall magnitude of the severe risk in most areas. Low (5%) severe 
risk areas may be needed in later outlooks where late-day 
destabilization can be maximized. 


.Cook.. 09/21/2017 


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