U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 200042 
Storm Prediction Center ac 200040 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0740 PM CDT Wed Sep 19 2018 

Valid 200100z - 201200z 

..there is a marginal risk of severe thunderstorms this evening 
across parts of the Central Plains...and through tonight across 
parts of the middle Missouri Valley into upper Midwest... 

Strong storms may continue to impact a corridor from the Central 
Plains into the upper Midwest through tonight, accompanied by at 
least some risk for severe weather. 

..01z outlook update... 
Low-level convergence along a quasi-stationary frontal zone across 
southwest through central Nebraska has been sufficient to support 
the initiation of thunderstorms in the peak late afternoon/early 
evening boundary layer instability. Beneath modestly steep 
mid-level lapse rates associated with warm elevated mixed-layer air, 
a narrow corridor of dew points near 70f is contributing to sizable 
cape of 2000-3000 j/kg. This corridor appears south of the stronger 
mid-latitude westerlies, but veering of winds with height appears to 
be contributing to sufficient deep layer shear for organized 
convection, including supercells. Ongoing development has not been 
particularly rapid, and convection may still be suppressed by 
mid-level inhibition, which probably will increase through 02-04z, 
with the loss of daytime heating. However, a period of thunderstorm 
intensification may still not be out of the question, before this 
results in weakening convective trends. Stronger storms may become 
capable of producing severe hail and strong wind gusts. 

Later this evening, into the overnight hours, forcing for ascent 
associated with strengthening lower/mid tropospheric warm advection 
is expected to maintain and support increasing thunderstorm 
development along a corridor from southeastern South Dakota/ 
northeastern Nebraska through northern Iowa and perhaps southwestern 
Wisconsin/northwestern Illinois. This is roughly along the 
northeastern periphery of the plume of warmer and more strongly 
capping elevated mixed layer air, where lapse rates and shear 
profiles above/to the cool side of the surface frontal zone may 
remain supportive of a risk for severe hail threat into tonight. 

.Kerr.. 09/20/2018 


Mesoscale Discussion

acus11 kwns 192231 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 192231 

Mesoscale discussion 1480 
National Weather Service Storm Prediction Center Norman OK 
0531 PM CDT Wed Sep 19 2018 

Areas affected...portions of southwestern/central/northeastern 

Concerning...severe potential...watch possible 

Valid 192231z - 200030z 

Probability of watch issuance...40 percent 

Summary...storms will likely initiate along a slow moving front 
within the next hour or two. Large hail and damaging wind gusts will 
be the primary threats. A ww is possible. 

Discussion...an area of agitated cumulus, on visible satellite 
imagery, near the slow moving front in southwest Nebraska has 
persisted over the past hour. Given the long residence time of 
parcels within the zone of ascent as compared to the warm front 
farther east, this appears to be the most likely area for initial 
storm development. Storm Prediction Center objective analysis shows mid-level lapse 
rates of 7.5-8 c/km and 50-70 kts of effective deep layer shear. 
Given this environment, a few instances of large hail are possible. 
Isolated severe wind gusts will also be possible with the strongest 
cells. While enhanced srh near the warm front is supportive of 
low-level rotation, storms that form are expected to quickly move 
north of the boundary which should limit the overall tornado threat. 
A ww is possible. 

.Wendt/Thompson.. 09/19/2018 

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


Latitude...Lon 41110130 41720060 42349869 42629698 41769634 41229835 
40550046 40260106 40380146 40790168 41110130