U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 201610 
Storm Prediction Center ac 201609 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1109 am CDT Thu Sep 20 2018 

Valid 201630z - 211200z 

..there is an enhanced risk of severe thunderstorms from northern 
Iowa to southern Minnesota and western Wisconsin... 

Severe thunderstorms over parts of the upper Midwest this afternoon 
and evening should produce damaging wind gusts, tornadoes and 
sporadic large hail. 

..upper Midwest... 
Only minor changes have been made to the ongoing forecast today. 
Morning visible satellite imagery and surface analysis show the 
primary warm front extending across central Iowa. This boundary will 
develop/move northward through the day and into parts of Minnesota/WI by 
this evening. Intense low-level shear profiles will develop along 
and north of the boundary, resulting in significant conditional 
potential for supercells and associated tornadoes. However, 12z cam 
solutions show very limited discrete convective development in this 
region prior to the approach of a severe squall line this evening. 
Nevertheless, will maintain the ongoing tornado risk probabilities 
due to conditional concerns, and due to potential for 
qlcs-associated tornado threat this evening. 

By late afternoon, an intense squall line will develop along the 
cold front sweeping into eastern NE, western Iowa and southwest Minnesota. 
Strong wind fields, ample instability, and significant large-scale 
forcing all support the risk of rather widespread damaging winds 
with the storms. The activity should spread quickly eastward across 
the enh risk area through mid-evening. Damaging wind gusts are the 
main threat, but hail and a few tornadoes will also be possible - 
especially near the synoptic low track over southeast Minnesota and western 

.Hart/coniglio.. 09/20/2018 


Mesoscale Discussion

acus11 kwns 200426 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 200426 

Mesoscale discussion 1481 
National Weather Service Storm Prediction Center Norman OK 
1126 PM CDT Wed Sep 19 2018 

Areas affected...northeast Colorado...northwest Kansas and western 

Concerning...severe potential...watch unlikely 

Valid 200426z - 200600z 

Probability of watch issuance...20 percent 

Summary...storms will pose a risk for isolated large hail through 
about 06z over the Central High plains region. Due to the expected 
limited duration of the threat, a ww issuance is not expected. 

Discussion...late this evening a slow-moving cold front extends from 
eastern Colorado into central NE. Forcing for ascent associated with a 
strengthening low-level jet interacting with the sharpening 
baroclinic zone is fostering the development of storms from 
northeast Colorado through western NE. These storms are within an 
environment characterized by 1500-2000 j/kg MUCAPE, 7.5 c/km 700-500 
mb lapse rates and 45-50 kt effective bulk shear supportive of some 
supercell structures. However, updrafts are elevated above a stable 
layer north of the front which should limit primary threat to large 
hail, though a few strong downdrafts could also reach the surface. 
Activity will continue developing northeast this evening, but 
tendency may be for storms to begin to congeal into clusters with 
time, which should eventually mitigate the overall hail threat. 

.Dial.. 09/20/2018 

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


Latitude...Lon 41500075 40630094 39260237 39320292 41350226 42010132