U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 200509 
Storm Prediction Center ac 200507 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1207 am CDT Wed Sep 20 2017 

Valid 201200z - 211200z 

..there is a marginal risk of severe thunderstorms from Upper 
Michigan southwestward into northeast Kansas... 

A few strong storms are possible from northeastern Kansas 
northeastward to the Upper Peninsula of Michigan this afternoon and 

..synopsis and discussion... 
A potent shortwave trough will quickly exit the northern 
plains/upper MS valley in a negatively tilted fashion with an 
occluded front moving from northern Minnesota into western Ontario. To the 
south, a weak boundary will remain, extending from WI into eastern 
Kansas. To the east of this front, a moist and unstable air mass will 
exist with upper 60s dewpoints. 

Scattered storms are likely to develop during the afternoon along 
this front, especially over northern areas including WI, eastern IA, 
and the u.P. Of Michigan where convergence is strongest. However, forcing 
for ascent will generally be weak as the primary shortwave trough 
continues away from the region. Capping will be a concern for much 
of the day, with only a narrow zone of storms expected near the 
front. Marginally severe hail and wind are most likely, and a 
brief/weak tornado cannot be ruled out given a moist boundary layer 
and modest low-level shear. 

.Jewell.. 09/20/2017 


Mesoscale Discussion

acus11 kwns 200604 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 200603 

Mesoscale discussion 1661 
National Weather Service Storm Prediction Center Norman OK 
0103 am CDT Wed Sep 20 2017 

Areas affected...Minnesota 

Concerning...Severe Thunderstorm Watch 488... 

Valid 200603z - 200730z 

The severe weather threat for Severe Thunderstorm Watch 488 

Summary...the ongoing line of storms probably will tend to weaken 
diminish as it progresses east of watch 488, but it is still 
possible that another watch will be needed to account for a 
lingering risk of potentially damaging wind gusts. 

Discussion...the more vigorous southern flank of the pre-cold 
frontal squall line is progressing east northeastward around 40 kt, 
and appears likely to progress east of the watch into east central 
portions of Minnesota (including the Minneapolis/St. Paul Metro 
area) during the 07-09z time frame. However, even within the narrow 
corridor of higher pre-frontal surface dew points (mid 60s), 
forecast soundings suggest that activity is likely rooted above a 
fairly deep (1-1.5+ km) surface based stable layer, supported by 
low-level warm advection. While the stronger mid/upper forcing for 
large-scale ascent pivots northward into the international border 
area, and inflow of drier more stable air becomes an increasing 
issue with eastward extent, general weakening trends seem probable 
within the next few hours. Even now, it is not particularly clear 
that the risk of strong downdrafts reaching the surface is 
particularly high, but until more substantive weakening of 
convection begins, heavy precipitation could continue to aid at 
least the downward Transfer of higher momentum to the surface, 
accompanied by localized potentially damaging wind gusts. 

.Kerr.. 09/20/2017 

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


Latitude...Lon 45499468 46269454 46209348 45339305 44129320 43539442 
43619518 44469484 45499468