U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 221618 
Storm Prediction Center ac 221616 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1116 am CDT Fri Sep 22 2017 

Valid 221630z - 231200z 

..there is a slight risk of severe thunderstorms over parts of the 
Upper Peninsula of Michigan... 

..there is a slight risk of severe thunderstorms across portions of 
northwest Minnesota and northeast North Dakota... 

..there is a marginal risk of severe thunderstorms over parts of 
the plains states and upper Midwest... 

Isolated severe hail and wind storms are possible from the upper 
Midwest to the southern High Plains, mainly during the late 
afternoon and evening. The greatest threat today is over parts of 
the Upper Peninsula of Michigan. This afternoon and evening, the 
threat will focus over northwest Minnesota. 

..WI/Michigan today... 
An ongoing cluster of strong/severe storms has been affecting 
northern WI for several hours. This cluster is in an environment 
where continued weak low level warm/moist advection and daytime 
heating will help destabilize the boundary layer. Given the 
persistent nature of the storms, and the slowly increasing mesoscale 
organization of the activity, have added a slight risk for wind damage 
across parts of the Michigan Upper Peninsula. 

The overall forecast for this region remains unchanged. A very 
moist/unstable air mass will develop this afternoon in the warm 
sector of a low over northwest Minnesota and eastern ND. Vertical shear 
profiles will also be quite strong across this region with forecast 
soundings suggesting a conditional risk of supercells and tornadoes 
in warm sector. However, virtually no 12z cam guidance shows robust 
convective initiation in the warm sector. Instead, all storms form 
along and northwest of the warm front, affecting northwest Minnesota and 
northeast ND this afternoon and evening. Supercell storm structures 
are still expected in the elevated-convective regime, with stable 
near-surface lapse-rates likely limiting the tornado and wind damage 
potential. Large hail is expected to be the main threat. 

.Hart/picca.. 09/22/2017 


Mesoscale Discussion

acus11 kwns 221521 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 221520 

Mesoscale discussion 1665 
National Weather Service Storm Prediction Center Norman OK 
1020 am CDT Fri Sep 22 2017 

Areas affected...northern Wisconsin and the Upper-Peninsula of 

Concerning...severe potential...watch unlikely 

Valid 221520z - 221645z 

Probability of watch issuance...20 percent 

Summary...ongoing thunderstorms will continue through the morning. 
These thunderstorms will have the potential to produce hail and 
gusty thunderstorm winds. 

Discussion...thunderstorms will continue this morning within the 
warm-air advection regime of weak mid-level wave moving northeast 
within the larger-scale long-wave trough across the west. 
Mid-level-lapse rates greater than 7 and surface dewpoint 
temperatures in the upper-60s to lower-70s are supporting 
surface-based cape values in excess of 2000 j/kg (and most-unstable 
cape in excess of 3000 j/kg) along and ahead of the ongoing 
thunderstorms, despite surface temperatures only being in the 
mid-70s. Additionally, deep-layer shear is being augmented by the 
mid-level wave resulting in values around 40 knots. Although the 
thunderstorms appear to be outrunning the better shear, the result 
is still an environment supportive of ongoing convection and a 
hail/wind threat. As the thunderstorms move east and deep-layer 
shear weakens, a transition to outflow dominant/bowing segments will 
be possible. 

Although the initial development appeared to be elevated, recent 
radar trends depict new new thunderstorm development to the south of 
the longer-lived thunderstorm cluster. This new development is being 
watched for potential transition from elevated to 
surface/near-surface based convection which would increase the 
likelihood of strong/gusty winds being realized at the surface. 

Despite the ongoing thunderstorms, current thinking is that the 
limited spatio-temporal nature of the severe threat will not require 
a watch. This will be monitored. 

.Marsh/Hart.. 09/22/2017 

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


Latitude...Lon 45379186 45809153 46549040 47228850 47068639 45598631 
45258816 45089029 45379186