U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 221959 
Storm Prediction Center ac 221958 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0258 PM CDT Fri Sep 22 2017 

Valid 222000z - 231200z 

..there is a slight risk of severe thunderstorms over parts of 
northwest Minnesota and northeast north dakota`... 

..there is a marginal risk of severe thunderstorms surrounding the 
slight risk across the upper Midwest to the southern High Plains... 

..there is a marginal risk of severe thunderstorms across eastern 
Upper Michigan and a small part of northern lower Michigan... 

Isolated severe hail and wind storms are possible from the upper 
Midwest to the southern High Plains, mainly during the late 
afternoon and evening. There remains some severe threat this 
afternoon across a small portion of the upper Great Lakes. A 
separate severe threat will focus over northwest Minnesota this 
afternoon and evening. 

..Upper Michigan and northern lower Michigan... 
Mid afternoon mosaic radar imagery showed a band of storms, located 
along the leading edge of an MCS, advancing to the east at 35-40 kt 
into the eastern portion of Upper Michigan. Although the downstream 
environment is moderately unstable, including across northern lower 
mi, objective analyses indicated surface-based inhibition was 
sufficiently strong to potentially limit stronger wind gusts from 
reaching the surface. High-level clouds spreading east ahead of the 
storms should result in weaker diabatic heating and further preclude 
the reduction of inhibition for surface-based storms. This data 
suggests a marginal severe risk is warranted across portions of 
eastern Upper Michigan and also into northern lower Michigan in vicinity of 
stronger instability that could allow a locally strong wind gust, 
but aforementioned reasoning suggests this threat is quite low. 

The severe-weather threat and much of the potential for 
thunderstorms through the remainder of the forecast period across 
western Upper Michigan and northwest WI has diminished, as the environment 
has generally stabilized in the wake of the eastward-moving mesoscale convective system. 

..ND/Minnesota and elsewhere... 
No changes are needed with the rest of the forecast severe risk 

.Peters.. 09/22/2017 

Previous discussion... /issued 1116 am CDT Fri Sep 22 2017/ 

..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. 


Mesoscale Discussion

acus11 kwns 222058 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 222057 

Mesoscale discussion 1667 
National Weather Service Storm Prediction Center Norman OK 
0357 PM CDT Fri Sep 22 2017 

Areas affected...portions of Nebraska northeastward to far southwest 

Concerning...severe potential...watch unlikely 

Valid 222057z - 222300z 

Probability of watch issuance...20 percent 

Summary...thunderstorms are expected to gradually develop from 
central Nebraska northward into South Dakota by early evening. 
Isolated storms may be capable of large hail and damaging winds. 
While a watch is not currently anticipated, organizational trends 
will be monitored for a potential uptick in the overall severe 

Discussion...visible satellite data illustrate developing cumulus 
along a cold front from central Nebraska northeastward into South 
Dakota this afternoon. Ahead of this front, temperatures have risen 
into the 90s, owing to strong surface heating. While deep mixing has 
lowered dew points into the 50s across parts of Nebraska, 
boundary-layer moisture remains greater farther north and east. In 
tandem with steep mid/level lapse rates, these boundary-layer 
conditions are yielding around 2000-3000 j/kg of MLCAPE. 

Forcing for ascent is not particularly strong across the region and 
relatively dry 850-700mb conditions may stunt nascent updrafts 
initially; however, broad/weak ascent and further heating will 
likely be sufficient for isolated/widely scattered convective 
initiation. Southerly surface winds around 15-20 kt, veering to 
southwesterly mid/upper flow around 40-50 kt, should offer adequate 
effective shear for organization/rotation in the most robust cores, 
with an attendant large hail and damaging wind threat. Additionally, 
while large surface T/TD spreads will likely limit the tornado 
threat, favorable low-level hodographs suggest a tornado may be 
possible, primarily across far eastern South Dakota and far 
southwestern Minnesota later this evening. 

Although watch issuance is not currently expected, one may be 
considered if greater storm organization/coverage is observed. 

.Picca/Hart.. 09/22/2017 

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


Latitude...Lon 41480022 42399925 43339801 44879765 45189707 45159624 
44509574 43309597 41709720 40959843 40759907 40729974 
40960020 41160031 41480022