U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 200546 
Storm Prediction Center ac 200544 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1244 am CDT Thu Sep 20 2018 

Valid 201200z - 211200z 

..there is an enhanced risk of severe thunderstorms this afternoon 
and evening across the upper Midwest... 

Severe thunderstorms are expected to develop across the upper 
Midwest this afternoon and evening. In addition to potential for 
damaging wind gusts, it appears that this will include the risk for 
a few tornadoes, with a couple possibly strong. 

Models suggest that much of the northern tier of the U.S. Will come 
under the increasing influence of a generally zonal belt of 
westerlies emanating from the mid-latitude Pacific. A couple of 
significant short wave troughs are embedded within this regime, 
including one forecast to accelerate east/northeast of the northern 
rockies through much of the upper Mississippi Valley by late 
tonight. As this occurs, the center of a prominent subtropical high 
is forecast to shift across the Tennessee Valley through the 
southern mid Atlantic coast region. 

It appears that the short wave trough may provide support for fairly 
strong cyclogenesis along a surface frontal zone now curving across 
portions of the Central Plains into portions of the Ohio Valley to 
the south of the Great Lakes. Models generally suggest that most 
rapid deepening of the low may be across parts of the upper Midwest 
through the upper Great Lakes region late this afternoon through 
tonight. In its wake, a reinforcing intrusion of cool air appears 
likely, with the front advancing through the remainder of the 
Central Plains and upper half of the Mississippi Valley by 12z 

..plains into the Great Lakes region... 
Strong thunderstorm activity with perhaps at least some severe 
weather potential is possible along much of the frontal zone. 
However, the most appreciable severe weather potential seems likely 
to accompany the deepening surface low, particularly near peak 
daytime boundary layer destabilization. This appears most probable 
in a corridor east of the middle Missouri Valley through the upper 

Warm sector destabilization could be hindered at least somewhat by 
relatively warm air at mid/upper levels, with cold air aloft 
associated with the short wave trough forecast to lag to the west 
through the period. However, seasonably high moisture content, 
including dew points in the lower 70s, may still contribute to cape 
up to around 2000 j/kg with daytime heating. The moist surface air 
mass will initially be confined to areas southwest of a convectively 
reinforced front likely extending across eastern Iowa through 
northern Illinois and Indiana at the outset of the period. During 
the day, models indicate that this boundary will shift northward 
with the approach of the deepening low. 

Near the intersection of the developing low and this boundary, it 
appears that destabilization of the boundary layer will coincide 
with enlarging clockwise curved low-level hodographs. This is 
expected to include wind speeds increasing to 40-50+ kt in the 
850-700 mb layer. Coupled with forcing for ascent associated with 
low-level warm advection, the environment may become conducive to 
the development of discrete supercells capable of producing 
tornadoes. A couple of these could be strong, particularly across 
parts of southeastern Minnesota and adjacent northern Iowa into 
parts of southwestern/west central Wisconsin. 

Warm sector thunderstorm development remains otherwise somewhat 
unclear. In the wake of the aforementioned warm advection activity, 
forcing along the cold front in relatively close proximity to the 
cyclone may be accompanied by an evolving line of storms late this 
afternoon into this evening. This activity could pose a risk for 
potentially damaging surface gusts, mainly across portions of 
southeastern Minnesota and northern Iowa into Wisconsin, before 
diminishing this evening in weaker/waning instability. 

.Kerr/Wendt.. 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