U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 201651 
Storm Prediction Center ac 201649 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1149 am CDT Sat Jul 20 2019 

Valid 201630z - 211200z 

..there is an enhanced risk of severe thunderstorms across portions 
of MN/IA/WI/mi/IL... 

..there is a slight risk of severe thunderstorms across the upper 
Midwest/Great Lakes and Front Range/Central High plains... 

Severe thunderstorms capable of damaging wind gusts are expected 
this afternoon into tonight across portions of Iowa, southern 
Wisconsin into Southern Lower Michigan and far northern Illinois. 

..Iowa/southern Minnesota/Wisconsin/lower Michigan/Illinois... 
An intense Bow echo capable of a heightened damaging wind risk/brief 
tornado potential continues to quickly spread eastward at midday 
across far east-Central/Northeast Wisconsin toward Lake Michigan, 
with this activity likely to cross the lake and continue into lower 
Michigan this afternoon, with other peripheral development expected 
to the south within a destabilizing air mass. 

Farther the west, an eastward-accelerating squall line continues to 
progress eastward along the Iowa/Minnesota (roughly 50 miles either 
side) border. This will continue to favor the roughly west/east 
outflow-reinforced boundary where the combination of 
buoyancy/vertical shear is maximized. This complex will likely favor 
a corridor near/south of the prior early-day mesoscale convective system (first paragraph), 
with an increasing damaging wind risk expected from southeast 
Minnesota/northeast Iowa into southern Wisconsin this afternoon. 

Other potentially severe storms may develop/expand southward tonight 
across Iowa/northern Illinois near the southward-advancing front 
where A Reservoir of ample moisture/buoyancy will exist. Some hail 
will be possible, but damaging winds may be the most probable risk 
even with a tendency for elevated storms late tonight. 

..Wyoming/Colorado Front Range to Central High plains... 
Across the Central High plains, the initial frontal push will 
support north-northeasterly surface winds this afternoon, with 
upslope flow into the Front Range and Raton Mesa. Mid/upper-level 
winds will be weak from the west-southwest. Initial storm 
development and intensification is expected along/into the 
foothills/I-25 corridor this afternoon with some hail/wind 
potential, but a slow eastward-moving cluster may evolve over 
east-central/southeast Colorado with an increasing damaging wind 
risk. This threat should eventually diminish towards late evening as 
convection becomes increasingly undercut/elevated with strengthening 
low-level stability behind a secondary frontal surge. 

..lower Great Lakes/northeast states/New England... 
In a moist/unstable air mass, isolated strong to severe 
thunderstorms capable of localized downbursts will be possible this 
afternoon with maturing thunderstorm development in the Lee of the 
Great Lakes and/or near a northeast states surface trough. Other 
strong storms could move into northern New England late tonight. 

.Guyer/Wendt.. 07/20/2019 


Mesoscale Discussion

acus11 kwns 201900 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 201859 

Mesoscale discussion 1546 
National Weather Service Storm Prediction Center Norman OK 
0159 PM CDT Sat Jul 20 2019 

Areas affected...portions of northern Illinois and chicagoland area 

Concerning...severe potential...watch possible 

Valid 201859z - 202100z 

Probability of watch issuance...40 percent 

Summary...as thunderstorms/outflow near northern Illinois, intense 
convection is possible given the extreme buoyancy in the area. 
Uncertainty remains as to how far south ongoing/newly developing 
convection will push into northern Illinois. A ww is possible should 
convective trends become more clear. 

Discussion...a relatively narrow corridor of undisturbed airmass 
remains in far southern Wisconsin and northern Illinois. 
Temperatures have risen into the upper 80s f to low 90s f with 
dewpoints holding in the upper 70s f. Two ongoing linear segments in 
southwestern and southeastern Wisconsin will likely continue to sag 
slowly southward with time. Given the extreme buoyancy -- MLCAPE of 
4000+ j/kg -- intense convection is possible with damaging winds 
being the primary threat. With the low-level shear vector oriented 
nearly parallel to the outflow, it is uncertain how much development 
will occur southward into Illinois. However, hcrs are currently 
visible on visible satellite imagery in northeastern Illinois. Even 
subtle lift from the outflow boundary may allow parcels to reach 
their level of free convection. A ww is possible should trends reveal convection is more 

.Wendt/guyer.. 07/20/2019 

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


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