166
FXUS63 KGRB 201955
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
255 PM CDT Mon Jul 20 2026

.KEY MESSAGES...

- Level 3 of 5 risk of severe storms today. Damaging winds, large
  hail, and a few tornadoes all possible. Highest risk is mid
  afternoon to early evening (3 to 8 PM).

- Drier and cooler for mid-week.

&&

.DISCUSSION...
Issued at 255 PM CDT Mon Jul 20 2026

MLCAPE of 2500-3000 J/KG is building into north central WI at
this time on swly flow ahead of a weak sfc trough. Meanwhile 0-8km
shear has increased to 50-60 kt across the area. 994 mb low
pressure over north central Ontario will deepen slightly 991 mb
as it moves near the srn tip of James Bay by 12Z Tue. The pre-
frontal sfc trough over nw WI will move across the nrn half of
WI late this afternoon and early evening. Sfc convergence along
this feature will be modest, but aided by the main upper wave
over sw Ontario moving over nrn Lake Superior for late
afternoon, and then deepen slightly as it moves across ern Lake
Superior this evening. Overall 12hr 500 mb ht. falls of 60-80
meters are expected across the nrn half of WI. Without strong
forcing, believe the CAMs are correct with discrete supercells
after mainly 21Z into the early evening. Cannot rule out some
consolidation of cells farther to the east toward Lake MI and
will forecast 50-70 percent chances for showers and storms.
Effective SRH of 200 m2/s2 is forecast with 0-1KM SRH of 100-150
m2/s2. LCLs and LFCs will be sufficiently favorable for
tornadoes along with the aforementioned low level wind shear.
DCAPE of 1200-1600 J/KG will be a limiting factor for tornadoes,
and a stronger factor for damaging winds. Overall though, all
hazards, possibly significant severe, are forecast.

The actual cold front will then pass late tnt on increasing nwly
winds. Breezy nwly winds and cold advection will prevail into
Tue nt. Broken stratocumulus, relatively cold air aloft and lake
effect rain showers from Lake Superior may lead to a few
showers over far north central WI late tnt-Tue. Relatively cool
temps are forecast Tue-Wed but with less clouds for Wed when
high pressure approaches from the west.

Nwly flow aloft will then prevail into the weekend. High
pressure over MN and IA on Wed will track across the Ohio Valley
and into New England for Thu-Fri. Temperatures will climb back
to normal for Thu-Fri then above normal for the weekend as swly
winds return. Chances for showers and storms return to the
forecast for Sat nt-Mon as a weak Canadian cold front will pass
or stall over the region.


&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1240 PM CDT Mon Jul 20 2026

Skies have been clearing early this afternoon with gusty
southwesterly winds developing. Expect scattered to numerous
storms to form ahead of a cold front, peaking between 21Z and
03, potentially resulting in brief IFR periods. Behind the
front, winds will gradually become northwesterly with some gusts
exceeding 20 kts by Tuesday morning. A period of MVFR stratus
may occur behind the front Tuesday morning, with the highest
chances across central into north-central Wisconsin (60-90%).
The HRRR smoke model suggests some wildfire smoke may advect
south behind the front overnight, but confidence in reduced
visibility is low at this time.

.KOSH...

Expect MVFR visibility restrictions to improve this afternoon
with gusty southwest flow developing. Attempted to provide some
timing detail with the greatest threat for storms between 00Z
and 02Z. As confidence increases, will include additional
timing/weather details in the TAF. There`s a 20-40% chance for a
period of MVFR ceilings at KOSH behind the cold front,
especially after 12Z Tuesday.

&&

.MARINE...
Issued at 255 PM CDT Mon Jul 20 2026

Breezy sly winds this evening will become breezy nw winds Tue-
Tnt as deep low pressure moves across srn Canada. A Small Craft
Advisory is in effect into tnt but will likely need to be
extended for Tue-Tue nt. A Beach Hazards Statement will continue
through tnt for the Lake MI beaches.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
WI...Beach Hazards Statement through Tuesday morning for WIZ022-040-
     050.
MARINE...Small Craft Advisory until 1 AM CDT Tuesday for LMZ521-522.
     Small Craft Advisory until 7 AM CDT Tuesday for LMZ541>543.

&&

$$

DISCUSSION...MG
AVIATION...JM
MARINE...MG

NWS GRB Office Area Forecast Discussion