173
FXUS63 KGRB 210450
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1150 PM CDT Mon Jul 20 2026

Updated aviation portion for 06Z TAF issuance

.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...for 06Z TAF Issuance
Issued at 1150 PM CDT Mon Jul 20 2026

VFR conditions should prevail through the night. Westerly
winds gusting to near 20 knots should diminish to sustained
winds around 10 knots. On Tuesday, it will be blustery with
northwest of 10 to 20 knots with gusts around 25 knots. Winds
will diminish Tuesday evening. Otherwise, stratocumulus
deck develop by/around 15z. Much of the day will be in the VFR
category, however there could be a brief period of MVFR CIGS
during the morning, generally from 2,500 to 3,000 feet. Also,
would not be surprised if there were a few sprinkles across
northeast WI.

&&

.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...
Beach Hazards Statement until 7 AM CDT Tuesday for WIZ022-040-
050.
&&

$$

DISCUSSION.....MG
AVIATION.......Eckberg
MARINE.........MG

NWS GRB Office Area Forecast Discussion