653
FXUS63 KGRB 210304
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1004 PM CDT Sat Jun 20 2026

Updated aviation portion for 06Z TAF issuance

.KEY MESSAGES...

- Scattered non-severe storms this afternoon and early evening,
  mainly across northeast Wisconsin.

- Next more widespread rainfall potential (70-90% chance) late Tuesday
  and Wednesday, but severe weather is unlikely.

- Near to below normal temperatures and comfortable humidity
  will prevail through next week.

&&

.DISCUSSION...
Issued at 1215 PM CDT Sat Jun 20 2026

Tonight-Sunday: Scattered showers/storms this afternoon

A weak mid-level trough embedded in northwest flow aloft will swing
across the Upper Great Lakes this afternoon. With steep low level
lapse rates beneath cool mid-level temps, isolated to scattered
showers/storms will develop, with the greatest focus across far
northeast Wisconsin. These will quickly diminish after sunset. With
limited instability (MLCAPE < 500 J/kg), the threat for severe
weather is quite low. Lightning and brief downpours will be the
primary hazards, although with the cool thermal profiles, very
small hail cannot be ruled out with any stronger cores.

Patchy fog may develop overnight with weak boundary layer flow
and clearing skies, but widespread dense fog is not anticipated.

Sunday continues to trend dry with precipitation associated
with a shortwave trough and surface low expected to remain
mainly south of the area. However, although instability is even
weaker than today, an isolated shower could not be ruled out
across northern Wisconsin during the afternoon, but chances are
less than 20 percent.

Monday-Saturday: Cool, highest rain chances mid-week

A northwest flow pattern will persist across the region this
week into the coming weekend. This flow pattern will keep
temperatures and humidity levels relatively low for late June
with highs mainly in the 60s and 70s.

The highest rain chances (70-90%) over the coming week right now are
forecast to be late Tuesday into Wednesday as a trough of low
pressure passes across the Great Lakes. With the relatively dry,
cool airmass, significant destabilization appears unlikely, keeping
the risk of severe weather low. The risk of excessive rain also
looks low, given the limited instability, but there are at least low
to medium (20-50%) chances for 1/2 inch of rain or more. Lower (30-
40%) chances for showers and storms persist into Thursday as
cyclonic flow aloft and cool mid-level temps reside over the Great
Lakes.

Beyond this time, confidence is lower with respect to pattern
details and precipitation potential for late week. However, there
are some indications that the belt of stronger mid-level flow (80th
percentile 500 mb winds relative to climo), which will be focused
south of the area across the central plains to Mid-Mississippi
Valley for much of this week, may retreat northward in conjunction
with greater moisture/instability towards the Upper Midwest and
northern plains late next weekend into the following week, providing
a more favorable environment for storms.

However, overall, for this time of year, at least over the next
week, the risk for more impactful severe weather is low.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1004 PM CDT Sat Jun 20 2026

With mostly clear skies and light winds during the overnight
period, favorable conditions will exist for ground fog to develop,
particularly in the higher elevations of central and north-
central WI and areas which received measurable rainfall today. No
changes to forecast visibilities in the tempo groups at RHI, AUW,
and CWA to IFR in the 08-12Z time period.

Ground fog will burn off early Sun morning, likely by 13Z. Then
fair weather cumulus will pop in the 15-17Z time frame. Convective
allowing models indicate scattered shower activity will be
possible across north-central WI in the afternoon. Coverage should
not be as widespread as today, but an isolated shower could
impact RHI.

Light winds (under 10 kts) are expected through the TAF period.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION.....JM
AVIATION.......MPC

NWS GRB Office Area Forecast Discussion