860
FXUS63 KGRB 202325
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
625 PM CDT Tue Oct 20 2026

Updated aviation portion for 00Z TAF issuance

.KEY MESSAGES...

- Patchy dense fog will develop late tonight into Wednesday
  morning across east-central Wisconsin and areas west of Green
  Bay, locally dropping visibilities below 1/2 mile.

- Dry and mild conditions across the region through Saturday
  afternoon with inland high temperatures warming into the middle 60s
  to lower 70s.

- A 20 to 30 percent chance of light rain showers Saturday night
  into Sunday across central and northern Wisconsin.

- Cooler air filters for early next week, dropping daytime high
  temperatures into the 50s on Monday and Tuesday.

&&

.DISCUSSION...
Issued at 1147 AM CDT Tue Oct 20 2026

Current surface analysis shows strong high pressure centered over
northern Ontario and Quebec, maintaining large-scale subsidence
and light winds across northern Wisconsin. Upper-air analysis
displays broad mid-level ridging over the central United States
flanked by troughs over the western and eastern CONUS. Morning fog
and low stratus have been slow to erode across portions of
northeast Wisconsin, but further daytime heating will support
clearing through early this afternoon. High pressure will linger
through Wednesday before a weak, moisture- starved cold front
crosses the region late Wednesday into Wednesday night.

Patchy Dense Fog Tonight And Wednesday Morning: Clear skies and
light winds will create good radiational cooling conditions late
tonight into Wednesday morning across east-central Wisconsin and
areas west of Green Bay. Boundary layer relative humidity will
approach saturation after 10 PM this evening, promoting
fog development that will persist through mid- Wed morning. High
confidence exists in fog dissipation by 10-11 AM Wednesday as
diurnal solar heating drives boundary layer mixing.

Prolonged Mild and Dry Late Week Pattern: Persistent atmospheric
ridging and deep-layer subsidence will lock in tranquil weather
across north- central and northeast Wisconsin through Saturday
afternoon. Daily high temperatures inland will warm into the upper
60s and lower 70s, while onshore lake breezes hold shoreline
communities in the lower 60s. NBM temperature percentiles show
exceptionally tight 10th to 90th percentile spreads of 2 to 4
degrees F, reflecting high forecast confidence.

Weak Weekend Front Rain Probabilities: A weak shortwave trough
moving across southern Canada will push a trailing cold front into
the Upper Mississippi Valley Saturday night into Sunday. Weak
dynamic forcing and limited moisture transport ahead of the
boundary will restrict precipitation coverage, yielding low-end 20
to 30 percent rain chances across central and northern Wisconsin.
NBM probabilistic guidance shows 90th percentile 24-hour liquid
accumulations remaining under 0.25 inches across the forecast
area so not expecting a potential soaker. The chance of showers
will taper off Sunday afternoon with partly sunny skies returning
by Sunday evening.

Early Next Week Cooling Trend: Post-frontal cold advection will
usher in a cooler and more seasonable airmass for Monday and
Tuesday as surface high pressure builds south from Manitoba.
Daytime high temperatures will fall into the lower to mid-50s
across north-central Wisconsin and upper 50s across the Fox
Valley. Clear skies and light winds beneath the building high will
promote radiational cooling at night, dropping low temperatures
into the low to mid-30s.

&&

.AVIATION...for 00Z TAF Issuance
Issued at 625 PM CDT Tue Oct 20 2026

Another round of fog is expected late this evening through mid-
Wednesday morning. Confidence is highest for impacts over east-
central WI (KMTW, KGRB, KATW), remaining lower farther west. LIFR
to VLIFR will be possible between 08-13z Wed. Indications still
present that ground fog will develop as early as 02-03z at KMTW.

Besides the fog, after mostly clear skies tonight, scattered mid
and high clouds will pass across the region on Wednesday. Winds
will be light at less than 10 kts through the period.

&&

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

&&

$$

DISCUSSION.....MPC
AVIATION.......JLA

NWS GRB Office Area Forecast Discussion