792
FXUS63 KGRB 271831
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
131 PM CDT Sun Sep 27 2026

Forecast discussion for routine afternoon forecast issuance

.KEY MESSAGES...

- Patchy dense ground fog will develop late tonight into Monday
  morning across central and north- central Wisconsin, locally
  reducing visibilities below 1/2 mile before clearing by 10 AM.

- Rain shower and thunderstorm probabilities increase to 60 to 80
  percent Tuesday night into Wednesday as a slow-moving upper-
  level trough and surface cold front traverse Wisconsin.

- Cooler conditions filter in behind the cold front Thursday
  through Saturday, with high temperatures falling into the low-60s
  and overnight lows dropping into the mid-30s across north-
  central Wisconsin.

&&

.DISCUSSION...
Issued at 131 PM CDT Sun Sep 27 2026

The current surface pattern features a sharp high pressure system
centered over northern Quebec and extending down the length of
Lake Michigan, keeping dry air with precipitable water values 40
to 60 percent of normal over eastern Wisconsin. Meanwhile, a
deeper moisture plume and scattered light showers over western
Wisconsin are slowly shifting eastward against the Great Lakes
ridge axis. While these showers will likely dissipate this
afternoon as they run into the very dry airmass, cloud cover will
likely keep temperatures cooler than yesterday. Focus of this
forecast remains on cloud cover and dense fog potential tonight.

Patchy Dense Fog Tonight And Monday Morning: Cloud cover will
remain fairly widespread through the evening. But after 3-5 AM,
radiational cooling beneath light winds and thinning cloud cover
will favor nocturnal ground fog development into Monday morning
across central and north-central Wisconsin. Surface dewpoints
gradually rising into the upper 40s to near 50 degrees this
afternoon will promote boundary layer moisture saturation between
3-10 AM Monday. NBM statistics and API hourly guidance indicate a
40 to 60 percent probability of localized dense fog reducing
visibilities below 1 mile near Rhinelander, Wausau, and Merrill.
High confidence exists in rapid fog dissipation by 15Z Monday as
morning solar heating drives boundary layer mixing.

Prolonged Dry And Mild Early Week Pattern: Deep-layer atmospheric
subsidence associated with the persistent surface high pressure
ridge will lock in dry and tranquil weather across the forecast
area through Tuesday afternoon. Inland high temperatures will warm
into the mid-70s on Monday, while light onshore lake breezes keep
Lake Michigan shoreline communities slightly cooler in the upper
60s. NBM ensemble guidance exhibits high confidence with tight
10th to 90th percentile temperature spreads of only 2 to 3 degrees
F for daily highs through Monday.

Midweek Frontal System And Widespread Rainfall: Deterministic and
ensemble guidance consensus tracks a slow-moving northern stream
trough and advancing surface cold front into the Upper Mississippi
Valley Tuesday night into Wednesday. Strong low-level moisture
transport will elevate rain probabilities to 60 to 80 percent
Tuesday night through Wednesday afternoon. NBM probabilistic
guidance indicates a 50 to 80 percent probability of 48-hour QPF
exceeding 0.50 inches across the region, with NBM 50th percentile
liquid accumulation ranging between 0.50 and 1.25 inches.
Surface- based CAPE values under 300 J/kg and poor shear- moisture
overlap will keep thunderstorm activity isolated and mitigate
severe weather potential.

Late Week Cooling Trend And Frost Potential: Post-frontal cold air
advection will usher in a cooler and drier airmass Thursday
through Saturday, dropping daytime highs into the low to mid-60s
area-wide. As high pressure builds southward from Canada, light
winds and clearing skies will promote efficient radiational
cooling on Friday night. NBM temperature percentiles show
overnight minimum temperatures dropping into the mid to upper 30s
across Vilas, Oneida, and Forest counties Friday night. This setup
yields a 15 to 30 percent probability of frost in sheltered low-
lying spots across the Northwoods before high pressure begins
shifting east over the weekend.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1227 PM CDT Sun Sep 27 2026

THROUGH 08Z MON: High pressure maintains light winds. Deeper
moisture slowly pushing east will lower VFR CIGS to MVFR (020-030)
over central and north-central WI (KRHI, KAUW, KCWA) around
23-01Z. Eastern sites (KGRB, KATW, KMTW) remain VFR with mid-level
clouds.

08Z TO 15Z MON: MVFR CIGS west will dissipate late, leaving
abundant low-level moisture. This sets the stage for locally dense
FG and IFR/LIFR VIS between 08-15Z. Confidence in FG is highest
at KRHI, and lower at KAUW and KCWA. Eastern WI sites have a low
FG risk due to persistent mid-level clouds.

AFTER 15Z MON: FG burns off by 14-15Z. All terminals will evolve
into a VFR fair-weather cumulus field for Mon afternoon.

Forecast confidence is medium.

OUTLOOK: Widespread rain arrives late Tue - Wed.

&&

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

&&

$$

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