921
FXUS63 KGRB 281825
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
125 PM CDT Mon Sep 28 2026

Forecast discussion for routine afternoon forecast issuance

.KEY MESSAGES...

- Patchy dense fog will develop late tonight into Tuesday morning
  across north-central and eastern Wisconsin, dropping visibility
  below 1/2 mile locally before clearing by 10 AM.

- Rain shower and thunderstorm probabilities increase to 70 to 90%
  Tuesday night into Wednesday night with a widespread, soaking
  rain expected.

- Drier and cooler weather returns Thursday night through Saturday
  under building surface high pressure, bringing a threat for
  patchy frost to northern Wisconsin.

&&

.DISCUSSION...
Issued at 121 PM CDT Mon Sep 28 2026

The current surface analysis reveals high pressure centered over
the western Great Lakes, delivering large-scale subsidence and
clear to partly cloudy skies across northeast Wisconsin. Low-
level moisture continues to return with dewpoints rising into the
50s, while broad warm advection and widespread low stratus advance
across the Central Plains. Upstream, a northern stream shortwave
is triggering convection over the Dakotas on the nose of a low-
level jet. High pressure will slowly shift east tonight into
Tuesday as an upper-level trough deepens over the Plains.

Patchy Dense Ground Fog Tonight Into Tuesday: As surface high
pressure shifts east late tonight, light winds and saturating
boundary layer conditions will promote ground fog development
across north-central and eastern Wisconsin between 3 AM and 10 AM
Tuesday. REFS guidance reflects a 30 to 50% probability of
localized dense fog dropping visibility below 1 mile before mixing
burns off the stratus layer by 10 AM. High clouds and a
tightening pressure gradient spreading in from the west may
mitigate widespread dense fog, maintaining moderate overall
confidence. A Dense Fog Advisory may be needed if clouds or winds
do not impact fog development.

Heavy Rainfall Tuesday Night Through Early Thursday: A
slow-moving cold front will advance across Wisconsin Tuesday
night through early Thursday, bringing widespread rain as it
interacts with an anomalously moist airmass. Ingestion of tropical
moisture originating from the eastern Pacific will push
precipitable water values to 200 to 250% of normal across the
Upper Midwest. NBM probabilistic guidance shows a 50 to 90% chance
of exceeding 1.00 inch, and a 30 to 60% chance of exceeding 2.00
inches in central and east-central Wisconsin. Rainfall rates will
peak Wednesday into Wednesday night before the frontal boundary
exits east Thursday morning.

Despite robust moisture transport and strong synoptic forcing
along the front, instability across the region remains extremely
weak. NBM surface-based CAPE values remain capped under 100 J/kg
throughout the event, preventing the development of organized
severe convection. Probability of thunder remains low at 10 to 20%,
indicating that precipitation will primarily take the form of
widespread stratiform rain with only embedded thunderstorms.
Forecast confidence is high that severe weather hazards will not
materialize with this system.

Cool Conditions and Late Week Frost Potential: Post-frontal high
pressure builds back over the Great Lakes Thursday night through
Saturday, ushering in a cooler and drier airmass. Clearing skies
and decoupling winds will support efficient radiational cooling
Friday night into early Saturday morning across the Northwoods.
NBM temperature guidance indicates a 30 to 60% probability of
minimum temperatures falling to 36 degrees or colder around Land O
Lakes, Eagle River, and Rhinelander. High pressure shifts east
late Saturday, bringing a return to mild temperatures and a 20-30%
chance of light rain Saturday night.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1156 AM CDT Mon Sep 28 2026

Through 08Z Tue: Scattered to broken MVFR ceilings late this
morning will lift and dissipate by early to mid afternoon leaving
VFR prevailing across KGRB, KATW, KMTW, KAUW, KCWA, and KRHI
under surface high pressure. Light winds 3-7 kt becoming calm
tonight.

08Z-15Z Tue: Radiational cooling drives patchy dense ground
fog/stratus. Periods of LIFR/IFR vsby <1SM and cigs <500 ft AGL
at KRHI/KGRB. MVFR/IFR visibilities possible at KAUW/KCWA/KMTW.
REFS shows 30-50% of visibilities falling below 1 mile over far
northeast WI and the lakeshore.

After 15Z Tue: Morning heating burns off fog by 15Z, returning
all terminals to VFR. S winds 5-10 kt with gusts to 20 kt at KRHI.
Increasing mid/high clouds.

Outlook: High confidence in widespread IFR/MVFR cigs in moderate
rain Tue night through Wed.

&&

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

&&

$$

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