989
FXUS63 KGRB 011932
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
232 PM CDT Mon Sep 1 2025

Forecast discussion for routine afternoon forecast issuance

.KEY MESSAGES...

- Patchy ground fog is expected again late tonight, with locally
  reduced visibility.

- High confidence of rain and some thunderstorms (60-80% chance) Tuesday
  night through Wednesday. No severe storms are expected.

- Record to near-record cold high temperatures remain likely for
  Thursday.

&&

.DISCUSSION...
Issued at 232 PM CDT Mon Sep 1 2025

Short Term...Tonight and Tuesday

The latest RAP analysis and satellite/radar imagery indicate a
persistent high-pressure system remaining anchored from the Great
Lakes to the Canadian Maritimes. A dry airmass is currently in
place across the region, with only a smattering of fair weather
clouds across the state. Thunderstorms have developed along the
lake breeze over the central Upper Peninsula. Trends will be
monitored to add a chance storms to northern Vilas county, but
thus far, they have remained north of the border. Upstream, a
strong cold front is progressing southeast across central Canada,
with south winds increasing moisture over the northern Plains and
southern Canadian prairies ahead of it. As this front approaches
and moves into the region late on Tuesday, rain chances and
timing, along with thunderstorm potential, become the primary
forecast concerns.

Dense Fog: Patchy ground fog is expected to develop late tonight,
potentially leading to localized reductions in visibility to under
1 mile. This is a recurring pattern observed over the past few
nights under the influence of the strong high-pressure system and
light winds.

Shower and Thunderstorm Potential: High pressure will remain in
place across the region tonight, leading to clear skies after
diurnal fair weather clouds dissipate by sunset. On Tuesday, warm
advection will increase from the west ahead of the cold front
diving southeast across Minnesota. Clouds will increase throughout
the day, with rain chances beginning to move into the I-39/WI 51
corridor by mid to late afternoon. A small chance of thunderstorms
will be possible. Most Unstable CAPE (Convective Available
Potential Energy) values are expected to be in the 200-300 J/kg
range, which is sufficient for isolated, weak thunderstorms.
However, with limited instability and forcing, no severe weather
is anticipated.

Temperatures: Highs in the middle to upper 70s are expected on
Tuesday afternoon.

Long Term...Tuesday Night Through Monday

Strong energy will be digging into the region on Wednesday and
Thursday and forming an upper low over the northern Great Lakes.
This upper low will slowly lift northeast from the Great Lakes to
northern Quebec during the weekend. The latest model guidance
remains quite agreeable with this amplified pattern.

Rain & Thunderstorm Potential: The cold fronts` arrival remains
on track for late Tuesday night, ushering in a period of more
widespread shower activity. In a modest warm advection pattern
ahead of the front, showers are expected to move into central and
north-central Wisconsin during Tuesday evening, spreading over
eastern Wisconsin overnight. Strong height falls combined with
most unstable CAPE values of 300-500 J/kg will result in
widespread showers and isolated thunderstorms (60-80% chance)
along the front on Tuesday night and Wednesday. The progressive
nature of the front, coupled with weak instability and
precipitable water values only reaching around 120-130% of normal,
suggests a very low risk of heavy rain. Probabilities for 0.75
inches of rain have only risen slightly into the 15-30% range.
Trailing energy will bring another round of light showers to the
area on Thursday night into Friday morning. Beyond Friday, the
predictability of rain chances is low due to uncertainty in finer
details, but the evolving pattern will remain conducive for
intermittent light shower activity at times through the weekend.

Near Record Cold: Behind the frontal passage, a significantly
cooler airmass will arrive. 850mb temperatures are forecast to
fall to 10-12 degrees Celsius below normal by Thursday morning.
Thursday appears to be the coldest day, with NAEFS 850mb
temperatures forecast to be below the 0.5 percentile from 06Z
Thursday to 00Z Friday, potentially approaching the climatological
minimum for the date. As a result, high temperatures on Thursday
are expected to approach or fall shy of record low maximums for
the date. Temperatures are expected to remain below normal through
early next week.

Boating Hazards: Unstable conditions over northern Lake Michigan
are expected to create favorable conditions for waterspouts and
potentially necessitate Small Craft Advisories from Wednesday
through Friday. The most favorable times for waterspout
development appear to be with the frontal passage on Wednesday and
also late Thursday night into early Friday with the trailing wave.
However, confidence in waterspout development remains low at this
time. The timing for potential Small Craft Advisories will need to
be refined in subsequent forecast cycles.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1209 PM CDT Mon Sep 1 2025

High pressure will remain in place across the region through the
TAF period, resulting in good flying weather. Maintained the patchy
ground fog (IFR-MVFR vsby) at GRB/RHI late tonight. FEW to SCT
fair weather clouds this afternoon will feature bases around 4-5 kft.
The clouds will dissipate after sunset this evening. Winds will
be light once again.

Clouds will slowly increase on Tuesday. The chance of showers
should not start to ramp up until mid to late afternoon over
RHI/AUW/CWA.

&&

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

&&

$$

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