478
FXUS63 KGRB 171108
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
608 AM CDT Thu Sep 17 2026

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Light rain over northern and central Wisconsin will diminish
  this morning, with skies gradually clearing this afternoon.

- A period of heavy rainfall is expected Friday afternoon through
  Saturday, with the heaviest rain occurring Friday night. A 40 to
  60 percent chance of rainfall exceeding 2 inches exists roughly
  south of Highway 29.

- A quieter and drier weather pattern will settle into the region
  early to mid next week.

&&

.DISCUSSION...
Issued at 1256 AM CDT Thu Sep 17 2026

The latest RAP analysis and satellite/radar imagery show light
rain spreading east across northern Wisconsin within the right
entrance region of a 150 kt jet streak and ahead of a positively
tilted shortwave. A cluster of thunderstorms is pushing east
across west-central Wisconsin on the nose of an elevated moisture
transport axis and moisture convergence. Instability is nearly nil
across northeast Wisconsin, so expectations are for this cluster
of storms to diminish prior to reaching central Wisconsin. At the
surface, high pressure is centered over the northern Plains, while
the next weather system for Friday is currently impacting the
central Rockies.

Early Morning Showers and Quiet Weather Tonight: The ongoing
showers will continue to track east across northern Wisconsin
early this morning and provide up to a quarter inch of
precipitation. Clouds with spotty showers will likely stick around
for much of the morning, particularly across east-central
Wisconsin where low-level moisture is most plentiful, before
partial clearing occurs in the afternoon. High temperatures will
be seasonable today, ranging from the upper 60s to middle 70s.
Relatively quiet weather is expected tonight as the surface high
pressure builds into the region. Patchy fog may develop over far
northeast Wisconsin late tonight, before mid and high clouds above
15 kft begin to increase from west to east overnight ahead of our
next system.

Heavy Rainfall Potential Friday into Saturday: High pressure
departs the region on Friday morning as shortwave energy and
associated low pressure move across the central Plains. Rain
within a strong frontogenesis region on the nose of a 30 to 40 kt
low-level jet looks to spread into north-central Wisconsin as
early as late Friday morning, which is slightly earlier than
forecast yesterday, before spreading across the rest of the area
in the afternoon. The heaviest rain will likely fall on Friday
night as impressive dynamics in the right rear quadrant of a jet
streak, mid-level frontogenesis, and moisture convergence align
above the returning warm front. Precipitable water values are
forecast to climb to 1.80 inches, pushing into the 98th percentile
for mid-September. Deep warm cloud depths over 12 kft will lead
to efficient rainfall production. The risk for severe
thunderstorms is low, with most unstable CAPE values only up to
500 j/kg.

Confidence in heavy rainfall has held steady over the
past 24 hours. Probabilities of rainfall in excess of 2 inches
generally range from 40 to 60 percent south of Wausau and Green
Bay, roughly south of Highway 29. However, there has been a slight
southward shift in the axis of heaviest rainfall in the latest
model runs. It is common for the heaviest rainfall to be displaced
southward from consensus model guidance, making the exact
location of the heaviest rainfall one of the greatest
uncertainties with this forecast. There will also likely be a
rather sharp cutoff to the northern edge of the precipitation
shield, so far northern Wisconsin may see considerably less rain
than current forecasts indicate.

In terms of flooding impacts, the EFI has held steady across
central and east-central Wisconsin. Probabilities of reaching
flash flood guidance are generally under 10 percent in a 6-hour
period, so the potential for flash flooding looks isolated at
best. This will behave more as a long- duration rainfall event
that will lead to stream and river rises, accumulating water in
low-lying areas such as ditches and culverts, and ponding of water
on roads. The heaviest rainfall is projected to impact some of
the same areas that received 1 to 3 inches of rain earlier in the
week and are the areas that will be most susceptible localized
flooding. The rain will likely end for most locations by Saturday
evening.

Marine and Beach Hazards Friday Night through Sunday: Gusty
easterly winds ahead of and during the passage of the
aforementioned low pressure system will build wave heights
significantly on Lake Michigan from Friday night through the
weekend. This will create hazardous conditions for small craft and
increase the risk for dangerous rip currents along area beaches
through at least Sunday and possibly into Monday.

Quiet Weather Pattern Next Week: Shortwave energy from the
western North American trough will slowly migrate east over the
center of the country through the middle of next week. The
associated low pressure system and rainfall footprint have trended
further south over the past 24 hours. Therefore, it appears that
the bulk of the precipitation will remain south of the region.
Only small chances of rain, up to 30 percent, are forecast for
Monday. For the remainder of the week, strong high pressure
settling over northern Ontario looks to funnel ample dry air
southward across northern Wisconsin. Rain chances are expected to
remain low for much of next week under the influence of this
Canadian high.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 608 AM CDT Thu Sep 17 2026

Through 00Z Fri: Light rain and patchy fog will exit northeast WI by
14Z, with brief IFR/MVFR reductions in visibility in the rain.
Scattered LIFR ceilings and visibilities may also impact RHI, AUW,
CWA through 14-15Z. Thereafter, MVFR/VFR BKN ceilings around
2500-5000 ft will persist before scattering out late afternoon.
Winds will be NE under 10 kts. Confidence is medium in cloud bases
lifting and clearing by early this evening.

After 00Z Fri through 12Z Fri: High pressure builds in, bringing
prevailing VFR conditions. Patchy fog may develop late, but
confidence of fog significantly impacting the TAF sites remains
low. Mid and high clouds above 15 kft will arrive late in the
period. Winds will be light and variable.

Outlook: Rain and IFR conditions are expected to develop Friday
afternoon through Saturday.

&&

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

&&

$$

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

NWS GRB Office Area Forecast Discussion