743
FXUS63 KGRB 160349
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1049 PM CDT Mon Sep 15 2025

Updated aviation portion for 06Z TAF issuance

.KEY MESSAGES...

- Patchy dense fog late tonight into early Tuesday morning may
  impact travel across far northeast and east-central WI.

- Isolated showers and storms central to north-central Wisconsin
  through Tuesday.

- Chances for showers and thunderstorms Wednesday into Thursday as
  cold front crosses the region. Additional scattered showers and
  isolated storms at times Friday through Monday. No severe storms
  expected.

- Temperatures about 10 to 15 degrees above normal through mid-
  week, with a return to near normal conditions expected from
  Friday into the weekend.

&&

.DISCUSSION...
Issued at 317 PM CDT Mon Sep 15 2025

Near term through Tuesday...High pressure centered over eastern
Canada continues to lead to dry conditions with nighttime fog.
Pattern beginning to break down though. Forcing with negative
tilted trough lifting north across MN interacting with instability
gradient over the Upper Mississippi river valley has resulted in
clusters of showers and storms over western WI, that are moving
south to north. Have added small chance pops and sprinkles through
Tuesday over our far western zones that will be in the proximity
of right entrance region of mid-level jet and closer to southerly
weak low-level jet and gradient of instability. Farther east it
should stay dry through Tuesday. Fog again tonight as that part of
the pattern persists. Once again greatest signal for lower vsby
is showing up over far northeast WI to northern edge of the Fox
Valley and on east to the lakeshore. Fog coverage not expected to
justify an advisory, but will keep the mention in HWO. Late season
warmth persists. Based on 850mb temps starting day 15-16c, expect
highs on Tuesday to reach the mid 80s over much of central and
northeast WI. Southeast winds will keep the lakeshore as cool as
the mid to upper 70s. Yet, even these cooler values are above
climo for mid September. Normal high for 9/16 at GRB is down to 72
and has now slipped to 70 at RHI.

Precipitation trends beyond Tuesday...Large ridge over the Great
Lakes shifts east while dampening, then tries to re-emerge briefly
Wednesday into Thursday as troughing occurs over the plains. At
the same time, cold front will drop across the Upper Great Lakes
late Wednesday and continue crossing the Great Lakes on Thursday.
As the front arrives from the north on Wednesday, larger area of
scattered showers and storms will impact especially central and
north-central, far northeast WI during peak heating on Wednesday.
Mean MLCAPEs are forecast above 1500j/kg, but as it has looked
like for a while now, wind fields aloft and resulting effective
shear is weak, thus organized severe weather risk on Wednesday is
on lower side. However, isolated strong to severe storm not out
of question given the instability and the front arriving during
peak heating, especially if clouds are at a minimum before the
front arrives. NSSL machine learning guidance hints at at least a
small potential for severe storms.

Shower chances decrease some with loss of instability on Wednesday
night especially as main low-level forcing shifts more east. On
Thursday, chances for showers and storms increase again during the
afternoon as instability builds during the afternoon. MLCAPEs not
as elevated as Wednesday afternoon. Joint probability of shear
over 20 kts and CAPE over 500j/kg have increased over last couple
days, but are mainly focusing over IA into southern MN closer to
the low-level jet axis near the plains sfc low.

Moving into Friday and the weekend, there remains uncertainty on
Friday as it is not certain how far east the plains sfc low will
progress. There likely will be a gradient from southwest to
northeast across our area in where greatest coverage of showers
will be. As the plains trough crosses the western Great Lakes for
the weekend, chances for scattered showers and at least isolated
storms (MLCAPEs less than 500j/kg) increase. Chances for showers
continue on Monday as another warm front lifts toward the region.
Overall, compared to the regime we have been in recently, expect a
more unsettled and active weather pattern beginning later this
week.

Temperature and wind trends beyond Tuesday...Wednesday will feature
well above normal temps again, with greatest potential for highs
in the mid 80s far northeast WI to the Fox Valley and the lakeshore.
Temps trend downward on Thursday as the front crosses, then bottom
out in the 60s north and near the lakeshore, to the lower 70s
elsewhere, behind the front Friday into the weekend. Could see
temps ending up lower on Friday than forecast given what will be
gusty easterly winds off of Lake Michigan, especially if there are
more clouds and showers around. These gusty winds will linger into
Saturday, before becoming lighter from the south Sunday into Monday.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1049 PM CDT Mon Sep 15 2025

VFR conditions will continue late this evening with a few mid and
high clouds spreading across central and north central WI. A few
showers/sprinkles and a stray storm will continue to remain west
of CWA/AUW/RHI overnight. Some fog is expected again overnight
into early Tuesday, focused mainly across far northeast WI to the
lake shore. GRB and MTW look to be the sites impacted, so will
continue to have the IFR or lower conditions. A little ground fog
can`t be rule out at the other TAF sites overnight. Any ground fog
will quickly burn off after sunrise on Tuesday, with any bigger
areas of fog burning off by 14-15z. VFR conditions are expected on
Tuesday after the fog burns off. A shower or storm could approach
CWA/AUW/RHI Tuesday or Tuesday evening, but chance under 20% so
will not include. Patchy fog is possible again Tuesday night,
mainly after 06z.

Winds will be light (under 5 kts) tonight, mainly out of the
east- southeast. Winds under 10 kts are expected most of Tuesday,
mainly out of the south-southwest, with a few gusts to 15 kts
possible in central and north central WI in the afternoon. A lake
breeze should turn the winds to the southeast near Lake Michigan
in the late morning/afternoon. Light south-southwest winds are
expected Tuesday evening/night.

&&

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

&&

$$

DISCUSSION.....JLA
AVIATION.......Bersch

NWS GRB Office Area Forecast Discussion