072
FXUS63 KGRB 170301
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1001 PM CDT Wed Sep 16 2026

Updated aviation portion for 06Z TAF issuance

.KEY MESSAGES...

- Light rain chances (20 to 50%) expected tonight into Thursday
  morning, though rainfall totals will remain light at under one
  tenth of an inch.

- Confidence is increasing for a significant heavy rainfall event
  late Friday into Saturday, focused across central and east-
  central Wisconsin.

- Another round of showers and potential thunderstorms towards
  early next week.

&&

.DISCUSSION...
Issued at 141 PM CDT Wed Sep 16 2026

Light rain chances (20 to 50%) return tonight into Thursday
morning: Cloud cover will increase this evening as weak
overrunning moisture moves in ahead of a shortwave moving east
across southern Canada. Rain chances tonight generally range from
20 to 50 percent, but precipitation will be light with 90th
percentile amounts only up to 0.10 inches. Spotty light showers or
drizzle will linger into Thursday morning before clearing
develops Thursday afternoon. Seasonable high temperatures ranging
from the upper 60s to middle 70s are expected today and Thursday.

Heavy rainfall potential Friday through Saturday: Heavy rainfall
late Friday into Saturday represents the most significant weather
hazard during the 7-day forecast period, with increasing
potential for localized flooding across central and east-central
Wisconsin. Shortwave energy advancing across the Rockies will
trigger a 30 to 40 knot low-level jet directed into central
Wisconsin Friday afternoon into Friday night. Strong dynamic
forcing will overspread the region, characterized by upper-level
divergence in the right rear quadrant of an upper jet streak,
robust mid-level frontogenesis, and strong moisture convergence
above the returning warm front. Precipitable water values are
forecast to climb to around 1.70 inches, which sits near the 96th
percentile for mid- September. Warm cloud depths in excess of
12,000 feet will favor efficient rainfall processes.

Models continue to struggle with where the heaviest axis of heavy
rain will occur. There are some signs that an initial band across
the far north on Friday afternoon associated with 700mb
frontogenesis would subsequently push the second band on Friday
night further south away from our area. However, there are still
signs the main band of heavy rain would drive through the heart of
our area. This makes predicting rainfall amounts an issue as the
NBM still seems too high with a mean value around 3 inches. The
previous forecast amounts of 1 to 3 inches along with the
probabilities still seem reasonable with the caveat that these
features need to be resolved before higher confidence can be
assigned to where the swath of heavy rain will end up. If the same
locations that received heavy rainfall earlier in the week get
the swath of heavy rainfall, antecedent soil moisture remains
high, elevating the threat of localized urban and small stream
flooding. Severe weather risk remains low due to meager
instability, with most unstable capes generally under 500 J/kg.

Additional rain chances early next week: Following the departure
of the aforementioned system, a brief break in precipitation is
expected Sunday as weak surface ridging passes over the Great
Lakes. However, the progressive mid-level flow will keep the
region in an unsettled regime heading into early next week.
Another upper shortwave trough and associated surface system will
approach from the Great Plains toward Sunday night and Monday,
bringing additional chances for rain showers and scattered
thunderstorms through Tuesday.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 943 PM CDT Wed Sep 16 2026

No significant changes to the forecast late this evening. A band
of showers is expected to move across the area late this evening
into the overnight hours. An isolated thunderstorm is possible
across central and east-central WI. Patchy fog is also possible
towards sunrise across central and north-central WI. Any lingering
fog should burn off by 14z with VFR conditions expected Thursday
afternoon. The most difficult portion of the forecast Thursday
afternoon will be wind direction as there some significant
difference in the numerical guidance, although winds are on
the lighter side.
 .
&&

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

&&

$$

DISCUSSION.....Kurimski
AVIATION.......Eckberg

NWS GRB Office Area Forecast Discussion