635
FXUS63 KGRB 161841
AFDGRB

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

Forecast discussion for routine afternoon forecast 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 18Z TAF Issuance
Issued at 1209 PM CDT Wed Sep 16 2026

FEW-SCT diurnally driven VFR CU will continue this afternoon then
dissipate around sunset. Light winds below 10 knots are also
expected. Clouds will increase and lower tonight as a shortwave
approaches. Light rain chances increase after 04Z Thu over central
and north-central WI (KAUW, KCWA, KRHI) and after 07Z Thu over
eastern WI (KGRB, KATW, KMTW). Handled with PROB30 groups given
the low probability for rain to hit the individual sites. CIGS
will deteriorate to MVFR across central and east-central WI
overnight, becoming VFR late tonight into Thursday morning.
Mostly clear skies and light winds will then prevail across the
region on Thursday.

&&

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

&&

$$

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

NWS GRB Office Area Forecast Discussion