NWS-Green Bay Forecast Discussion

						FXUS63 KGRB 121843
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
143 PM CDT Sun Apr 12 2026

Updated aviation portion for 18Z TAF issuance

.KEY MESSAGES...

- Periods of heavy rainfall and thunderstorms expected through the
  middle of the week. Expecting widespread 1.5 to 2.5 inches of
  rain, highest along and south of Hwy 29, where local higher
  amounts in excess of 3 inches could occur.

- A round or two of strong to severe thunderstorms will be
  possible during the early to mid part of next week. The greatest
  (Slight) risk of severe storms occurs south of a line from
  Tomahawk to Oconto to Algoma Monday evening. Hail up to golf
  ball size and damaging winds are the main threats. Additional
  lower probability severe events are possible across central and
  east central WI Tuesday afternoon and evening, and over eastern
  WI Wednesday afternoon.

- Much above normal temperatures expected, with highs in the 60s
  and 70s from today through Friday.

- Minor flooding will continue along the Wolf River through the
  upcoming week. Combined runoff from melting snow in the Upper
  Peninsula and heavy rain will likely result in flooding on the
  Menominee River by Tuesday or Wednesday, with moderate flooding
  in some locations toward the end of the work week and weekend.

- More rain and storms possible Friday into Saturday.

&&

.DISCUSSION...
Issued at 325 AM CDT Sun Apr 12 2026

Showers and embedded thunderstorms were ongoing across the region,
as a H8 warm front and 50-55 knot low-level jet interacted with a
marginally unstable (MUCAPE 400-800 j/kg) and moist (PWATs 1-1.25
inch) air mass. Heavy rainfall was occurring with this activity,
with a few spots in Marathon County already reporting over 1
inch. Isolated stronger cells were likely also producing small
hail. This batch of showers and storms will shift northeast of the
region by around daybreak, but additional storms developing near
the surface warm front along the MN/IA border should expand and
move through the forecast area this morning.

Models show a general decrease in precipitation after the warm
front lifts north this afternoon. The lull continues tonight into
Monday, as H8 flow turns westerly (parallel to the baroclinic
zone), PWATs drop to less than 1 inch and instability wanes.

The front sags south and stalls across central or southern WI
by Monday evening, and provides the focus for another thunderstorm
complex as a southerly low-level jet redevelops, strengthens and
ascends the frontal zone. Instability peaks across the forecast
area Monday evening, with mid-level lapse rates of 7.5-8.5 C/km
and H8 LI's of -4 to -8 C expected. SPC has placed roughly the
southwest half of the forecast area in Slight Risk, with large
hail (up to golf ball size) and damaging winds the primary
threats. If the surface front sets up over the far southwest part
of the CWA, there could be a low-end threat for a tornado, but the
evening timing of the event is not particularly favorable. Heavy
rainfall will also be a concern, especially south of Hwy 29.

After another lull in the precipitation late Monday night into
Tuesday morning, another period of showers and storms is expected
over mainly central and east central WI later tuesday afternoon
and evenings. A lower-end severe threat is possible during this
period, and SPC has placed parts of central and east central WI in
a Marginal risk of severe. Another low confidence severe event
could occur on Wednesday afternoon, but only if the surface low
tracks far enough west for us to get in the warm sector.

See the Hydrology section for information on heavy rainfall and
flooding potential.

Milder temperatures are expected through the period, with highs
reaching into the 60s and 70s from today through Friday.

Medium-range models show another frontal system approaching late
in the week (Friday-Saturday), with potential for additional
rainfall and thunderstorms.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 142 PM CDT Sun Apr 12 2026

Rain and thunderstorms will continue for the next 1 to 2 hours,
mainly in east-central and far northeast WI as this batch of
rain/storms moves off to the east. Precipitation will be minimal
the rest of the afternoon and overnight, but a few stray showers
could brush the far southeast or far north-central portions of
the area.

Meanwhile, flying conditions will remain quite variable with a mix
of VFR-LIFR flight conditions area-wide. Although, it was primarily
LIFR as of 18Z due to very low clouds at 200-300 ft and fog in
the wake of the morning rain.

More fog development is anticipated tonight, which will keep
flight conditions in a variable state, mainly MVFR/IFR, but some
dips to LIFR vsbys could occur. Guidance also shows LIFR cigs
returning to north-central WI late tonight.

During the mid-late morning on Monday, conditions will begin to
improve as fog dissipates and clouds lift and scattered out. Quiet
weather is likely for the afternoon followed by thunderstorms,
some likely strong to severe, Monday evening and overnight.

Winds will be S/SW through the evening, then gradually veer W
overnight. Gust of 15 to 30 knots are expected at times.
Additionally, LLWS at 2kft will continue until late tonight.

&&

.HYDROLOGY...
Issued at 325 AM CDT Sun Apr 12 2026

Minor flooding is expected to continue on the Wolf River through
the next week. Even though total rainfall amounts of 1.5 to 2.5
inches, highest along and south of Hwy 29 (with local 3+ inch
amounts), are expected through Wednesday, there will be
significant lulls between each period of heavy rain. In addition,
rainfall amounts over Upper Michigan will be a bit lower (1-1.5
inches) over the regions where a rapid snowmelt is expected to
occur. The runoff from Upper Michigan is expected to bring
minor flooding on the Menominee River by Tuesday or Wednesday,
with possible moderate flooding in spots by the end of the work
week and weekend.

Have opted to hold off on any Flood Watches for the Menominee
River and adjacent tributaries, as HEFS graphics showed a slower
trend with the flooding and lower probabilities of moderate to
major flooding than yesterday.

&&

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

&&

$$

DISCUSSION.....Kieckbusch
AVIATION.......KLJ
HYDROLOGY......Kieckbusch