NWS-Green Bay Forecast Discussion

						FXUS63 KGRB 201738
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1238 PM CDT Mon Apr 20 2026

Updated aviation portion for 18Z TAF issuance

.KEY MESSAGES...

-Minor to moderate flooding continues on several rivers, with
 major flooding expected on the Wolf River through Tuesday. Water
 levels have crested in most areas, and due to an extended period
 of dry weather, a gradually drop is anticipated through the week.

-Showers and thunderstorms are expected late Thursday afternoon
 into Thursday night. Heavy rainfall may accompany any
 thunderstorms. There is a 30-50% chance of rainfall in excess of
 1/2 inch, but localized amounts to 1-2 inches could occur.

-Temperatures will be be below normal today, with much above
 normal temperatures in the 60s and 70s expected away from Lake
 Michigan Tuesday through Friday.

&&

.DISCUSSION...
Issued at 219 AM CDT Mon Apr 20 2026

Dry and cool conditions continue today as upper ridging builds over
the region. A short-wave, currently over northern Manitoba will work
east across Canada today dragging a cold front across the western
Great Lakes this evening. This will lead an increase in cloud cover
this afternoon, however, with such a dry air mass in place the
chance for even a sprinkle is less than 10%. Forecast soundings this
afternoon do show a deeply mixed PBL which will lead to low relative
humidity values of 20-30 percent, lowest across northern WI. Deep
mixing is expected again on Tuesday and Wednesday with RHs similarly
bottoming out around 20-30 percent. Whiles fuels are rather wet from
recent rain these lower RHs will help dry them out and may lead to
an uptick in wildfire activity through mid-week.

Main focus of the extended forecast is a round of showers and
thunderstorms expected Thursday evening into Friday. Increasing
southerly winds Wednesday night and Thursday will draw Gulf moisture
into the area with PWATs reaching 1.2 to 1.5 inches, which is near
the climatological max. Widespread ascent will overspread the region
as a trough ejects out of the norther Plains Thursday reaching the
western Great Lakes Thursday night. This will lead to scattered to
widespread showers with embedded thunderstorms. Ensemble mean MUCAPE
values increase to 500-600 J/kg. This could support isolated
stronger storms, however, deep layer shear is expected to be
maximized west of forecast area which may keep the severe weather
threat generally over MN and western WI. Precipitation amounts will
likely vary widespread due to localized convection with this system.
Current NBM probs show 30-50% chance for greater than 0.5" across
most of the forecast area with high end amounts (95th percentile)
around 1.2-2" which would is reasonable to expect in areas that see
the strongest storms.

Zonal flow becomes more favorable behind the cold front Friday into
next weekend which may lead to a period of cooler temperatures and on
and off showers.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1233 PM CDT Mon Apr 20 2026

The main aviation concern is LLWS, which will develop this
evening and continue until late tonight. This will occur as SW
winds aloft increase to 40 to 45 kts.

VFR conditions will prevail through the TAF period. SCT high-
based cumulus clouds and increasing high level clouds are expected
this afternoon and early evneing, along with gusty S-SW winds. A
weak surface trough will move through the area on Tuesday, but
probably won't reach the eastern TAF sites until after this TAF
period. The main impact will be a wind shift to the northwest.
Lake breeze development during the daytime periods will result in
gusty SE winds at times near Lake Michigan.

&&

.HYDROLOGY...
Issued at 219 AM CDT Mon Apr 20 2026

Minor to moderate flooding continues on many rivers, with major
flooding expected to persist on parts of the Wolf River through
Tuesday. Most of the river levels have crested, and with a
prolonged stretch of dry weather anticipated through Thursday,
water levels should gradually subside through the week.

Those living near rivers should continue to monitor the latest
Flood Warnings and statements from the National Weather Service
in Green Bay.

See the discussion section above for information pertaining to
precipitation forecasts for the Thursday to Friday period.

&&

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

&&

$$

DISCUSSION.....GK
AVIATION.......Kieckbusch
HYDROLOGY......GK