NWS-Green Bay Forecast Discussion

						FXUS63 KGRB 170750
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
250 AM CDT Fri Apr 17 2026

Forecast discussion for routine morning forecast issuance

.KEY MESSAGES...

-Severe storms with a risk of tornadoes (possibly strong), large
 hail and damaging winds are increasingly likely this afternoon
 and evening (3 to 10PM), with the highest risk across central
 Wisconsin.

-Thunderstorms today will produce locally heavy rainfall. If this
 occurs over the flood sensitive areas of central and east-
 central WI additional flooding may occur.

-Minor to major flooding will continue into next week with the
 greatest impacts expected on the Menominee and Wolf Rivers where
 major or record breaking flooding has been observed.

-No significant precipitation is expected from Sunday through at
 least the middle of next week.

&&

.DISCUSSION...
Issued at 249 AM CDT Fri Apr 17 2026

Fog this morning...

A stable boundary layer has allowed widespread dense fog to develop
across eastern WI early this morning. Once the sun comes up fog will
start to clear out from west to east. Most areas should be clear by
about 10AM this morning, however, fog along the Lake Michigan
shoreline may linger into this afternoon.

Severe Thunderstorm and Flood Potential This Afternoon and Evening...

A strong upper trough, currently digging into the northern Plains,
will spur on surface cyclogenesis across the upper Midwest this
morning. This will lead to increasing southerly surface winds and
northward moisture transport. Currently a plumb of 60+ dew points
extends as far north an northwest MO. These anomalously high dew
points are forecast to nose into western and central WI early this
afternoon. At the same time an EML, currently visible on mid-level
WV imagery over western KS, will spread over the region resulting in
mid-level lapse rates steepening to 7.5-8 C/km. The result will be a
significant increase in instability with MLCAPE values of 2000-2500
J/km across central WI. CAMs generally show the nose of these higher
CAPE values extending as far north as Wood, Portage and Marathon
counties which will be the region to monitor or convective
initiation this afternoon. Shear profiles this afternoon are also
impressive with strongly curved hodographs showing 25-30kts of shear
and 200-300 m2/s2 SRH in the 0-1km layer. This all points to a
tornado (possibly EF2+) and large hail (2+") threat with any
discrete super cells that develop this afternoon. One question that
remains is the evolution of storm mode. CAMs show few hour window
from about 3-6PM where discrete to semi discrete super cells will be
possible before stronger forcing arriving in the form of a cold
front likely leading to rapid upscale growth this evening leading to
an increase in the damaging wind threat as storms move east. As
storms move further into northeast and east-central WI they will out
pace the better instability likely leading to a downward trend in
severity, however, damaging winds and hail will possible all the way
to the lakeshore. CAMs generally show storms exiting the region by 8-
10PM this evening.

Along with the severe threat localized heavy rainfall will create
flooding and flash flooding concerns. HREF mean precipitation totals
are about 0.5-0.75" across the entire forecast area. However,
localized areas that see multiple storms may see rainfall amounts
closer to 95th percentile of 1.5-2". Flash flooding concerns are
maximized along the urban I-41 corridor from Oshkosh to Green Bay
where 1hr flash flood guidance is about 1". Additionally areal and
river flooding concerns are maximized along near the Wolf River where
major river flooding is ongoing and MRMS soil saturation is
maximized. Overall, precipitation amounts have trended down slightly
over recent models cycles due to the more progressive nature of the
cold front which does lower to flood risk outside of those more
sensitive areas already mentioned.

Cooler temperatures this weekend...

The cold front will be through the region by Saturday morning. In
it's wake dry and seasonally cool temperatures are expected through
the weekend. Breezy northwest winds will keep highs in the middle
30s to middle 40s at most locations. Overnight lows are forecast to
drop into the 20s Saturday and Sunday nights. If you have any cold
sensitive plants outdoors make sure to bring them inside or cover
them.

Dry and warmer next week...

Ensemble upper level flow patters points towards ridging dominated
the upper Great Lakes for much of next week. A few ensemble members
(about 10%) bring some light rain to region with a frontal passage
Monday night, but otherwise a much needed dry stretch is expected
through the middle of next week. The next chance for widespread
precipitation comes towards the end of next week as an upper trough
lifts a surface low across the Northern Plains. Synoptically it does
look like much of Wisconsin will get into the warm sector with this
system meaning thunderstorms may possible, however, it't to early to
evaluate the severe weather potential during this period.
Temperatures are expected to gradually trend warmer through next
week.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1152 PM CDT Thu Apr 16 2026

A fog bank with LIFR/VLIFR conditions continues to creep west late
this evening making its way to KGRB as of TAF issuance. The dense
fog is expected to make it as far as the Fox Valley, including
KATW, but generally stay east of the central Wisconsin TAF sites.
The fog should then mix out between 12-16Z from west to east
Friday morning; however, the models do indicate the fog may linger
near the lakeshore for much of the day including KMTW.

Southeasterly surface winds tonight and become gusty from the
south during the day on Friday. Gusts may exceed 25 to 30+ kts
during the afternoon on Friday. Before surface winds increase, a
period of LLWS may occur early Friday.

Thunderstorm chances will increase on Friday afternoon and Friday
evening as a line of storms tracks in from the west. The best
timing for thunderstorms has been placed in a TEMPO group at all
TAF sites indicating when the line is expected to track through
the region.

&&

.HYDROLOGY...
Issued at 249 AM CDT Fri Apr 17 2026

Flooding continues along multiple rivers due to the combination of
snow melt and recent heavy rainfall, including major flooding along
the Wolf and Menominee Rivers. If you live near a river or stream,
continue to monitor the latest hydro forecasts and Flood Warnings
from the National Weather Service in Green Bay.

Additional thunderstorms will occur this afternoon into this
evening. Amounts will be variable with localized high end amounts
exceeding 2 to 3" may occur where repeated storms occur. This may
lead to additional rises for impacted river basins, in addition to
exacerbating flood concerns in areas that received higher rain
amounts already this week and in urban areas.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Flood Watch from 1 PM CDT this afternoon through Saturday
morning for WIZ011>013-019>022-030-031-035>040-045-048>050-073-
074.

Dense Fog Advisory until 9 AM CDT this morning for WIZ039-040-
049-050.
&&

$$

DISCUSSION.....GK
AVIATION.......Kurimski
HYDROLOGY......GK