NWS-Green Bay Forecast Discussion

						FXUS63 KGRB 122318
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
618 PM CDT Fri Jun 12 2026

Updated aviation portion for 00Z TAF issuance

.KEY MESSAGES...

- Gusty west winds up to 25 kts will cause hazardous waves for
  small craft on the Bay and Lake Michigan through early evening.

- Embedded thunderstorms are expected to develop tonight across
  north-central and northeast Wisconsin. Severe weather is not
  expected but a few storms could contain small hail.

- Additional thunderstorms are forecast Saturday afternoon, with
  isolated strong or severe storms possible over east-central Wisconsin.

- A cooling trend takes hold Sunday through much of next week,
  accompanied by widespread rain chances on Tuesday and Wednesday.

&&

.DISCUSSION...
Issued at 129 PM CDT Fri Jun 12 2026

The latest RAP analysis and satellite/radar imagery show
Wisconsin positioned between two cold fronts, with one located
over the mid-Mississippi Valley and another dropping south over
central Canada. Between these two boundaries, weak cold advection
is occurring beneath flattened troughing aloft over the western
Great Lakes. A tight pressure gradient combined with steep low-
level lapse rates have supported a breezy afternoon across
northern Wisconsin, with west wind gusts reaching 25 to 35 mph.

Tonight and Saturday Thunderstorm Potential: The main focus for
remains on the thunderstorm potential as the cold front over
central Canada moves into the western Great Lakes tonight and
Saturday. As the front dives across northern Minnesota and Lake
Superior, backing winds over the northern Mississippi Valley will
support the generation of mid-level frontogenesis from central
Minnesota into northern Wisconsin tonight. This frontogenetical
band will interact with modest elevated instability of 300 to 500
J/kg and deep effective shear of 40 to 50 kts to support embedded
thunderstorms within the larger precipitation shield. Given the
strong shear aloft, an elevated strong storm cannot be entirely
ruled out overnight, even though convective allowing models show
little support for this in their updraft helicity fields. Hail
would be the primary threat with any robust elevated cell that can
develop.

Most of this initial overnight shower activity should diminish
from early to mid Saturday morning. After a brief lull in
precipitation from late Saturday morning through early afternoon,
the chance for thunderstorms is expected to increase once again
directly along the advancing surface cold front. There is some
meteorological concern that lingering morning cloud cover could
negatively impact the diurnal destabilization process along the
front, where low-level convergence is already forecast to be on
the weaker side. However, provided the area receives sufficient
surface insolation, surface-based instability is expected to build
to 1000 to 1200 J/kg. When combined with deep layer shear of 40
to 50 kts and steep mid-level lapse rates, this environment could
support a few strong to severe storms to develop from about 1 PM
to 5 PM. Convective allowing models focus the strongest convective
development from roughly Waushara to Manitowoc and points
southward, indicating that the true temporal and spatial window
for severe weather could be significantly shorter and more
isolated than broader national convective outlooks indicate.
Saturday will also be another breezy day with west winds gusting
25 to 30 mph, and surface highs ranging from the middle 70s to
lower 80s.

Extended Outlook: Behind the Saturday's cold front, a strong
cooling trend takes hold through much of next week. This is driven
by an unseasonably amplified synoptic pattern featuring an
intense West Coast ridge and the consolidation of a broad longwave
trough with heights well below average centered directly over the
upper Great Lakes. High temperatures will fall back into the mid
60s to mid 70s through late next week, with the absolute coolest
days expected to be Sunday, Monday, and Wednesday. Periodic
shortwave energies digging into the mean trough will keep the
pattern active. Widespread chances for showers and some storms
will return Tuesday into Tuesday night as a northern stream
shortwave digs into the region. Yet another quick-moving system
will bring a reinforcement of showers and storms on Wednesday,
with small chances for light rain lingering into the end of the
week under the cold pool aloft before the pattern begins to
flatten and turn more zonal toward next weekend.

&&

.AVIATION...for 00Z TAF Issuance
Issued at 617 PM CDT Fri Jun 12 2026

VFR conditions will continue this evening as fair weather cu
continues to scatter out across the region. Satellite imagery from
eastern SD and western MN show a developing cluster of thunderstorms
in response to confluent mid-level flow and a surface cold front. As
mid-level winds back around to the southwest this evening and the
front continues to push southeast expect these storms to move east
across MN eventually reaching central WI around or after 05Z. Along
with periods of MVFR visibility cigs will lower to around 5kft as
these showers and storms pass over central and east-central WI.
Precipitation should be exiting east-central WI by 12Z Sat. In the
wake of the departing precip an area of MVFR status may develop
across central WI early Sat morning, however, cigs should return to
VFR a few hours after sunrise.

A second cold front is progged to drop across the region Saturday
afternoon. This may bring another round of showers and storms to
east-central WI.

Marginal LLWS will develop across the region tonight at all TAF
sites for a brief time. Gusty winds to 20 kts are expected to
redevelop by 14-16Z Sat.

&&

.MARINE...
Issued at 129 PM CDT Fri Jun 12 2026

The current gusty west winds will continue to support the ongoing
Small Craft Advisory through 7 PM this evening across the Bay of
Green Bay and nearshore waters of Lake Michigan. West winds will
be breezy again on Saturday but gusts will be slightly less and
more marginal for a small craft advisory. Boaters on the Bay
should use caution with gusts to 20 kts expected.

&&

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

&&

$$

DISCUSSION.....MPC
AVIATION.......GK
MARINE.........MPC