NWS-Green Bay Forecast Discussion

						FXUS63 KGRB 091937
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
237 PM CDT Tue Jun 9 2026

Forecast discussion for routine afternoon forecast issuance

.KEY MESSAGES...

- Dense fog over Lake Michigan is expected to spread inland
  overnight. Visibilities below one half mile will create
  hazardous travel conditions across the lakeshore and far
  northeast Wisconsin.

- Multiple rounds of thunderstorms are expected Wednesday. There
  is an enhanced risk for severe thunderstorms late Wednesday
  afternoon and evening capable of producing damaging winds, large
  hail, and an isolated tornado.

- Very warm and humid conditions will develop Wednesday afternoon
  between rounds of storms. Heat indices are forecast to reach in
  the middle to upper 90s across parts of central and east-
  central Wisconsin.

- Additional strong to severe thunderstorms are expected Thursday
  afternoon into Thursday evening, with the potential for locally
  heavy rainfall.

&&

.DISCUSSION...
Issued at 237 PM CDT Tue Jun 9 2026

The latest RAP analysis and satellite/radar imagery show broad
low pressure sitting over the western Great Lakes early this
afternoon. Widespread cyclonic flow has maintained a blanket of
clouds across the region. However, as this cyclonic flow exits
over the lower peninsula of Michigan later today, partial clearing
is expected to arrive from west to east through the afternoon.
Eastern Wisconsin will have the most difficult time clearing out,
but turning wind trajectories to the north for a brief time by
late afternoon should help erode the cloud deck. Looking across
other areas of the country that will influence the region over the
next 36 to 48 hours, a potent upper level trough is pushing
across the northern Rockies into the northern Plains, which will
draw an seasonably hot and moist airmass toward Wisconsin late
tonight into Wednesday thereby setting the stage for severe
weather.

Weak high pressure will move across the area this evening,
creating a brief period of quiet weather before a warm front
approaches from the southwest late tonight. The combination of
light winds and a very moist boundary layer will become highly
favorable for marine fog to push inland from Lake Michigan late
this evening and spread through the overnight hours across eastern
Wisconsin, and possibly into north-central Wisconsin. Widespread
dense fog is expected to develop near the lakeshore, and a dense
fog advisory will likely be needed to account for significantly
reduced visibilities for motorists.

As the warm front approaches late tonight into Wednesday morning,
an intensifying 30 kt low-level jet poking into southwest
Wisconsin will arrive with a surge of elevated moisture and
instability. This will drive a first round of elevated showers and
thunderstorms into central WI late tonight or early Wednesday
morning. Given effective shear of 25 to 30 kts, elevated
instability of 1500 to 2000 J/kg, steep mid-level lapse rates of
7.5 to 8.0 C/km, and a strong low- level inversion, large hail
looks to be the main threat with these morning storms. Exact storm
coverage is uncertain, and the evolution of this morning
convection will heavily dictate how the rest of the day unfolds.

If the morning convection clears quickly and allows the warm
front to lift into the Upper Peninsula by early afternoon,
increasing winds aloft ahead of the potent upstream trough will
allow the atmosphere to destabilize much quicker than normal.
Between the morning and late afternoon rounds of convection, it
will become very warm and humid with heat indices soaring into the
90 to 95 degree range across parts of central and east-central
Wisconsin. While the increasing chances for storms have tempered
the potential for dangerous heat somewhat, the early-season nature
of this heat warrants continued messaging. A capping inversion
around 800 mb should support a temporary lull in thunderstorm
activity through mid-afternoon, but this cap remains on track to
erode by late afternoon.

A significant severe weather outbreak appears possible late
Wednesday afternoon through Wednesday evening as the main synoptic
cold front and upper trough approach from the west. Multiple
rounds of severe storms are indicated through the evening hours,
supported by boundary layer instability of 2000 to 3000 J/kg and
deep-layer shear of 30 to 40 kts. The strongest storms will be
capable of large hail and damaging winds through early evening,
along with a risk for isolated QLCS-type tornadoes if bowing
segments can become organized within the strengthening low-level
wind fields. The primary hazard will transition to widespread
damaging winds through the evening as storms consolidate into a
fast-moving linear squall line. Frontal passage will bring an end
to the storm chances and severe threat from west to east by late
evening.

The atmosphere will need time to stabilize again on Thursday. A
second and more potent shortwave is forecast to track toward the
region on Thursday afternoon, causing an associated surface low to
lift northeast directly into southeast or east-central Wisconsin
by Thursday evening. Widespread showers and thunderstorms will
accompany this system, fueled by a deep plume of moisture with
precipitable water values climbing well above normal. A severe
threat may develop late in the afternoon over east- central
Wisconsin where instability ticks up to 800 to 1200 J/kg under
strong deep layer shear of 50 to 60 kts.  Locally heavy rainfall
and minor urban or low-lying flooding will also be possible.
Convection will wind down by mid-evening following the passage of
a strong cold front, leaving behind a minor 20 to 30 percent
chance of final storm totals exceeding 1 inch over northeast
Wisconsin.

A much cooler and drier weather pattern will return to the
western Great Lakes Friday into early next week as a broad
longwave trough sets up from the Hudson Bay down into the region.
On the southern periphery of this cold pool aloft, spotty
diurnally driven showers and storms look possible on Friday and
Saturday afternoons. Saturday stands out as having the slightly
better potential for scattered afternoon activity due to the
passage of a secondary cold front and minor daytime instability.
By Sunday and early next week, dry air and persistent high
pressure will take hold, dropping temperatures slightly below
seasonal normals.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 112 PM CDT Tue Jun 9 2026

Broad low pressure over the western Great Lakes will slide east
this afternoon, allowing wind trajectories to turn briefly
offshore. Widespread MVFR to LIFR ceilings early this afternoon
will be slow to lift, but partial clearing from west to east
should bring a return to temporary VFR conditions at most
terminals later this afternoon. Flight categories will remain
lowest near KMTW where MVFR decks may persist.

Winds become light and variable late this evening and overnight
as weak high pressure shifts east and a warm front approaches from
the southwest. With a very moist boundary layer remaining in
place, low stratus and fog will quickly redevelop. Expect LIFR
ceilings and visibilities to spread westward across eastern and
north-central Wisconsin after 04z to 06z. Locally dense fog is
likely near Lake Michigan, severely impacting KMTW.

As the warm front nears, the chance for thunderstorms will
increase late tonight into Wednesday morning. Elevated instability
will support a risk of small hail and gusty winds within the
strongest storms. Confidence in exact timing and spatial coverage
remains low to medium, prompting the use of tempo groups for KAUW,
KCWA, and KRHI after 09z, and a prob30 group for KGRB, KATW, KMTW
after 12z. Expect conditions to improve to MVFR or VFR by late
Wednesday morning as rain chances temporarily diminish and diurnal
heating helps lift cloud bases.

A severe weather threat is expected to develop late Wednesday
afternoon and evening as a potent trough and cold front cross the
region, potentially bringing damaging winds and large hail.

&&

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

&&

$$

DISCUSSION.....MPC
AVIATION.......MPC