311
FXUS63 KGRB 200309
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1009 PM CDT Fri Jun 19 2026

Updated aviation portion for 06Z TAF issuance

.UPDATE...
Issued at 706 PM CDT Fri Jun 19 2026

A couple of splitting supercells are tracking towards Marathon and
Wood County early this evening. The cell over northern Clark has
been gradually weakening over the past 15-20 minutes while the
storm entering northwest Jackson County has taken over as the most
potent cell. These storms will be heading into Marathon and Wood
counties over the next 45-75 minutes.

The mesoanalysis shows strong deep layer shear (around 50 kts 0-6
km shear) but marginal instability (500-700 j/kg mixed layer
cape). Given the less favorable thermodynamic environment over
central WI than further west, it`s possible that the slow
weakening trend will continue as storms enter Marathon and Wood
counties. The strong to near severe storm threat will continue
over central WI for the next 1-3 hours. CAMS indicate that storms
will likely weaken prior to reaching east-central WI.

&&

.KEY MESSAGES...

- Scattered showers and thunderstorms will bring a 50-75% chance
  of rain this evening and tonight. Any storms will be capable of
  producing hail. Strong, gusty winds are also possible.

- Scattered showers are expected Saturday afternoon, with a few
  storms possible across far northeast Wisconsin, including Door
  County.

- Near to below normal temperatures and comfortable humidity will
  prevail through next week.

&&

.DISCUSSION...
Issued at 215 PM CDT Fri Jun 19 2026

Precipitation Chances...This afternoon`s water vapor and radar
imagery showed a shortwave moving eastward over eastern ND and
northern MN, producing scattered showers and a few storms. As the
shortwave continues to track east, the forecast area will see these
scattered showers and isolated storms move across the area early
this evening through late tonight. When the precip reaches central
WI late this afternoon/early this evening, a corridor of SBCAPE of
500-900 J/kg, steep mid-level lapse rates of 6.5-7.5 C/km, and deep-
layer shear of 35-45 kts, will be present, leading to a marginal
risk for a few strong to severe storms. Given the present cooler
airmass, the freezing level will be lower, around 7000 ft, leading
to a hail threat. In addition, a few models indicate inverted-v
soundings, suggesting a marginal wind threat as well. As the storms
move eastward, the storms will become elevated, but the severe
threat will decrease as deep-layer shear and lapse rates lessen. Any
showers and storms will exit east after midnight.

Northwest/cyclonic flow will be in place following the departure of
the shortwave on Saturday. With low-level moisture in place and then
diurnally increasing during the afternoon, CAMs are indicating
isolated to scattered showers developing during the afternoon, and
dissipating in the evening as the sun sets. Cannot rule out a few
rumbles of thunder, mainly in far northeast WI, where a couple
hundred J/kg of CAPE will be focused during the afternoon.

On Sunday, a strong system will move across the Middle Mississippi
Valley. While models continue to show the system remaining south of
the forecast area, a few ensemble members indicate the far southern
portions of the forecast area may see some rain on the northern
periphery of the system; elsewhere should remain dry.

Quiet conditions look to prevail for the start of the work week with
the next chance for showers and storms arriving sometime midweek.

Temperatures...With a relatively unchanged northwest flow expected
through next week, very little variance in temperatures is forecast.
As such, both afternoon highs (mainly in the 70s) and overnight lows
(upper 40s to low 50s) will hover slightly below normal.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1009 PM CDT Fri Jun 19 2026

As a cold front exits eastern WI early overnight, the threat of
storms will diminish at GRB/ATW/MTW, but light showers will likely
continue through 08-09Z.

As the rain ends overnight, confidence remains low with the
flight conditions as guidance generally disagrees in the
development of low clouds and/or fog. Considering winds and
climatology, continue to show a mention of low end MVFR ceilings
at AUW/CWA/RHI late tonight into Sat morning. With heat of the
day, these ceilings should rise to VFR by late morning or early
afternoon.

Scattered showers will develop again on Saturday afternoon, but
confidence of coverage near the TAF sites is low enough to forego
a prob30 group. Then clearing conditions are expected after 00-02Z
Sun.

&&

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

&&

$$

UPDATE.........MPC
DISCUSSION.....Kruk
AVIATION.......MPC

NWS GRB Office Area Forecast Discussion