328
FXUS63 KGRB 291839
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
139 PM CDT Sun Jun 29 2025

Forecast discussion for routine afternoon forecast issuance

.KEY MESSAGES...

- Scattered thunderstorms are expected during the remainder of
  this afternoon and evening. Some storms may produce strong or
  damaging wind gusts and hail up to one inch in diameter.

- Storms could bring locally heavy rainfall to the area late this
  afternoon and evening. Rainfall rates of 1 to 2 inches per hour
  may result in localized flooding, especially in urban and low-
  lying areas. The greatest risk of locally heavy rain is across
  northeast and east-central Wisconsin, including the Fox Valley.

- Temperatures and humidity will trend back above average today,
  with near to above average temps continuing into next week.

&&

.DISCUSSION...
Issued at 139 PM CDT Sun Jun 29 2025

Short Term...Tonight and Monday

The first band of thunderstorms exited northeast Wisconsin shortly
after sunrise this morning. In the wake of the complex of storms,
a hefty inversion was noted on the KMPX and KGRB soundings. This
inversion and no real trigger has inhibited any storms from
redeveloping during the late morning into the early afternoon.
However, the KGRB sounding indicated a convective temperature of
87F at Green Bay. Forecast high temperatures near 90 are expected
across central and northeast Wisconsin which should allow for
convection to redevelop.

The combination of heat and humidity, a cold front moving across
the area combined with an upper level disturbance should trigger
thunderstorm redevelopment during the mid to late afternoon and
lingering through the evening. Bufkit soundings indicated 2,000 to
3,000 J/KG of CAPE to support strong or severe storms. 0-6km
shear values are only around 20-25 knots, thus storms will be
more pulsy in nature with damaging winds as the main threat. A
few storms could produce hail up to one inch (quarters). Torrential
rainfall is possible with rainfall rates of 1 to 2 inches per
hour. The greatest risk of heavy rains is across northeast and
east-central Wisconsin, including the Fox Valley.

A new wrinkle in the complexities of summer forecasting is the
12Z NAM has come in dry for most of the area through 12z Monday.
The previous run had 1 to 2 inches of rain across the Fox Valley.
Current thinking is convection will develop across northeast and
east-central Wisconsin where these locations will approach or
reach the convective temperature. Also, boundaries from the
outflow from this morning or lake breeze may aid in the initiation
of convection. The main forcing mechanism late this afternoon and
evening will be the 500mb impulse moving east across the state.
This feature will be the main focus for convection as a band of
storms moves across the area. Once the main feature exits the
area around 1 am Monday morning, some lingering showers are still
possible but will be isolated in nature.

Long Term...Monday Night Through Sunday

Monday severe storm chances... An mid-level trough will amplify
as it approaches the Great Lakes Monday morning, with broad
ridging over the Intermountain West and closed low off the west
coast. This trough paired with favorable location under the right
rear quad of an upper jet will be the main drivers for our storm
chances late Monday morning through the afternoon. Current
thinking is that best chances for convection popping along the
front will be over east-central Wisconsin early Monday afternoon
as instability (~1,000 J/kg SBCAPE) builds within deeper moisture
and low-level lapse rates steepen to around 9C/km. Inverted-V
soundings would suggest gusty/downburst winds as the primary
threat. CAMs then hint at a scenario in which diurnally-driven
storms develop behind the front Monday afternoon. With synoptic
support provided by the aforementioned trough and jet streak, some
stronger pulse storms would be within the realm of possibility
during this time as well. Hail threat increases during this time
as post-frontal temperatures lower both the freezing level and wet
bulb zero heights (~7 to 9k ft). This would be further evidenced
by elongated hodographs produced by 40 to 45 knot bulk shear.

Rest of the week... Remainder of the week will be characterized by
low confidence storm chances as northwest flow regime sets up
along the leading periphery of persistent upper ridging. We will
thus be afforded a brief break from showers and storms by around
mid-week. Ridge rider activity then ramps up during the
Thursday/Friday timeframe, resulting in the return of storm
chances. However, resolvability of these features remains
volatile; will therefore continue to carry low-end PoPs and slight
chance thunder in the forecast. Otherwise, steady-state
temperatures in the 70s and 80s prevail through the end of the
work week. Any deviation to above/below average temperatures will
likely hinge on FROPA timing and cloud cover.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1228 PM CDT Sun Jun 29 2025

MVFR CIGS over portions of central and northeast Wisconsin will
rise into the VFR category by mid afternoon. Otherwise, another
round of thunderstorms are expected later this afternoon, arriving
at KRHI/KAUW/KCWA after 22z until 02z. A few storms could develop
near KGRB/KATW/KMTW around 21z, but the main storm activity is
expected between 00z and 04z. Some of the storms could contain
strong winds and hail along with blinding rain. Additional shower
activity may linger for an hour or two after the main complex
moves through the area. Otherwise, VFR conditions will prevail
later tonight except for some patchy fog in a few spots from
09z-13z. The chances of showers and storms return tomorrow morning
and continue into the afternoon. Gusty winds over 30 knots and
hail will be possible with the stronger storms.

&&

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

&&

$$

DISCUSSION.....Eckberg/Goodin
AVIATION.......Eckberg