462
FXUS63 KGRB 281724
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1224 PM CDT Sat Jun 28 2025

Updated aviation portion for 18Z TAF issuance

.KEY MESSAGES...

- Periodic shower/storm chances through the weekend with the
  highest chances (50-80%) Sunday afternoon/evening, when a few
  severe storms are possible. Heavy rain and localized flooding is
  also possible.

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

&&

.DISCUSSION...
Issued at 413 AM CDT Sat Jun 28 2025

Fog This Morning:

Clouds were slow to erode during the overnight hours in parts of
the area, but where clouds did clear out, the recent rain,
cooler/moist low-levels and light winds allowed areas of fog to
develop, some dense, early this morning, particularly over
central into north-central WI. Will issue an SPS to highlight the
fog and monitor for a possible advisory if it can become more
widespread. The fog will slowly lift/burn off after sunrise.

Thunderstorm & Flooding Potential This Weekend:

The northern side of a dying complex of storms will spread into
western WI this morning, reaching central and north central WI
after sunrise. The system will have outrun its instability, with
little to no MUCAPE in our area, so no impactful weather is
expected. But could get some sprinkles/showers or perhaps a storm
as this first area tracks east. More shower/storm development is
expected this afternoon and evening as the LLJ interacts with the
frontal boundary and a large pool of CAPE over 3000+ J/kg. Still
some uncertainty on the exact details on how this convective
evolution will play out as CAMs show differences in
timing/location and strength, especially as outflows from the
previous activity may provide for different areas of convective
initiation than the models are currently advertising. But do
expect at least one or two areas of convection to move across the
area, mainly north of Hwy 29. If the storms can become surface
based, some gusty winds will be possible. Otherwise, hail and
heavy rain will be the main threats.

Attention turns to Sunday as a mid-level trough digs into the
northern Plains and a cold front tracks across the state.
Impressive instability forecast to build ahead of the front, with
3000-4000 J/kg of SBCAPE. Deep layer shear (15-25 kts) and
overall forcing is still forecast to be on the weaker side, which
could limit overall organization/coverage of the storms. In
addition, outflows (or MCVs) from previous convective activity may
also play a role is where/when this round of convection is able
to initiate. Current forecast has a break in any activity for much
of the morning hours, with storms firing in the afternoon and
tracking eastward into the early evening. Large hail and damaging
winds from any bowing segments are the most likely severe weather
threats. The supercell and tornado threat appears to be low, as
the lowest LCLs are forecast to be closer to SW WI, with the
better shear likely to reside across far northeast WI. ML/AI
forecasts are highlighting damaging winds as the highest threat,
with any small tornado threat over far northeast WI where better
0-3 SRH should be located.

PWATs climb back to 1.4-1.9" at times this weekend (NAEFS
PWAT exceeding the 90-95th percentile), along with deep/warm
cloud depths up to ~13,000 ft and tall/skinny CAPE profiles,
leading to a heavy rain threat. But the overall threat for
widespread flooding appears to be low as each area/complex will
be moving fairly quickly. Only exception would be if a back-
building area of convection were able to develop along the nose of
the LLJ and boundary across central or northern WI late today
into tonight. If/where any places get multiple rounds of storms
and/or any training cells, flooding concerns would increase. This
will be especially the case for locations that received the
heavier rain over the past few days. The Yellow and Wisconsin
rivers are running high, so any additional heavier rain in these
basins would likely cause more flooding issues.

Temps & Humidity Levels This Weekend:

South/southwest flow will prevail across the western Great Lakes.
Highs in the upper 70s to mid 80s are expected today with
humidity levels climbing through the day as dewpoints climb into
the 60s to near 70, highest toward central WI. Winds look to turn
onshore this afternoon near Lake Michigan, keeping temps a little
cooler. It will be a warm and muggy night across the area, with
lows only falling into the mid 60s to low 70s. A warm and humid
day on Sunday is expected with highs in the 80s to around 90 with
dewpoints in the mid 60s to low (possibly mid) 70s. We should come
up a little shy of heat advisory criteria, but heat index values
in the 90s are expected across much of central and eastern WI.

Next Week:

As a large trough drops across the Great Lakes on Monday, expect
some clouds along with a few showers, mainly diurnal in nature,
in the northwest/cyclonic flow regime. Could get a few storms as
instability builds and lapse rates steepen, aided by cooler air
aloft, in the late morning and afternoon. Much of the mid to late
week period is looking dry as flow turns northwest and weak
surface ridging builds over the Great Lakes. However, a couple
shortwaves are likely to ride within this northwest flow, along
with the lingering surface frontal boundary somewhere in the
region, which could touch off some showers and storms.
Predictability regarding the details in the flow will is
challenging, so we end up with several periods of low-end
shower/storm chances. Temps look to be fairly seasonal, but
location of the front and any cloud/precip will determine if/when
we end up a little below or above normal.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1217 PM CDT Sat Jun 28 2025

Some MVFR CIGS will linger into the early afternoon across
central and north-central WI as a cumulus deck developed
across the area. For this afternoon through Sunday evening,
the main story will be the timing of thunderstorms at the
TAF sites. Confidence is high there will be a round or
two of storms during the period, however confidence is
low on the timing. Current thinking is a round of thunderstorms
will move into north-central Wisconsin around 05-08z Sunday and
the spread into northeast Wisconsin after 10z. This complex should
dissipate Sunday morning with another round of storms expected
later Sunday afternoon into Sunday evening. Strong storms with
gusty winds and hail are possible with the storms, along with
the potential heavy rainfall that will significantly reduce
visibilities for pilots.

&&

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

&&

$$

DISCUSSION.....Bersch
AVIATION.......Eckberg