581
FXUS63 KGRB 090401
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1101 PM CDT Fri Aug 8 2025

Updated aviation portion for 06Z TAF issuance

.KEY MESSAGES...

- A Heat Advisory remains in effect for parts of central, east
  central and far northeast Wisconsin until 7 PM tonight, where
  heat indices are expected to top out near 100 degrees.

- There is a slight risk of severe thunderstorms and excessive
  rainfall on Saturday. Strong gusty winds and flash flooding
  (mainly for urban areas) will be the primary impacts through
  Saturday night.

- The slight risk of excessive rainfall will continue into Sunday
  over parts of central and east-central Wisconsin.

- Increasing southerly winds and building waves may result in hazardous
  boating and swimming conditions on Lake Michigan from late
  tonight through late Saturday night.

&&

.DISCUSSION...
Issued at 236 PM CDT Fri Aug 8 2025

Short Term...Tonight and Saturday

The latest RAP analysis and satellite/radar imagery show a
weakening MCS moving across western Lake Superior and northwest
Wisconsin. A few spotty showers and sprinkles have developed over
far northern Wisconsin where enhanced moisture transport was
occurring, despite the warm front lifting northeast of the Upper
Peninsula border. Widespread cloud debris is gradually spreading
east across central and north-central Wisconsin. This has helped
temperatures and heat indices remain modest in the mid-80s to
low-90s through early afternoon. Further east, widespread sunshine
has supported heat indices climbing to around 95 to 100 degrees
by 1 p.m.. As a result, the Heat Advisory remains on track.
Looking to the west, a cold front is moving east over the Dakotas
ahead of a robust shortwave trough. As this cold front slides into
the western Great Lakes, severe thunderstorms, flash flooding,
and dangerous heat remain the top forecast concerns.

Thunderstorms

After a muggy but quiet evening, mid-level height falls over the
northern Mississippi Valley will help to trigger a convective
complex across Minnesota that the convective allowing models
(CAMs) track southeast overnight. This MCS is forecast to reach
north-central Wisconsin between 5 and 7 AM Saturday. This MCS will
likely be weakening as it approaches north-central Wisconsin as
instability decreases from lofty amounts (1000-2000 J/kg). The
severe weather risk looks relatively low due to effective shears
around or less than 20 kts. However, DCAPE around 1000 J/kg will
bring a gusty wind threat from the strongest cells. This weakening
MCS will move across north-central and northeast Wisconsin on
Saturday morning, but there is uncertainty about its eastward
extent.

The timing of the morning convection could impact the
redevelopment of storms later in the afternoon with the
approaching cold front. It would be reasonable to think that
northeast Wisconsin will have a conditional severe threat because
the impacts of the morning convection could shift the severe risk
south of the area. If the morning impacts are limited, the severe
risk would certainly increase, as most unstable CAPE could
potentially increase to 2000-2500 J/kg, which would complement
20-30 kts of deep layer shear. This would bring a greater risk of
damaging winds. However, the HREF is considerably more muted with
respect to instability on Saturday afternoon (up to 1000 J/kg),
which would decrease confidence in the severe risk and most likely
limit the risk to heavy rainfall. Heavy rainfall and flooding
will be the primary threat, primarily in urban areas, due to 99th
percentile precipitable water values and slow storm motion, which
is increasing confidence of training or slow-moving cells.

Heat

A very warm and muggy airmass will remain in place into Saturday.
However, the prospects for cloud cover and precipitation are
considerably higher than today and heat indices could approach the
middle 90s over northeast WI prior to the arrival of storms.
Therefore, confidence is increasing that a Heat Advisory will not
be needed for Saturday.

Marine

Persistent gusty south winds will cause waves to build to 4-6
feet on Lake Michigan starting early Saturday morning and
continuing through late Saturday night. This will likely result in
hazardous boating conditions for small craft and dangerous
swimming conditions. A Small Craft Advisory and Beach Hazard
Statement has been issued.

Long Term...Saturday Night Through Friday

The weather pattern over North America will consist of upper
troughing from central Canada to the central Plains, while
subtropical ridging will extend from east of the Mississippi River
to eastern Canada. This pattern will place the western Great
Lakes within a southwest flow aloft and a corridor of above-normal
temperatures at 850 mb for much of the next week. With a
persistent trough over the northern Plains, the pattern will be
favorable for a cold front to stall nearby, providing round after
round of precipitation. The focus in this part of the forecast
remains on thunderstorm potential through early next week.

There is low confidence in the details for Saturday night,
pending how convection evolves on Saturday. It is thought that the
Saturday afternoon thunderstorm activity will be progressive,
pushing much of the convection into southern Wisconsin by Saturday
evening. However, some redevelopment is likely late Saturday
night into Sunday morning as nocturnal activity increases.

Ahead of the front, a plume of instability (1000-2000 J/kg) will
remain present from central to northeast Wisconsin on Sunday. Deep
layer shear from 25 to 35 kts will be favorable for clusters or
linear segments of thunderstorms at times, some of which could be
strong. The extreme precipitable water values (PWATs) are forecast
to fall to the 90th percentile from the 99th percentile, but
there are still indications of potential for training cells due to
slow storm movement. Therefore, the threat of flash flooding will
persist, mainly over parts of central to east-central Wisconsin,
through Sunday and possibly into Monday as well. The flooding risk
could increase further on Sunday into Monday if the initial round
of heavy rainfall materializes on Saturday, especially since most
locations over northeast Wisconsin have not seen significant
rainfall in nearly two weeks.

By Monday, the multi-model ensemble mean suggests the instability
axis will reside south of the area, which could indicate a lower
threat of precipitation. However, with the front nearby, the
chance of rain will continue. Ensemble means suggest there could
be a brief break in the unsettled pattern around the Tuesday-
Wednesday timeframe before the front returns north on Thursday
and Friday.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1052 PM CDT Fri Aug 8 2025

Isolated thunderstorms developed during the evening across
central WI. Isolated storms will continue for a few hours
after 96z. Meanwhile, low level wind shear from 200-230 degrees
at 35 to 40 knots is expected to continue overnight and diminish
after 13-14z. A complex of thunderstorms is expected to move into
north-central Wisconsin around 13z and towards KAUW/KCWA around
14-15z. As the showers and storms move into the area, an outflow
boundary will race eastward that will impact KAUW/KCWA/KRHI.
The chances of storms for the eastern TAF sites has been pushed
back by a few hours based on the latest model trends. Showers and
storms will prevail across the area Saturday afternoon, with rain
chances diminishing during the late afternoon across north-central
WI. The chances of showers and storms will linger through 06z
across eastern WI. Later Saturday evening into Sunday morning,
fog may be an issue across north-central WI as latest guidance
suggest fog could be dense in a few spots.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Beach Hazards Statement from 7 AM CDT Saturday through late
Saturday night for WIZ022-040-050.
&&

$$

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