046
FXUS63 KGRB 051129
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
629 AM CDT Sat Jul 5 2025

.KEY MESSAGES...

- There is a 70 to 90 percent chance of thunderstorms this
  afternoon and evening across the region. A few severe storms
  are possible that could produce damaging wind gusts. Heavy
  rainfall is also possible that could produce localized urban and
  low- lying flooding.

- Very warm and muggy conditions will result in heat indices in
  the lower to middle 90s over central to northeast Wisconsin this
  afternoon. Those with outdoor plans should take precautions for
  the heat.

- Gusty winds and high waves will lead to conditions hazardous to
  small craft this afternoon and tonight on the Lake Michigan
  nearshore waters. Small craft should exercise caution on the Bay
  of Green Bay. Hazardous swimming conditions will also be present for
  the Lake Michigan beaches across Door, Kewaunee, and Manitowoc
  counties this afternoon and tonight.

&&

.DISCUSSION...
Issued at 234 AM CDT Sat Jul 5 2025

Short Term...Today...Tonight...and Sunday

The latest RAP analysis and satellite/radar imagery show
shortwave energy moving across the northern Plains and pushing a
cold front from central South Dakota to northwest Minnesota early
this morning. Showers and thunderstorms continue ahead of the
front, but intensity has been slowly decreasing through the
overnight hours. These showers and storms will slowly move east
and move into far northwest WI through the early dawn hours.
Otherwise, a quiet and muggy night continues over northeast WI
with temperatures in the lower to middle 70s. As the front and
weak surface wave move across the region today, severe and heavy
rainfall potential are the main forecast concerns.

Synopsis: A weak low-pressure system currently situated over
eastern South Dakota will track eastward, moving across western
Lake Superior today and the Upper Peninsula tonight. This system
will usher in a cold front, which is expected to sweep across the
forecast area this afternoon and evening, coinciding with peak
daytime heating. Behind the cold front, high pressure will build
in for Sunday.

Thunderstorm Threat - Timing and Ingredients:
As low pressure tracks across western Lake Superior, showers and
thunderstorms are anticipated to become more widespread between 1
PM and 3 PM, according to HREF guidance. Atmospheric conditions
will be conducive to thunderstorms, with instability values
ranging from 1000 to 2000 J/kg and deep-layer shear measuring 25
to 30 knots. These parameters suggest a moderate potential for
organized thunderstorm development. Machine learning models
pinpoint the greatest risk of severe storms from central to far
northeast Wisconsin. The primary severe weather threat will be
damaging winds. While downdraft CAPE values are relatively poor,
around 600 J/kg, isolated damaging wind gusts will be possible.
There is a marginal risk of severe storms across the entire area.

Instability is expected to wane this evening, leading to a
decrease in thunderstorm intensity. Therefore, the severe weather
threat is expected to be highest between 1 PM and 9 PM. Showers
and isolated storms are anticipated to persist overnight before
gradually ending by early Sunday morning.

Excessive Rainfall Potential:  There is a marginal risk of
excessive rainfall. Precipitable water values are exceptionally
high, exceeding the 99.5th percentile, indicating a deeply
saturated atmosphere. The warm cloud depth, extending from 12,000
to 13,000 feet, further enhances the potential for heavy rainfall.
Additionally, winds are forecast to be parallel to the cold
front, which could lead to the development of training
cellsthunderstorms repeatedly moving over the same areas,
significantly increasing rainfall totals. Consequently, there is a
40-60% chance of rainfall exceeding 1 inch across central and
north-central Wisconsin. Localized flooding is possible in urban
and low lying areas.

Dangerous Heat: Another warm and muggy day is expected over
northeast WI. With thunderstorms not expected to arrive until mid
to late afternoon, heat indices are likely to rise into the middle
90s. The rain should bring a reprieve to the heat over north-
central and central WI by late morning or early afternoon.

Sundays Outlook: High pressure will begin to build into the region
on Sunday, bringing about a significant improvement in weather
conditions. Cooler and drier air will filter in, resulting in more
comfortable humidity levels. High temperatures are forecast to be
near normal for this time of year, ranging from the mid-70s to the
lower 80s.

Marine Hazards: South winds are anticipated to increase through
the day and peak this afternoon, reaching sustained speeds of 10
to 20 kts with gusts up to 25 kts on Lake Michigan. These gusty
winds will precede the arrival of the cold front. Wave are
forecast to build to 3 to 6 feet, which will make it challenging
for boaters. In addition, the strong onshore flow will contribute
to an increased risk of dangerous rip currents. A Small Craft
Advisory and Beach Hazards Statement remain in effect from 1 pm
this afternoon until 4 am Sunday. Boaters on the Bay should
exercise caution.

Long Term...Sunday Night Through Friday

Relatively seasonal weather can be expected during the upcoming
week. Low amplitude, zonal flow will be present across the US-
Canadian border which will send shortwave impulses across the
region on Monday night/Tuesday and also towards Friday. These will
be the periods of higher shower and thunderstorm chances.
Convective parameters during these periods do not indicate
potential for a severe weather threat at this time.

Seasonable temperatures are forecast next week. Highs on most days
will range from the upper 70s to middle 80s. Lows each night will
range from the middle 50s to middle 60s.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 628 AM CDT Sat Jul 5 2025

Clouds will continue to overspread the region as a cold front
approaches from the west. This cold front will bring scattered
showers and thunderstorms from west to east later this morning
through the evening. Only made minor changes to timing of the
storms at all taf sites in this forecast. Expect IFR/MVFR
conditions in the heavier showers and storms. Gusty winds will
also be possible.

South to southwest winds will become gusty today. Gusts from 20
to 30 kts are expected. Low end low-level wind

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
WI...Beach Hazards Statement from 1 PM CDT this afternoon through
     late tonight for WIZ022-040-050.
MARINE...Small Craft Advisory from 1 PM this afternoon to 4 AM CDT
     Sunday for LMZ541>543.

&&

$$

DISCUSSION...MPC
AVIATION...MPC