506
FXUS63 KGRB 251748
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1248 PM CDT Wed Jun 25 2025

Updated aviation portion for 18Z TAF issuance

.KEY MESSAGES...

- Periods of showers and thunderstorms expected this afternoon
  through Friday morning, with heavy rainfall expected. The
  probability of 2+ inches of rain is highest (30-60%) across
  central WI. There is a slight risk of excessive rainfall and
  flooding through this period.

- There is a small window for severe thunderstorms, mainly south
  of Highway 29, Thursday afternoon and early evening. Another
  round of stronger storms is possible late Saturday into Sunday.

- Near/below temperatures continue through Thursday, then
  increasing heat and humidity into the weekend, with highs near
  90 by Sunday.

&&

.DISCUSSION...
Issued at 410 AM CDT Wed Jun 25 2025

Short Term...Today Through Thursday Night

Thunderstorm & Heavy Rainfall/Flooding Potential: After a dry
start to the day, look for showers and embedded storms to develop
later today (especially after noon into this evening) along/north
of a slowly advancing 850mb front, increasing mid-level FGEN and
as pieces of shortwave energy work across/into the region, with a
LLJ aimed at west-central WI. PWATs climb to between 1.5-2.0
inches today with deep cloud depths to ~12,000 ft will support
some area of moderate to heavy rain, especially where the
strongest FGEN sets up. A lull in the steadier rain should occur
overnight, then an additional round of showers and storms is
expected on Thursday as low pressure, the final shortwave,
additional FGEN, an impressive pool of instability builds
near/just to our south (up to ~3500 J/kg), and a cold front
track across the area on Thursday. PWATs climb a little more to
2.0-2.25 inches (200+ percent of normal and near or above
climatological max for GRB), along with mean specific humidities
at/near the climatological max, leading to areas of heavy rain
once again. The heavier rain/storm activity will end from west to
east Thursday night into Friday morning.

In general, rain totals between 0.75-2.25" are expected for most
of the area, but locally higher amounts are expected in spots.
Models differ slightly on exactly where the best FGEN will set up,
which will dictate where the heaviest rain will fall and where the
greatest flood threat will be. Highest probabilities (30-60%) for
2+ inches of rain continue to remain across central WI. With an
atmosphere ripe for heavy rain (highlighted by tall/skinny CAPE
profiles), any training heavier showers or storm activity will
lead to flooding concerns. Highest concerns will be where the
heavier rain fell the past 48 hours, namely Wood, Portage,
Waupaca, Waushara, and Winnebago counties, where flash flood
guidance is the lowest (as low as ~1.5" in 1 hour and ~2 inches in
6 hours). Many rivers/streams will rise due to the rain, with
some minor flooding possible, but HEFS guidance showing very low
chances of sites reaching flood stage. The 1-2" per hour rain
rates will also lead to some urban/rural/field flooding at times.
WPC continues to place all of the area in a slight risk for
excessive rainfall later today into Thursday. Will hold off on any
flood watch as pinpointing where the heaviest swath(s) of rain
remains difficult and there will be lulls in the heaviest rain.

Severe Weather Potential: Severe weather is not expected today or
tonight as we remain far enough north of the better surface and
elevated instability and warm cloud depths limit the hail threat.
A small window of severe weather is possible Thursday afternoon
and early evening, mainly south of Highway 29, assuming the warm
front can make it into the southern counties and we get a little
sun to get to the low-levels to destabilize. If we make it into
the warm sector, SBCAPE would climb to 1-2K J/kg, with sufficient
shear (0-6km bulk shear of 30-40 kts and 0-3km SRH of 100-300).
Will be fighting a capping inversion as well, along with eastern
flow off of Lake Michigan as any storms approach the lake. So
still some questions if severe storms will be able to form. If
they do, gusty winds and hail look to be the main threat.

Temps & Humidity: After a cooler night for most of the entire
region with lows dropping into the 50s and 60s (with even a few
40s where skies stayed clear over far north central WI), the
cooler / below normal temps will continue today with highs mainly
in the upper 60s and 70s. It will be less humid as well, with
dewpoints mainly in the 50s to low 60s. Temps on Thursday will be
highly dependent on where the surface warm front sets up, with the
cooler 60s prevailing north of the boundary and 70s to near 80
south of it. Humidity levels will also climb south of the
boundary on Thursday, where dewpoints will climb into the 60s to
near 70.

Long Term...Friday Through Tuesday

Behind a departing surface low and cold front the region should
turn mostly dry Friday afternoon through much of Saturday. There
is a low- end chance (10-20%) that thunderstorms initiating over
northern MN drift into north-central WI late on Saturday, however,
the vast majority of LREF members (+80%) keep the region dry
through Saturday night.

Sunday will be a day to watch for potentially stronger
thunderstorms. Long range ensembles bring an upper trough across
western Great Lakes which will overlap an increasingly moist (low
70s dew points) and unstable (SBCAPE >2000J/kg) airmass. One
limiting factor is relatively weak shear profiles, less than 30kts
0-6km shear. AI/ML convective outlooks are also highlighting
parts of the region Sunday with an increased risk for stronger
storms. There is still a lot of uncertainty given the day 5
timeframe, but will continue to monitor trends to pin down timing,
storm mode, and potential hazards over the coming days.

A drier more stable airmass is expected to then build into the
region for the early part of next week as a ridge develops off to
the west. This should bring a quieter period to end the month.
Temperatures this weekend will again be rather warm with highs in
the middle to upper 80s at most locations, however, it should not
feel near as hot as this past weekend. Temperatures are then
forecast to moderate near normal for the start of next week.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1243 PM CDT Wed Jun 25 2025

A large area of showers was pushing northeast across the forecast
area early this afternoon. As this continues, flight conditions
will drop to MVFR, with local IFR vsbys within heavier showers.
Later this evening and overnight, as showers taper off, widespread
IFR/LIFR conditions will develop, with only minor improvement
expected toward the end of the TAF period. The best chance of
storms looks to occur across C/EC WI this evening and into the
early overnight, so have added a TEMPO group for storms at
GRB/ATW/MTW between 03z-07z/Thu.

Winds will remain out of the NE-E through the TAF period, with
speeds generally 10 knots of less.

&&

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

&&

$$

DISCUSSION.....Bersch/GK
AVIATION.......Kieckbusch