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FXUS63 KGRB 261116
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
616 AM CDT Thu Jun 26 2025

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- There is a marginal risk for severe thunderstorms this afternoon
  through this evening across central and east-central WI. Strong
  wind gusts are expected be the main hazard, with a few weak
  tornados also possible. Another round of strong to severe
  storms is possible late Sunday afternoon and evening.

- Storms this afternoon and evening may also produce localized
  heavy rainfall and create a slight flood risk. Areas with
  greatest flood potential are the more urban areas in central and
  east-central WI.

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

&&

.DISCUSSION...
Issued at 354 AM CDT Thu Jun 26 2025

This Morning: A few isolated showers are moving through central WI
early this morning, however, much of the region has become dry.
There are also a few pockets of patchy fog mainly across central and
north-central WI where some of the heaviest rain fell yesterday. The
dry conditions will be short lived as the next shortwave, over
southeast SD is already producing scattered storms over western IA.
As this wave moves east today it will bring another round of rain
and some strong to severe storms to the region this afternoon.

Severe Storm and Heavy Rain Potential this Afternoon/Evening: There
remains a conditions threat for a round of strong to severe storms
late this afternoon and early evening depending on how far north a
warm front lifts. 00Z HREF shows this front setting up along or just
south of HWY 10 this afternoon. CAMs show a few discreet storms
developing along this front between 1-3PM ahead of a broken line of
storms forecast to develop over southeast MN/ western WI. This line
of storms will move east across the state through the late afternoon
and evening hours. Immediately south of the warm front SBCAPE values
increase to ~1000-1400 J/kg along with 40-45kts of 0-6km shear. This
supports at least a marginal risk for severe wind gusts.
Additionally 0-3km SRH values in the vicinity of the warm front are
250-400 m2/s2 which does bring the potential for a few weak tornados
to spin up. HREF 2-5km UH >75 m2/s2 paintballs also show a cluster
along the 10 HWY 10 corridor late this afternoon/early evening
highlighting the potential for rotating updrafts.

Given that the heaviest of yesterdays rainfall fell north of HWY 29
and the expected progressive nature of todays storms flash flood
potential is going to be primarily tied to storms producing higher
rainfall rates over the more flood prone urban areas of central WI
and the Fox Valley. HREF soundings show PWATs ~ 1.8-1.9", which is
around the climatological max for this time of year, along with a
deep warm cloud layer. This suggests that torrential downpours
producing 0.5 to 1" of rain per hour will be possible. Further north
steady light to moderate rainfall may lead to an addition 0.5 to
1.5" of rain this afternoon and evening, but given the sandy soil
don`t think the flash flood potential is as great north of HWY 29.
Will need to monitor some area river gages over the next 24-36 hours
as a few sites along the Wisconsin River are forecast to reach
bankfull and Babcock on the Yellow River is forecast to reach minor
flood stage early Friday morning.

Sunday Thunderstorm Potential: A weak cold front and low amplitude
upper-level trough are forecast to move across the region late
Saturday through Sunday. These features will overlap an increasingly
unstable airmass Sunday with peak SBCAPEs 3000-4000 J/kg. Result
will likely be a round of scattered thunderstorms with a few
potentially becoming strong to severe. Weak 0-6km shear ~ 20-30kts
may be a limiting factor in how organized any storms can become.
There remains a large degree of uncertainty with the finer details
during this period. Behind the cold front Sunday a more stable
airmass will move into the region Monday leading to mostly dry
conditions to start next week.

Temperature Trends: Near normal temperatures are expected through
the end of the week with a warming trend expected this weekend. By
Sunday most locations are forecast to reach the middle to upper 80s.
NBM probs show a 40-50% chance for areas in and around the Fox
Valley to reach 90 degrees Sunday. Prevailing northwest flow will
develop Monday which help to moderate temperatures back towards
normal through the early and middle parts of next week.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 615 AM CDT Thu Jun 26 2025

Generally poor flying conditions today due to low clouds, patchy
fog, periods of heavy rain and thunderstorms.

This morning most locations are IFR/LIFR due to a low status deck as
well as patchy areas of mist and fog. Vsbys across northern WI have
shown improvement over the past few hours and expect this trend to
continue further south through the morning. Cigs will also gradually
rise this morning, however, there is uncertainty as to weather or not
they will become MVFR before the next round of rain and storm
arrive.

North of a AUW to GRB line widespread light to moderate rain is
expected to spread in form the west late this morning and continue
through much of the afternoon and evening. IFR cigs and MVFR vsbys
will likely accompany this round of rain. South of this line there
is a better chance for scatter thunderstorms, some possibly severe,
this afternoon and evening. If any thunderstorms move over a
terminal expect cigs and vsbys to drop to IFR. Have framed out the
best timing for thunderstorms with TEMPO groups at all the TAF sites
except RHI where a PROB30 group was used because confidence in any
thunder across northern WI is low.

Behind the rain and storms expect IFR to LIFR conditions tonight as
patchy mist and fog redevelop.

&&

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

&&

$$

DISCUSSION.....GK
AVIATION.......GK