054
FXUS63 KGRB 231136
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
636 AM CDT Mon Jun 23 2025

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- The intense heat will come to an end from west to east today as
  a cold front, along with showers and thunderstorms spreads
  eastward across the area. The heat index will climb to around
  100 degrees over northeast Wisconsin before the rain arrives.

- Strong or severe storms are possible this afternoon into this
  evening with damaging winds and torrential rainfall the primary
  hazards. Localized flooding is possible.

- Periodic chances of showers and thunderstorms will continue
  Tuesday into next weekend. It is too early to determine if or
  when any of the storms would become severe.

- Locally heavy rainfall is possible at times on Wednesday and
  Thursday where probabilities of receiving over an inch is
  between 40-80%. Probabilities of receiving over 2 inches is
  20-40% over central and east-central WI and less than 20 over
  the north.

&&

.DISCUSSION...
Issued at 349 AM CDT Mon Jun 23 2025

Short Term...Today...Tonight...and Tuesday

595+ dam 500mb ridge centered over the Ohio Valley persists through
tonight. Meanwhile, a sharp trough over south central Canada shifts
toward Hudson Bay, lowering heights over the western Great Lakes
and pushing a cold front across the region. The front will bring
an end to the heat and generate showers and storms this afternoon
and evening. Some storms will be strong to severe, with damaging
wind gusts and torrential rain shaping up to be the main hazards.
Much cooler weather is expected behind the front on Tuesday.

Last day of the heat...Deja vu but once again very warm this
morning as 3 am temps are still in the mid 70s to around 80 over
much of the area. Cold front pushing in from the west could lead
to isolated showers and storms as early as this morning over
the far north, but overall the front will be slow enough to allow
for one more day of heat. The hottest temps in the lower 90s and
heat indices around 100 will occur from the Fox Valley to far
northeast WI and to the lakeshore, including Door County as
southwest wind direction will keep lake cooling to a minimum all
day today. No changes to heat headlines. Depending on how quick
the showers and storms develop central, likely will be able to end
the Heat Advisory there early. Heat indices all areas fall below
80 by late evening. Lows later tonight will fall into the 50s
north, but will remain in the upper 60s to 70 from the Fox Valley
to the lakeshore. Even though that is still warm, it will be
nothing like the last few nights.

Severe storms this afternoon and evening...Not a true uniform cold
front passage as sfc front moves through late afternoon into early
evening, while 925-850mb temp drop and wind shift occurs after
midnight. There also is really not strong shortwave trough, just
a gradually lowering of heights. This all adds up to limited pre-
frontal effective shear, likely less than 30 kts, which could
limit storm organization and longevity. Instead looking at more of
a pulse-type environment as MLCAPEs are still at least 1500j/kg if
not 2000-2500j/kg. Some CAPE in hail growth zone, but higher
freezing levels and weaker shear would tend to favor non-severe
hail sizes. Damaging wind potential is there with unidirectional
flow and sfc-delta theta-e values well over 20 in the lowest
15kft, suggesting downdrafts could accelerate when stronger cores
aloft develop. Low LCLs and outflow boundaries could lead to spin
up, but 0-1km/0-3km helicity values up to 100 are not too favorable.
Overall think wind is the main severe threat, supported by recent
NCAR NN output as well with most favored time between 3-8 pm.

Heavy rain parameters still look very favorable. PWATs rising up
over 2 inches (exceeding the daily max for the day) along with
warm cloud depths surging to 12-14kft and MBE vectors slowing to
less than 5kt are concerning if there are training storms. Supported
by hints from HREF probs over over 1"/hr, seems greatest risk for
heavy rain would be across the more vulnerable urban areas of the
Fox Valley. HREF/NBM probs for 1" and 2" of rain this afternoon
through tonight stay mainly southwest of our southern cwa, lined
up with slight risk of Excessive Rain from WPC. Yet, glance through
available models does support narrow swaths of 1-2" QPF over
areas generally south of highway 29. 1hr-3hr Flash Flood Guidance
that are lowest over Fox Valley point to needing only 1-2" in 3hr
to cause some issues, so this will need to be monitored later
today along with severe potential.

MUCH COOLER on Tuesday...Cold front fully moves through late
tonight with nw winds after midnight shifting northeast on Tuesday
morning. Northeast winds may gust to 10-20 mph for a time close
to the bay and along Lake Michigan. Mostly cloudy skies expected
and some post frontal showers and storms could develop south of
highway 29, but better chances will be south. Highs will be much
cooler, with readings reaching mainly low to mid 70s. Warmer
exception of near 80 would be over central WI if there is more
sun. Cooler exception will be near the immediate lakeshore where
the onshore winds may keep temps in the mid to upper 60s. As they
say, what a difference a day makes!.

Long Term...Tuesday Night Through Sunday

500mb ridge across the southeast United States is expected
to flatten some during the week, resulting in lower 500mb
heights. However, the western Great Lakes region will be in
the ring of fire with periodic chances of showers and
thunderstorms from Tuesday into next weekend.

The main focus for heavy rain is in the Wednesday and Thursday
time frame. Still a lot of uncertainty in where the axis of
heavy rain will set up, but current thinking is that central
and east-central Wisconsin currently have the greatest chance of
seeing the heaviest rainfall totals. Based on the NBM, the
probabilities of receiving over an inch is between 40-80%.
Probabilities of receiving over 2 inches is 20-40% over central
and east-central WI and less than 20 over the north. Determining
the risk of severe weather is difficult this far out, but current
thinking is that the front will be south of the area which would
lower the risk of severe weather. The ECMWF has been most
consistent with timing of the heavy rain which would support at
least two rounds if not three rounds of heavier rain from late
Tuesday night into Thursday night. It will be much cooler compared
to the last several days with highs in the 70s to lower 80s
Tuesday through Thursday, warming into the 80s Friday into next
weekend.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 636 AM CDT Mon Jun 23 2025

LLWS of 40-45 KT is expected early with gusty southwest winds
just off the surface.

Isolated showers and storms over western WI will slowly fill in as
the day progresses as a cold front moves in from the west. A line
of showers and storms will shift northwest to southeast across the
state this afternoon and evening. The storms should reach north-
central Wisconsin late this morning through midday, central
Wisconsin and northeast Wisconsin early afternoon, and east-
central Wisconsin late Monday afternoon. The showers and
thunderstorms will lower CIGs to MVFR at times, with MVFR
conditions expected behind the cold front. Some storms could be
strong to severe as they track through the area this afternoon and
evening. The primary hazards will be wind gusts to 50 kts and
torrential rain. Behind the front tonight, expect cigs to fall to
IFR over east- central WI (GRB/ATW/MTW).

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until noon CDT today for WIZ005-010-011-018-019.

Heat Advisory until 7 PM CDT this evening for WIZ012-013-020>022-
030-031-035>040-045-048>050-073-074.

Beach Hazards Statement through this evening for WIZ022-040-050.
&&

$$

DISCUSSION.....JLA/Eckberg
AVIATION.......JLA