422
FXUS63 KGRB 261120
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
620 AM CDT Fri Jun 26 2026

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Dry today and Saturday with near normal temps and tolerable
  humidity. Pattern change begins on Sunday.

- Prolong period of building heat and humidity will result in
  heat-related impacts next week.

- In addition to the heat, expect chances for strong to severe
  thunderstorms at times next week.

&&

.DISCUSSION...
Issued at 158 AM CDT Fri Jun 26 2026

Near term through Saturday...High pressure remains in control as
heights slowly build aloft. Patchy ground fog early today will
fade shortly after daybreak. May see repeat tonight into early
Saturday morning. Normal highs are near 80 and we`ll be there
today and Saturday. Humidity values will remain tolerable with
dewpoints in the 40s/50s.

Pattern change begins on Sunday...Surface warm front lifts toward
western Great Lakes by Sunday Night. Ahead of sfc front, strong
warm air advection and theta-e advection and increasing LLJ should
support potential of showers and storms just about anytime from Sunday
morning through late Sunday night. MUCAPE rising to 1000-2000J/kg
(up to 4000J/kg into MN and western WI) and effective shear at
least 30 kts points to potential for stronger to severe storms.
WBzero heights up to 13kft may limit the severe hail potential.
PWATs near 2.00 inches and very high warm cloud depths would
result in drenching downpours.

Temperatures/heat next week...500mb ridge with heights near max
for climo builds northward late weekend through next week. Ridge
peaks at 595-596dam early next week over the midwest (IL/IN/OH)
before retrograding west through late next week. 850 temps up to
24C on Monday over WI only drop to 21-23C rest of the week. Though
prolonged period of highs in the 90s is likely, could still a day
where nearby showers/storms or at least cloud debris limit highs.
In stark contrast to the comfortable humidity values we have seen
so far late this spring into first days of summer, humidity will
increase sharply with dewpoints through the week residing in the
70s much of the time. This has the potential to be draining heat
for many with multiple days in the 90s and little relief at night
as lows stay in the 70s. Heatrisk output is picking up on this
with major to extreme heat impacts pegged for Monday through
Thursday. If you have outdoor activities planned next week, start
making plans for this heat.

Thunderstorms next week...The heat will be with us, as too will
be the threat for thunderstorms, some of which could be strong to
severe. Main limiting factor especially early in the week is good
agreement that once strong warm front lifts through late Sunday
night into Monday, there will be stout mid-level capping as shown
by 700mb temps 13-16c. That is typically plenty strong enough to
limit daytime storm development especially with no boundaries
around and it is interesting that models, ensembles now show a
decent minimum in pops for Monday given that scenario. Quick
glance at GFS forecast soundings on Monday and Tuesday during the
day shows very strong capping with convective temp over 100 both
days.

Given the wealth of extreme instability (MUCAPEs 4000+J/kg) that
will be parked over our area with anomalously strong mid-upper
level winds not that far north and west, overnight into morning
MCS activity is still on the table early next week. Maybe getting
too specific as details are not there yet, but there are hints in
guidance that after warm front lifts through, the next favorable
time for showers and storms may end up holding off til midweek as
until then strongest theta-e advection that can drives MCSs will
be located well north of our area, likely even north of Lake
Superior. Possible that ridge rider shortwaves could try to build
into our area at times toward maximum instability (shown by some
models for at least northern WI), but the strength of mid-level
cap may not allow for that or at least dampen the extent in which
they move in. By midweek though, ridging flattens some, so that
would also argue for better chances for storms by that time.
Machine learning output is in fair agreement that Sun-Mon the
chances for severe are greater over northern Plains, then expand
into western Great Lakes Tue-Thu.

Wind/fog...Tightening pressure gradient Sunday into Monday may
result in breezy conditions either of these days. Gradient weakens
by Tuesday. With 70s dewpoints and water temps on Lake Michigan
still in the mid to upper 50s, fog is expected. Best chance for
impacts on land probably is Monday as winds are still S instead of
SW. Once fog forms on the waters, it probably will persist much of
next week, sloshing too and fro, as there will be minimal airmass
change to get rid of it.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 620 AM CDT Fri Jun 26 2026

Patchy ground fog (LIFR-IFR) is possible at RHI/MTW early on this
morning. Scattered to broken mid and high clouds stream across
through this afternoon with fair weather cu (040-060kft)
developing as well. Scattered mid clouds linger tonight over
northern WI, with skies becoming mostly clear elsewhere. Light
winds less than 10 knots are expected during the TAF period.

&&

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

&&

$$

DISCUSSION.....JLA
AVIATION.......JLA

NWS GRB Office Area Forecast Discussion