675
FXUS63 KGRB 270800
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
300 AM CDT Sat Jun 27 2026

Forecast discussion for routine morning forecast issuance

.KEY MESSAGES...

- High heat and humidity will result in heat-related impacts much
  of next week. Most oppressive conditions Monday and possibly
  Tuesday.

- Mainly dry this weekend with small chance of thunderstorms
  (20-30%) over central Wisconsin Sunday afternoon.

- Strong to severe storms possible Sunday night and again on
  Monday night, especially northern Wisconsin. Main hazards would
  be damaging winds along with heavy rain.

- Periodic chances for thunderstorms Tuesday through late this
  week. Some of these storms could become strong or severe.

&&

.DISCUSSION...
Issued at 300 AM CDT Sat Jun 27 2026

Near term through Sunday...Today will be the last pleasant day
with temps still a bit below normal and tolerable humidity.
Warming trend and gradually more humid conditions begin to ramp
up on Sunday, along with more breezy conditions (SSE winds). Highs
mid to upper 70s today with cooler readings near Lake Michigan.
Highs finally reach normal values around 80 on Sunday. Dewpoints
around 60 will arrive. Compared to what is coming early this week,
that will still be pretty comfortable.

Surface warm front lifts toward western Great Lakes by Sunday
Night, but still will be south of here by daybreak on Monday.
North of the sfc front, strong warm air/theta-e advection and
increasing LLJ will result in showers and storms mainly staying
west on Sunday, though a few showers, isolated storms may reach
central WI in the afternoon from the west. Better chances for
storms will be 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. Northern WI favored for most concentrated
coverage of storms. SPC Day2 marginal risk is only into Vilas
County.

Oppressive heat arrives Monday...500mb ridge with 596dam heights
nearing climatological maximum for the date is centered over Ohio
Valley Monday with ridge extending into the central Great Lakes.
The ridge is now forecast to shift farther east by midweek
compared to what was shown 24hr ago. Result is peak of heat and
humidity for our area likely occurs early this week. 850mb temps
rise to 23-24c on Monday once the warm front lifts through. Strong
low to the northwest should propel the front through instead of
it getting hung up. It will become breezy again and there is
really good agreement that once the front moves through, dewpoints
on Monday will rocket into the 70s by midday while temps away
from the Lake Michigan shore (SSE winds) will reach 92-96F. Monday
still looks very capped and hostile to convection as 700mb cap
with temps 12-16c builds overhead and convective temps are over
100. Heat index values could pretty easily reach 105-108F given
this setup. Potential for Excessive Heat Warning criteria being
reached is increasing. Heat headlines will be needed soon.

Beyond Monday, high temperatures get more uncertain, mainly from
potential showers/storms and outflow/cloud debris. Tuesday
certainly has a potential for mid 90s highs and continuing
oppressive humidity, but there also have been hints that storms
nearby on Monday night could produce an outflow that would disrupt
the heating, at least for part of the area. NBM/LREF data suggest
greatest percentage chance for hottest conditions will be Fox
Valley/far northeast WI all the way to the lakeshore (winds more
SSW there compared to the onshore flow on Monday). Again, this
will depend on nearby trends with storms. Compared to Monday, not
as confident that we`ll see excessive heat criteria reached on
Tuesday. No matter what the highs are though through the rest of
the week (upper 80s to low 90s most favored), nighttime lows will
provide no relief from the heat with readings in the 70s to near
80. As a result the NWS HeatRisk output continues to show
widespread major to extreme heat impacts the whole week. Overall
If you have outdoor activities planned this next week, start
making plans for this heat as it will impact everyone that does
not have adequate cooling and hydration.

Thunderstorms through the week...The heat will be with us, as too
will be the threat for thunderstorms, some of which could be
strong to severe. Beyond Monday when capping should restrict
storms, the pattern could become much more active. Monday night
would be the first opportunity for this. Would expect storms to
develop Monday evening northwest of here (northwest WI/MN/western
Lake Superior) farther away from cap and in area of strongest low-
level theta-e advection ahead of northern plains low. If that
occurs, possible that MCS or more focused area of storms would
track E-SE toward reservoir of mid-level lapse rates around 8C/km
and MUCAPEs over 4000J/kg that will be present over the western
Great Lakes. 850-300mb thicknesses can be a proxy for where an MCS
would track and that would be more east than south, but with LLJ
aimed into WI, could see focus trying to build south into our
northern areas on the edge of cap. Tough to say for sure, but it
fits this overall pattern. If that occurs, would have to watch for
outflow boundaries left by this on into Tuesday which could serve
to develop more sfc based storms. Any storms Monday night into
Tuesday could be severe (main hazard damaging winds as freezing
levels will be very high) with such high instability present and
effective shear of 30-40 kts. There will be heavy rain potential
as well with PWATs of 1.8 over the 90th percentile, very high warm
cloud depths, and slow storm motions.

Beyond Tuesday, with ridge and cap shifting east and LLJ aimed
into the western Great Lakes ahead of approaching front, would
expect multiple rounds of strong to severe storms at least into
Thursday. Ridge rider shortwaves will dictate where/when main
times of storms will occur which is impossible to say at this
point. Given multiple round of storms, moderate to heavy rain
could occur even as axis of higher PWAT shifts east by this time.

Late this week, mid-level ridging reorients shifting from the
eastern Ohio valley more toward the mississippi river valley into
the western Great Lakes, taking our region out of the true ring
of fire. We`ll have to see how this carries forward into the 4th
of July holiday weekend. That is just tough to say this far out.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1136 PM CDT Fri Jun 26 2026

VFR conditions should prevail through the TAF period. The only
exception may be for some patchy MVFR VSBY between 07z-13z
overnight into Saturday morning due to light fog. Otherwise, east-
southeast flow expected through Saturday evening. High clouds will
stream in at times at night, with some fair weather cumulus clouds
expected later Saturday morning and into the afternoon.

The next chance of storms will not be until late Sunday afternoon
and Sunday night. Hot weather is expected at times next week with
highs in the 90s across much of the area away from the bay and
Lake Michigan.

&&

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

&&

$$

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

NWS GRB Office Area Forecast Discussion