159
FXUS63 KMQT 090001
AFDMQT

Area Forecast Discussion
National Weather Service Marquette MI
801 PM EDT Fri Aug 8 2025

.KEY MESSAGES...

- Heat and humidity increases this weekend. The chance of heat
  indices exceeding 90 degrees is greater than 50% through
  Saturday for most areas.

- A line of thunderstorms, some strong to severe, is expected to
  move across the west-central UP tonight through early
  Saturday.

&&

.SHORT TERM /THROUGH SATURDAY/...
Issued at 349 PM EDT Fri Aug 8 2025

GOES-East satellite imagery shows primarily clear skies over the
east half with some fair weather cu mixed in. Over the west, some
high cirrus and low stratus is observed flowing ahead of a dying MCS
over northern WI/western Lake Superior, MN Arrowhead. Aloft, SWerly
flow over the UP is seen at 500mb with a deep cutoff low over
Manitoba and an inverted trough over the Lower Mississippi Valley
between a cutoff high over the Desert Southwest and a ridge over
Bermuda. This flow is funneling warm, moist air from the Plains
over the UP, leading to dew points lingering around the 70
degree mark throughout the short term forecast. Being downstream
of the larger trough is also providing PVA for potential
convection, though the 12Z CAMs have backed off on potential
thunderstorms today, with only the NAM Nest, HRRR, and RAP
showing any shower potential over the Lake Superior shores and
the Keweenaw this afternoon and evening as the cap holds
elsewhere. The main story then for today will be the heat, as
the aforementioned hot and humid air moving in from the
southwest will drive heat indices into the 90s for most,
especially in areas that downslope towards Lake Superior. The
12Z HREF shows chances of exceeding 95 degree heat index is only
isolated to the Ontonagon to Baraga/L`Anse region with little
to no chances of exceeding 100 degree heat index, so no
headlines will be issued with this forecast package. However,
with plenty of outdoor recreation ongoing heading into this
weekend, it is still advised to keep hydrated and cool as low
temperatures overnight in the low 70s (!) will provide little
relief from the heat for those without access to air
conditioning.

Attention then turns to shower and thunderstorm activity in
northwest Wisconsin, western Lake Superior, and the MN Arrowhead
tomorrow early morning as CAMs show a consistent signal for at least
some complex of storms approaching the western UP around 08Z-12Z.
Steep lapse rates aloft will lead to some massive elevated CAPE
profiles with the mean MUCAPE of the Euro Ensemble being over 3000
J/kg for the Ironwood area at 06Z Saturday. Shear quickly runs out
as the convective system leaves the high-shear environments out west
and enters the ~20 kt bulk shear environment over the UP (12Z HREF
shows 0-3km SRH values mostly sub-75 m2/s2 east of the US-45
corridor). This leads to the majority of CAM solutions showing a
weakening trend in convection as it marches across the western UP by
15Z (the HRRR being a notable exception). Some attention will also
need to be given to the afternoon period as the ARW WRF, HRRR, and
FV3 WRF show strong convection along the state line through around
21Z, though with CAPE values decreasing across much of the UP in a
post-convective environment, only a few remnant non-severe showers
and thunderstorms are expected by 00Z Sunday. With the abundant
cloud cover from the disturbance, diurnal heating will be somewhat
limited relative to prior expectations with highs in the low 80s now
expected and heat indices primarily peaking in the mid 80s to 90
degree mark. Sustained south to southwest winds will create waves up
to 4-5 feet along the Lake Michigan shores, which will create
conditions ripe for rip currents and hazardous swimming conditions.

&&

.LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...
Issued at 349 PM EDT Fri Aug 8 2025

00Z Sunday through 00Z Monday, the cutoff upper low will lift into
the Hudson Bay, with ensembles showing variances in the presence of
embedded shortwaves that keep PoPs in the forecast but confidence in
the precise details low. A reinforcing shortwave looks to drop down
from Manitoba late Monday into Tuesday, providing another mechanism
for active weather, though significant differences are observed in
the global deterministic suite with the GFS showing a very flat and
weak wave while the Canadian shows a quite deep cutoff low and the
Euro falling in between. Instability will be present but not nearly
to the degree of Saturday, with the 75th percentile of SBCAPE in the
LREF being sub-1000 J/kg by Tuesday afternoon. Persisting southwest
flow aloft will keep warm, moist air in the forecast, but the
present cloud cover will keep high temperatures mainly around the 80
degree mark even as low temperatures struggle to fall below 60
degrees thanks to the ever-present humidity (NAEFS humidity charts
are almost always above the 90th climate percentile into next
weekend). Ridging looks to follow for next midweek, giving a dry
period, but spread increases further going towards the end of the
week. While ensemble mean daily precip charts show some chances of
rain just about every day, the chances of over an inch of rainfall
in a day are 10 percent or less after Tuesday.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 758 PM EDT Fri Aug 8 2025

Primarily VFR conditions are expected at all sites through the
duration of the 0Z TAF period. Where this will not be the case is
during periods of TSRA expected late tonight into Sat. There still
are some discrepancies in model guidance regarding the arrival time
of the first round and how well it holds together as it progresses E
across the UP, but it likely will impact IWD first then CMX prior to
sunrise on Sat. These TSRA reach SAW closer to sunrise with some
preluding SHRA possible. Additional SHRA/TSRA Sat afternoon is
possible, but timing is even more spread at that point. Given the
uncertainty, kept most precip mention as PROB30 groups but did add a
TEMPO group for the first round at IWD. Impacts from the TSRA
include brief periods of LL turbulence and LLWS, frequent lightning,
and heavy downpours lowering VIS to MVFR. Otherwise S to SW winds
stay elevated around 10-12 kts with gusts into the low 20 kt range.

&&

.MARINE...
Issued at 349 PM EDT Fri Aug 8 2025

Ahead of a line of thunderstorms passing over the western end of the
lake tonight, wind gusts out of the will increase to 20-25 kt over
the eastern half of the lake, with gusts near 20 kt for the west
half of the lake Saturday. Once gusts fall below 20 kt behind the
system, the only remaining chance of gusts above 20 kt is Sunday
when southwest wind gusts increase to above 20 kt when they funnel
between Isle Royale and Thunder Bay. Waves will primarily be 4 ft or
less. Some thunderstorms tonight through Saturday will have frequent
lightning and locally gusty/erratic wind gusts.

&&

.MQT WATCHES/WARNINGS/ADVISORIES...
Upper Michigan...
  Beach Hazards Statement from Saturday morning through Saturday
     evening for MIZ014.

Lake Superior...
None.
Lake Michigan...
None.

&&

$$

SHORT TERM...GS
LONG TERM...GS
AVIATION...Jablonski
MARINE...GS