120
FXUS63 KMQT 211128
AFDMQT

Area Forecast Discussion
National Weather Service Marquette MI
728 AM EDT Thu Aug 21 2025

.KEY MESSAGES...

- Patchy, locally dense fog may cause hazardous driving conditions
for the morning commute.

- Dry weather is expected through this evening before the next
chances of showers and thunderstorms return to the UP Friday. There
is currently a Marginal Risk (level 1 of 5) for severe weather.

- A seasonably strong system moving north of the Great Lakes will
bring hazardous marine conditions to Lake Superior this weekend.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 433 AM EDT Thu Aug 21 2025

GOES-East Night Fog Channel Difference product and METARs show
patches of fog for various parts of the UP, though lacking the
spatial and temporal consistency necessary for a Dense Fog Advisory.
HREF visibility charts show an expanding trend in coverage of fog,
so will be worth keeping an eye on throughout the morning hours as
some of the fog has forced visibilities below 1/4 mile. Given the
patchy nature of the fog, be vigilant on the morning commute as
conditions can change significantly over the course of the commute,
turning a benign morning drive into a potentially hazardous one.
With the significant density of the fog and the sharply changing
visibilities, elected to issue a SPS describing the fog and safety
information. With temperatures already at their dew points (thus,
fog), not expecting temperatures to fall much below their current
temperatures in the mid to upper 50s and lower 60s.

As the sun comes out today, fog dissipates and a mostly clear day is
expected under a high pressure near 1020mb. High temperatures in the
70s are expected today with light southerly winds, though lake
breeze circulations may cause variable winds in the afternoon.

Tonight, the HREF shows a 20-40 percent chance of a return to foggy
conditions for the eastern third of the UP. Attention turns upstream
to an approaching cold front. PoPs begin increasing prior to 12Z
Friday across the western UP as some isolated to scattered pre-
frontal showers are shown in the CAMs. Steepening mid to upper level
lapse rates are allowing for some elevated instability, with
HREF mean MUCAPE values approaching 1000 J/kg over the west
coast of the UP by 12Z, so a few thunderstorms may be possible,
though forcing is uncertain and shear is somewhat lacking given
that the primary synoptic feature, a 500mb deep cutoff low, will
still be over Manitoba by 12Z Friday. Expect low temperatures
(and dew points) tonight to be around the 60 degree mark as
despite mostly clear conditions ahead of the potential shower
activity, low level moisture is still elevated (NAEFS low level
humidity in the top 10 percent of climatology for this time of
year).

&&

.LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/...
Issued at 326 PM EDT Wed Aug 20 2025

After a quiet Thursday, a seasonably deep trough is forecast to dive
southeast across Sask/Manitoba into the northern Great Lakes this
weekend. The first round of associated weather impacts comes in the
form of a cold front set to move west to east into Upper Michigan
sometime Friday evening into early Saturday morning. A few strong
thunderstorms may accompany the frontal boundary given
afternoon/evening MUCAPE values pushing 1000 j/kg (potentially
higher given your choice of model), though weak upper level lapse
rates and bulk shear <30 kts may limit severe potential. For these
reasons, SPC has included much of Upper Michigan in a Day 3 Marginal
Risk (level 1 of 5) for severe weather.

The subjectively more interesting and potentially just as impactful
weather follows the cold frontal passage into this weekend and early
next week. By Saturday, deterministic models are in decent agreement
that the deep closed upper low and associated sfc pressure
reflection will be placed solidly atop north-central Ontario,
however, they begin to diverge into Sunday, with the GFS suite
conjuring a stronger secondary trough diving into the N Plains and
western Lakes into Monday. Meanwhile, the Euro/Canadian members
maintain the closed low spinning stop James Bay through Monday
before pushing northeast into Quebec. Regardless, stout NW flow
aloft into the late weekend will send fall-like 850mb temps crashing
into the single digits (10th to 2.5th percentile via NAEFS
climatology). 00z LREF suggests a 30-50% chance for 850 temps <5C
across northern Lake Superior by Sunday evening, increasing >70% by
Mon/Tues. With the average Lake Superior temperature around 17-18C
(near the climatological summertime peak), the incoming cool airmass
will support efficient momentum transfer as well as drive lake
effect/enhanced clouds and precipitation outside of showers driven
by embedded shortwaves pinwheeling around the deep upper low.

The big question is whether or not we can achieve the Gales of
August on Lake Superior this weekend. Ensembles are hit or miss,
with the ENS suggesting a <40% chance in the western lake and
Keweenaw along the initial push of CAA Saturday night into Sunday.
However, NBM 24hr max gust >34 kts depicts a broad 50-70% chance in
the open waters west of the Keweenaw followed by 30-50% chances both
east and west of the Keweenaw through Sunday. Probability for gusts
>22 kts are nearly 100% across the entire lake through the weekend.

With the cooler airmass overhead, expect fall like temps in the 60s
Sunday into the coming week and overnight lows in the 40s and 50s.
NBM 25th percentile points towards what could be our first frost
headline Monday/Tuesday night, though this will be contingent on
cloud cover and wind. Tuesday night looks to be a better shot with a
slackened pressure gradient across the region as high pressure
expands across the Plains.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 727 AM EDT Thu Aug 21 2025

Patchy dense FG has affected the TAF sites throughout the morning,
and while SAW and IWD are currently reporting VFR conditions, a
transient fog bank cannot be ruled out until about 14Z this morning,
so TEMPO MVFR groups have been added. FG has been more consistent at
CMX, and the VLIFR will scatter out throughout the early morning to
VFR. Light winds and VFR conditions then define the weather through
the day today at all sites. Tonight, chances of FG returning at SAW
are about 20 percent. Further west at CMX and IWD, chances of rain
showers as soon as 10Z are about 25 percent, with about 20 percent
chances of thunder at CMX, so a PROB30 group has been included, even
though better chances of TSRA occur at all sites tomorrow afternoon.

&&

.MARINE...
Issued at 326 PM EDT Wed Aug 20 2025

Light and variable winds less than 15 kts continue today and much of
Thursday as high pressure builds overhead. Southerly winds increase
Thursday night as troughing develops to the west, becoming southwest
up to 20 kts for Friday as high pressure shifts to the Lower Great
Lakes. A low pressure system then tracks over northern Ontario this
weekend, sending a cold front across the lake Friday into Saturday
and reintroducing west becoming northwest winds into the 20-30 kt
range for Saturday and Sunday as a seasonably cold airmass settles
in. Significant wave heights are expected to increase to 4-6 ft as a
result of persistent winds. Gales to 34 kts are becoming more
likely, though uncertainty still remains high. Ensemble model
guidance are hit or miss, with the ENS suggesting a <40% chance in
the western lake and Keweenaw along the initial push of CAA Saturday
night into Sunday. However, NBM 24hr max gust >34 kts depicts a
broad 50-70% chance in the open waters west of the Keweenaw followed
by 30-50% chances both east and west of the Keweenaw through Sunday.
Probability for gusts >22 kts are nearly 100% across the entire lake
through the weekend.

&&

.MQT WATCHES/WARNINGS/ADVISORIES...
Upper Michigan...
None.
Lake Superior...
None.
Lake Michigan...
None.

&&

$$

SHORT TERM...GS
LONG TERM...BW
AVIATION...GS
MARINE...BW