891
FXUS63 KMQT 210523
AFDMQT

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

.KEY MESSAGES...

- Patchy, locally dense fog returns tonight into Thursday morning.

- Dry weather is expected through Thursday before the next
  chances of showers and thunderstorms return to the UP Friday
  evening. 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 THURSDAY/...
Issued at 326 PM EDT Wed Aug 20 2025

GOES satellite reveals mostly cloudy skies overhead slowly clearing
as high pressure inching southward across Lake Superior ushers in a
drier airmass aloft. So far today, fog and drizzle have clung to the
central UP amidst light, moist upsloping, though the drizzle is
finally starting to taper off. Temps have managed to climb to the
mid to upper 60s save for the far east where breaks in the clouds
have provided enough sunshine for some low 70s. Tonight, skies
continue to clear while low level moisture hangs around, creating
the ideal setup for radiational fog. 12z HREF paints a 60-90% chance
for visibility less than 1 SM from roughly 3-9 AM EDT across the
northern tier of the UP, particularly area affected by northerly
upsloping. Probability for dense fog decreases to the south, but
conceptually I would not be surprised to see widespread fog in the
early morning hours across much of the interior south and west.
Otherwise, low temps will fall into the 50s. Once the fog breaks
Thursday morning, expect mostly sunny skies and light winds through
the day with afternoon temps pushing into the mid 70s to near 80.

&&

.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 /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 123 AM EDT Thu Aug 21 2025

Light upslope to variable winds overnight under high pressure and
abundant low level moisture will support fog development as
temperatures cool. Fog has already been observed at all sites, with
persistent airport minimums observed at KSAW. Less steady fog has
been observed at KIWD/KCMX. This trend will continue overnight, with
temporary LIFR conditions at KIWD/KCMX and at or below airport
minimums being commonplace at KSAW. Conditions are expected to
improve with daytime heating Thursday, with scattered low clouds
potentially lingering through the day.

&&

.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...BW
LONG TERM...BW
AVIATION...JTP
MARINE...BW