572
FXUS63 KGRR 180542
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
142 AM EDT Sat Oct 18 2025

.KEY MESSAGES...

- Wet and Stormy Saturday/Saturday night, and Wet and Windy Sunday

- Another Wet and Windy System Monday Night-Tuesday

- Unsettled and Cool Weather Wednesday-Friday

&&

.DISCUSSION...
Issued at 318 PM EDT Fri Oct 17 2025

- Wet and Stormy Saturday/Saturday night, and Wet and Windy Sunday

A lot of weather to impact Southern Lower Michigan through the
weekend, especially given the quiet late summer and fall over the
past few weeks.

We are in the relative calm before the storm this afternoon, that
will continue through most of the night after a few early morning
showers that moved across Central Lower. We are in a warm sector in
the wake of the elevated warm front that moved through earlier, with
the cold front still well west of the area over MN and IA this
afternoon. In addition, Gulf moisture is just starting to be drawn
northward into the system. Sfc dew points in the 40s and a lack of
moisture below 7-9k ft are keeping the area dry. This will continue
through much of tonight.

We will see showers and some isolated storms arrive across the NW
corner of the forecast area toward/after midnight, and then further
south and east Saturday morning. The arrival of the showers and few
storms will be with the 850 mb jet axis nosing over the area. This
looks to bring a line of showers and embedded storms toward the
lakeshore toward 12z Saturday.

These showers and storms will initially be quite benign as they will
be elevated in nature, with just a little instability to support
weak updrafts. We do see a decent potential for the line of showers
and isolated storms to strengthen a bit through the afternoon for
the SE portion of the area. HREF mean MU CAPEs are forecast to
increase to over 500 J/kg. This is likely due to low level dew
points increasing into the low to mid 60s, and some diurnal heating
ahead of the line of rain.

If the instability of 500-1000 J/kg can be realized, we could see
some strong to maybe even marginally severe storms for a few hours
Saturday afternoon. Deep layer effective shear of 40-50 knots is
quite favorable for some organization. If the instability can not be
realized, the shear would likely overcome the instability, and not
much would be expected. Mid level lapse rates are good around 6.5-
7.0 C/km to support some hail. The low level jet over the SE corner
of the area will provide for some decent low level shear for some
wind, and maybe even rotating storms. This will have to be
monitored.

The heavy rain potential really looks to be centered on Saturday
night, especially from around 03-09z. This will be the result of the
deepening sfc low to our east with the long wave trough supporting
it, and strong short wave energy currently over AZ lifting NE ahead
of the long wave trough. This short wave and jet energy will help to
provide a fairly strong FGEN signature just NW of the low in the
TROWAL/deformation zone. This will wring out the very moist air mass
(250% of normal pwats for date) that will move in. Thankfully with
the lack of rain and drought conditions over the past few months, we
have some room for it to go. Please see the Hydrology section below
for more detail.

Finally, the sfc low will move by the area on Sunday, but we will
see additional showers likely, while adding some strong winds to the
mix.

We will see the strong gradient on the backside of the strengthening
low move over the area just before daybreak on Sunday along the
lakeshore. This will then spread across the entire area through
Sunday. The highest winds look to be just inland from the lakeshore
where 40-50 mph winds are likely. Inland areas will be a little less
with more friction from the ground, and the better gradient being
over the lake. Showers will continue all day as the upper trough
will be slowly approaching all day, with additional short wave
energy rotating through.

- Another Wet and Windy System Monday Night-Tuesday

After a brief respite Sunday night and Monday, we will see more
windy and wet weather move in over the area by Tuesday morning. High
pressure between the strong departing system, and the next one
moving in from the NW will be short lived.

Stronger winds from the SW will arrive late Monday, and rain will
arrive ahead of the system Monday night. This system will not be in
the process of strengthening like the weekend system. It will be a
fairly strong upper low however, leading to strong dynamics.
Moisture will not be nearly as rich with this system, but it will
eventually be pulling some moisture northward from the Gulf.

- Unsettled and Cool Weather Wednesday-Friday

More widespread rain will give way to instability showers on
Wednesday under the cold pool and cyclonic flow aloft. We could even
see some heavier lake effect rain showers develop along the
lakeshore as 850 mb temps drop to as low as -2C, giving delta t`s
mid to upper teens C with water temps still 16-18C.

The Tuesday/Wednesday upper low will set the stage for general long
wave troughing to persist over the area through the end of the week.
We will see some short wave ridging build in centered around
Thursday, but another short wave dives in by Friday giving the area
another round of showers, and keeping the air mass cool.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 142 AM EDT Sat Oct 18 2025

This forecast period is starting out with mainly high clouds in
place at all of the terminals. Showers are starting to form over
Lake Michigan and are ongoing over Wisconsin. This axis of shower
activity with some isolated storms will arrive at KMKG by 11z or
so, and will continue to move across most of the rest of the
forecast terminals, except maybe staying north of KJXN. MVFR
conditions are likely, and there may be some periods of IFR with
the heavier showers/storms. This first batch will move out by
about 18z.

A few showers may linger, but there will be a relative lull in
the more widespread shower activity. Later in the afternoon and
into the evening, the I-94 corridor is likely to see additional
scattered showers and isolated storms develop. Then beginning
around 03z, widespread rain will move over the I-94 corridor and
spread north through 05-06z. IFR conditions are forecast to
develop in this widespread rain. Some isolated storms will remain
possible.

&&

.MARINE...
Issued at 318 PM EDT Fri Oct 17 2025

Small Craft Advisory conditions through Saturday evening.
Gales likely on Sunday.
Gales possible late Monday through Tuesday.

Hazards to small craft will be frequent through at least Wednesday
next week. Winds 10 to 25 knots from the south-southwest are
expected over the water through Saturday, slowly subsiding into a
temporary lull Saturday night. Today and tonight, those winds will
build 5 to 8 ft waves from Grand Haven toward Manistee.

Strengthening low-pressure system tracking from SW to NE across
Lower Michigan early Sunday will have strong winds on the back side
of it along with cooler air flowing in amid northwest winds.
Ensemble models, particularly the European ensemble system, continue
to indicate about 75 percent of solutions favor gale force gusts
over the southern half of Lake Michigan aimed at the Michigan shore.
A gale watch has been posted for Sunday morning through evening.

Somewhat relaxed winds within a high pressure ridge Monday morning
will be fairly short-lived also, as another developing low-pressure
system appropriate for the season consolidates over the western
Great Lakes. Any lull in hazardous conditions for small craft Monday
morning will be temporary. Depending on the strength and track of
the center of the low, there is a chance of southwest gales Monday
night, but west-northwest gales are more probable on Tuesday. The
European ensemble system, though potentially underdispersed this far
out (giving us overconfidence), have about 70 percent of members
favoring gales over Lake Michigan during Tuesday.

&&

.HYDROLOGY...
Issued at 318 PM EDT Fri Oct 17 2025

Significant rainfall is expected over the next several days across
much of Lower Michigan. The total amount of rain we are expecting
continues to slowly nudge upwards as forecast details come into
focus. The rain will be spread out over several rounds, primarily
spanning Saturday and Sunday, with the heaviest rain currently
expected before/during sunrise on Sunday. Overall averages across
much of the area will probably end up in the 1 to 2 inch range, with
some multi-county streaks of 2-3"+. The good news is that because
we`re in a drought, the rivers have plenty of room to put this
water. Some of the midsize rivers in the area like Sycamore Creek
(Holt), Buck Creek (Grandville), and the Looking Glass River (Eagle)
are the most likely to see rises close to bankfull, but no river
flooding is expected unless this storm seriously overperforms our
expectations. It would probably take 4"+ over a fairly large area to
begin flooding the midsize tributaries mentioned above.
Nevertheless, with water levels currently running lower than normal
due to the drought, notable within-bank rises are expected across
most of the area. Anyone with personal property near the waters edge
leftover from the summer should prepare for rising water.

While river flooding isn`t a big concern, if some slow-moving heavy
rain bands park over some of our urban areas we could see some
localized minor flooding develop in the classic low spots and other
poor drainage areas. It`s hard to pin down which urban areas would
be most at risk of this happening, but recent model trends suggest
the eastern edges of our area are most at-risk, which would include
Lansing and Jackson areas.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...Small Craft Advisory until 8 PM EDT this evening for LMZ844>849.
     Gale Watch from Sunday morning through Sunday evening for
     LMZ844>849.

&&

$$

DISCUSSION...NJJ
AVIATION...NJJ
MARINE...CAS
HYDROLOGY...AMD