983
FXUS63 KGRR 232357
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
757 PM EDT Sat Aug 23 2025

.KEY MESSAGES...

- Here comes Fall!

&&

.DISCUSSION...
Issued at 328 PM EDT Sat Aug 23 2025

- Here comes Fall!

Isolated showers and possibly a thunderstorm will attend the
passage of a cold front across the southern and eastern zones late
this afternoon and evening. Although deep layer shear is about as
strong as any seen in the past several weeks, moisture and
instability are generally lacking so severe weather is not
expected.

The big story is still the trend towards much cooler temperatures
as persistent upper troughing across eastern Canada and the NE
CONUS over the next 10 days will mean a continued northwest flow
of Canadian air masses, along with occasional chances for showers
from cold front passages and lake/diurnal enhanced showers.

The upshot in terms of grid forecasting is to adjust the POPs,
clouds and QPF upward across much of the forecast area, especially
the western half, into Tuesday as shortwave troughs rotate
through the broad cyclonic flow extending south of the upper low
spinning over James Bay.

The rest of the week looks dry and cool as surface high pressure
builds in and the upper trough axis shifts eastward.

&&

.AVIATION /00Z TAFS THROUGH 00Z MONDAY/...
Issued at 757 PM EDT Sat Aug 23 2025

Thunderstorms are now distant E-SW of JXN moving E and it should
stay that way tonight. During the daytime Sunday, conditions
become increasingly favorable for lake-effect rain showers with
cloud bases between 3,000 and 5,000 feet. Cloud tops will tend to
be around 8,000 ft Sunday morning, rising closer to 12,000 feet in
the afternoon. Temporary visibility drops are possible as showers
pass through. MKG may have a 30 percent chance of a thunderstorm
after 20 Z.

&&

.MARINE...
Issued at 328 PM EDT Sat Aug 23 2025

Two main marine concerns exist over the next few days. First is
several days of northwest winds bringing conditions hazardous to
boaters and swimmers. Waves around 4 feet have already been observed
at a few sites this afternoon with waves expected to persist through
the day and beyond. 3-6 foot waves are likely through late
Monday/Early Tuesday along with 20-25 knot gusts.

Given the solid signal in short range guidance and favorable setup,
am extending the Small Craft Advisory/Beach Hazards Statement
through Monday evening. South of Grand Haven, an extension into
Tuesday morning may be needed as we get short range model guidance
in later packages. There is a signal for a brief lull in winds and
waves Sunday Afternoon into the moderate risk range north of
Pentwater, however given its borderline headline criteria,
especially south of Big Sable, and waves and winds jump again in the
evening there`s enough risk to keep them in headlines through the
period.

The other concern is for thunderstorm/waterspout development over
the waters beginning Sunday afternoon. A much colder airmass moving
over the lakes will lead to conditions favorable for shower and
thunderstorm development. In addition, this will lead to the
potential for waterspouts late Sunday into Monday, especially as we
get convergence along the shoreline thanks to the stronger winds
over the waters. Have continued a slight chance of waterspouts in
the going forecast.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement through Monday evening for MIZ037-043-
     050-056-064-071.
MARINE...Small Craft Advisory until 8 PM EDT Monday for LMZ844>849.

&&

$$

DISCUSSION...Ostuno
AVIATION...CAS
MARINE...Thomas