626
FXUS63 KGRR 211140
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
740 AM EDT Thu Aug 21 2025

.KEY MESSAGES...

- Dry through Friday, some fog tonight

- Showers and thunderstorms Friday night into Saturday

- Significant cooldown Sunday; lake effect rain into Monday

&&

.DISCUSSION...
Issued at 316 AM EDT Thu Aug 21 2025

- Dry through Friday, some fog tonight

No changes to previous forecast except to add patchy fog Friday
morning based on recent guidance. This is based on weak moisture
advection from Lake Huron/Saginaw Bay. Fog could end up being
locally dense around Jackson and perhaps Lansing as a result.
Otherwise, highs today and Friday will be in the lower to mid 80s.

- Showers and thunderstorms Friday night into Saturday

Models have been quite consistent regarding PoPs and QPF for this
first round of precipitation. The most recent SPC Day3 outlook is
valid through 12Z Saturday and does advertise general thunder for
our northwest forecast area. Thunder chances spread across the rest
of Lower MI during the course of Saturday. MUCAPE of greater than
1000 J/kg will be possible over the forecast area and could exceed
1500 J/kg in our eastern zones in the afternoon. The chances for
severe weather are low overall, but would not be surprised if a
Marginal Risk gets assigned to parts of southeast Lower MI with
later SPC updates.

- Significant cooldown Sunday; lake effect rain into Monday

Went above NBM guidance PoPs POPs Sunday night and Monday as model
guidance advertises significant conditional instability developing
as H8 temperatures around 5C overspread a warm Lake MI and yield
MUCAPE values on the order of a few hundred J/kg. This plume should
overspread much of southern Lower MI, so lake effect rain could move
pretty far inland around the I-94 corridor. As noted previously,
thunderstorms with small hail are highly plausible given the
thermodynamics in place.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 740 AM EDT Thu Aug 21 2025

Ceilings have over-performed in terms of coverage and flight
category this morning. JXN and AZO were reporting LIFR ceilings
at 11Z and cloudiness extended as far northwest as GRR and
currently is encroaching on MKG. Guidance is pretty quick with
raising/eliminating ceilings in the late morning/early timeframe.
Have decided to adopt this general stance as well with this
update; however, it should be noted that ceilings have tended to
be a bit more persistent than guidance over the last few days, so
a plausible scenario is that timing will need to be bumped out a
bit with future amendments. Ceiling heights should gradually
increase through this morning, though, and expect IFR restrictions
to be gone after 14-15Z with fuel alternate ceiling restrictions
not lasting much longer than that.


&&

.MARINE...
Issued at 316 AM EDT Thu Aug 21 2025

Have cancelled the beach hazards and small craft advisory a bit
early as winds and waves have subsided. Similar to what happened
Wednesday, today we do expect another surge in northerly winds
this afternoon/evening in the Muskegon/Grand Haven/Holland
vicinity. This should yield a moderate risk of beach hazards and
not be quite as intense as yesterday. The north flow as of late
has induced quite a bit of upwelling; lake surface temperatures
south of the Sable Points are now in the 50s according to IR
satellite. So, not a particularly enticing day for getting in the
water in the first place.

After today`s moderate concerns for small craft and beachgoers,
Friday looks quieter. As noted previously, winds/waves will
probably return to advisory criteria by Saturday with the passage
of a cold front.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...None.

&&

$$

DISCUSSION...TJT
AVIATION...TJT
MARINE...TJT