498
FXUS63 KGRR 211914
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
314 PM EDT Fri Aug 21 2026

.KEY MESSAGES...

- Scattered showers/thunderstorms today into Saturday

- Cooler and dry Sunday through Tuesday, midweek rain chances

&&

.DISCUSSION...
Issued at 314 PM EDT Fri Aug 21 2026

- Scattered showers/thunderstorms today into Saturday

Scattered thunderstorm cells have developed along an inland
surface trough / confluence zone east of US-131 and will move east
this evening. These are ahead of diminishing convection from
Wisconsin with midlevel MCVs apparent on radar. The storms are in
an area of MLCAPE 500-1000 J/kg. Modeled effective shear is no
more than 20 knots, though it`s possible the MCVs could boost the
shear values somewhat and provide greater longevity to the cells.
Small hail and wind gusts to 40 overall are possible today.

Additional scattered storm cells are likely tonight, with elevated
bases via 850 mb moisture, then may become more surface based as
they move toward Mid Michigan later Saturday. CAPE values will
generally be 500-1500 J/kg and effective shear is likely to remain
no more than 20-25 knots. Lapse rates both at low and mid levels
should be fairly modest during much of this activity which should
also help keep severe wind/hail potential low.

QPF mean is about 0.10 to 0.25, though more realistically, amounts
will be more hit or miss, with a few isolated totals closer to 1
inch possible.

- Cooler and dry Sunday through Tuesday, midweek rain chances

Northwest flow Sunday and Monday will provide slightly cooler,
drier, more comfortable air as the upper-level trough turns cutoff
low over the eastern/northeastern Great Lakes region. A minor
return of moisture and warmer air occurs from late Tue into Wed,
with scattered thunderstorms again possible with the next frontal
wave.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 107 PM EDT Fri Aug 21 2026

Key Changes...

- Added details on forecast ceiling and visibility reductions
  overnight

Overall, no significant changes to the ongoing forecast. Still
favoring scattered showers and storms this afternoon into tonight,
with coverage low enough to warrant maintaining PROB30s for now
rather than going TEMPO/Prevailing. Will monitor and amend as
needed. Can`t rule out gusty winds over 20 knots in any storms.
Low chance (15-20%) of additional storms late tonight, but will
handle those in future packages given extended time range and low
confidence. Otherwise, high boundary layer moisture will favor
patchy fog and low stratus overnight, which should mix out mid to
late Saturday morning.

&&

.MARINE...
Issued at 314 PM EDT Fri Aug 21 2026

Ongoing Beach Hazards Statement / Small Craft Advisory for near
Big and Little Sable Points through Fri evening is fairly low
confidence given latest data, as south winds 10-15 knots may only
build up 2 to 4 foot waves with a moderate swim risk before
subsiding tonight. Scattered thunderstorms are possible over the
lake tonight with a low chance for gusts to 30 knots.

More likely hazards from wind, waves and currents for this weekend
as north-northwest winds increase behind a cold front. There
remain some differences in the models regarding the strength of
winds at times, but overall, waves should build to 3 to 6 feet at
most beaches later Saturday morning. Then after a slight lull
Saturday evening, winds likely strengthen and waves build again
Saturday night into Sunday.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement until 8 PM EDT this evening for MIZ037-
     043.
     Beach Hazards Statement from Saturday morning through Sunday
     evening for MIZ037-043-050-056-064-071.
MARINE...Small Craft Advisory from 8 AM Saturday to 11 PM EDT Sunday for
     LMZ844>849.
     Small Craft Advisory until 8 PM EDT this evening for LMZ848-849.

&&

$$

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