300
FXUS63 KGRR 241040
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
640 AM EDT Mon Aug 24 2026

.KEY MESSAGES...

- Areas of Fog Tuesday Morning

- Thunderstorms Possible Wednesday, Less Likely Thursday

- Active Pattern Could Develop By This Weekend

&&

.DISCUSSION...
Issued at 310 AM EDT Mon Aug 24 2026

- Areas of Fog Tuesday Morning

With a surface high building into the region later today into
tonight, winds will gradually tail off after dark and likely go
calm for many areas. GFS MOS guidance along with NAM soundings do
support areas of fog developing after 06z Tuesday as dew point
depressions approach or reach zero in some spots and an inversion
above the surface develops. Scattered clouds could be present,
however, so a widespread fog event does not appear likely. Still,
there is enough of a signal to put areas of fog in the forecast.
Fog may develop especially in low lying areas, which could have
some localized impacts to the Tuesday morning commute.

- Thunderstorms Possible Wednesday, Less Likely Thursday

Midweek rain chances remain on track as upper troughing is set to
move through the Great Lakes during peak heating on Wednesday.
Low level moisture advection ahead of this trough is shown by
model guidance, with surface dew points starting the day (12z) in
the upper 50s, then climbing to the mid to perhaps upper 60s by
late afternoon. SBCAPE values shown by the RRFS are forecast to
reach 1500 J/kg. Some model guidance supports up to 2000 J/kg,
though this would likely be the higher end of what materializes.
While deep layer shear looks to be about 25 kts, thus leading to
quasi-organized convection, one supporting factor to some locally
stronger storms is RRFS modeled DCAPE values of 800-1000 J/kg.
We`re expecting at least some scattered showers/storms to develop
primarily after 18z and lasting to about 06z. Better coverage may
get going after 21z. A scattered damaging wind gust is not out of
the question Wednesday afternoon and evening, so will monitor
trends.

The GEM and ECMWF support a slight chance for a few afternoon
showers and thunderstorms Thursday mainly east of US 131 as
residual surface based instability could remain underneath upper
troughing. The GFS dries out the low levels quicker and would
largely end the threat. The NBM POPs are quite low on Thursday, so
rain is not a high threat at this point.

- Active Pattern Could Develop By This Weekend

Both ensemble and deterministic model guidance is signaling a
potential shift in the synoptic pattern by this weekend into early
next week. In fact, both the 00z ECE and CMC ensemble mean 500 mb
heights are in the upper 580s dm across Lower MI by Sunday. This
could potentially place the Great Lakes in stronger mid level flow
with mid level jet cores moving overhead. The positioning of the
upper ridge will be key to the location of convective activity. At
this point we may be in an active corridor or close to it. PWAT
values of 1.50"-2.00"+ are supported by ensemble guidance. So,
these are some interesting trends to keep watching. We may round
out the month of August and enter September with some periods of
storminess, especially Sunday and beyond.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 640 AM EDT Mon Aug 24 2026

SCT to even BKN cloud bases around 3k ft have developed across
portions of western and northwestern Lower MI, poorly handled by
model guidance. With daytime mixing this morning, these cloud
bases should lift toward 5k ft or so. Either way, not a major
impact to aviation. Focus turns to the 08z-12z Tuesday time frame
with possible fog development occurring. This is shown by NAM
soundings with an inversion setting up above the surface and
temp/dp depressions near zero at times. MAV/LAV guidance does
suggest 2sm or below for the terminals with the exception of MKG,
possibly dense fog at times for locations like JXN. Will continue
assessing model guidance today to better pin down this threat.

&&

.MARINE...
Issued at 310 AM EDT Mon Aug 24 2026

Waves should generally stay in the 2 to 4 foot range today as NW
flow will be a bit weaker than Sunday, and also decreasing later
in the day. Winds and waves should by and large stay below Small
Craft Advisory criteria, though some 3 to 5 footers can`t be ruled
out early afternoon mainly south of Pentwater. A moderate swim
risk will be in place given these waves. Will keep an eye on buoy
trends today to see if waves overperform, and if so a Beach
Hazards Statement and Small Craft Advisory may be needed for some
areas. At this point we will hold off on any headlines.

&&

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

&&

$$

DISCUSSION...Hoving
AVIATION...Hoving
MARINE...Hoving