735
FXUS63 KGRR 231125
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
725 AM EDT Sat Aug 23 2025

.KEY MESSAGES...

- Spotty coverage of showers today, isolated storms possible

- Cooler temperatures expected with lake effect rain Sun-Tue

- Moderating temperatures late week with less certain rain chances

&&

.DISCUSSION...
Issued at 332 AM EDT Sat Aug 23 2025

- Spotty coverage of showers today, isolated storms possible

Latest QPF guidance shows even less precipitation expected today
than before.  Precipitation in question extends from the western
Michigan UP to SE MN and is looking much less impressive than even a
couple hours ago as nocturnal stabilization takes hold. This
supports the idea of spotty precipitation over much of our forecast
area later today. However, some guidance such as the 00Z HRRR does
show convection re-intensifying this afternoon in the US-127 area
before it exits into SE Lower MI.

- Cooler temperatures expected with lake effect rain Sun-Tue

Have continued to go above guidance PoPs for Sunday night through
Tuesday. Conditional instability will be impressive as early as
Sunday night, with the 23/00Z HRRR advertising MUCAPE values greater
than 500J/kg overspreading much of southern Lower MI. With this in
mind, we expect an especially good coverage of rain showers in the
Sunday night/Monday timeframe with thunder also in play closer to
the lakeshore based on forecast soundings modified for the current
warm lake temperatures.

After lake effect rain tapers off early Tuesday across far southwest
Lower MI, the rest of the day looks drier as PV forcing for upward
vertical motion will be shifted south of the forecast area and flow
aloft is more N-NW in orientation.

- Moderating temperatures late week with less certain rain chances

From Wednesday onward, our forecast is generally quite dry, but
this comes with the caveat that episodic rain showers are not out
of the question during this time in light of growing forecast uncertainty.

We are reasonably confident about a rebound in temperatures given
that the mean trough position shifts eastward towards the eastern
Great Lakes. However, there will be a succession of PV maxima
rotating through the mean trough, and there are questions regarding
the position of these features as well as the amount of moisture
and diurnal instability they will have to work with. By this
weekend, the northern stream becomes increasingly unphased, which
will add to the uncertainty.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 725 AM EDT Sat Aug 23 2025

Showers associated with a prefrontal trough are over the northwest
CWA and are moving east while gradually dissipating. The actual
cold front is farther west moving into western WI. Thunderstorms
will be possible, primarily at LAN and JXN, when this cold front
reaches our area this afternoon. Went with a PROB30 group for LAN
and VCTS at JXN. Some model guidance suggests that a storm could
develop early this afternoon in the vicinity of AZO and BTL, but
this is too low probability to include at this time. Any
precipitation that occurs should be quick hitting with rapid
improvements to visibility and ceilings afterwards.


&&

.MARINE...
Issued at 400 AM EDT Sat Aug 23 2025

Latest WW3 guidance that just rolled in is significantly earlier
with onset of Small Craft Advisory (SCA) criteria with thresholds
already being met this morning at the Sable Points. Higher waves
should work their way down the lakeshore today and tonight.

Went with a monolithic SCA and Beach Hazards statement for the
entire marine forecast area, which is admittedly a bit simplistic,
but this will keep us covered for now and the day shift can
refine as necessary.

Looking ahead to Sunday night and Monday, have included the
mention of waterspouts. Thunderstorms will also be possible during
this time due to significant instability associated with cooler
air moving over the warm lake.

&&

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

&&

$$

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