857
FXUS63 KGRR 201131
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
731 AM EDT Wed Aug 20 2025

.KEY MESSAGES...

- Decreasing clouds today and dry through Friday

- Rain chances Friday night-Saturday, better chances north

- Cooler temperatures beginning Sunday with intermittent rain chances

&&

.DISCUSSION...
Issued at 325 AM EDT Wed Aug 20 2025

- Decreasing clouds today and dry through Friday

As noted previously, we remain under the influence of boundary layer
moisture sourced from Lake Huron/Saginaw Bay. This will keep clouds
in the forecast through this morning with gradual clearing in the
afternoon and evening. After this, high pressure will be firmly in
control. Highs Thursday and Friday have trended up a bit and are
now in the lower/mid 80s, which is slightly above normal.


- Rain chances Friday night-Saturday, better chances north

The last several iterations of ECMWF ensemble have not changed much
with respect to how much QPF is expected with this system. As noted
previously, amounts should be less than 0.2 inches and probably more
in the ballpark of 0.10. Q-G forcing looks a bit stronger to the
north and thus expect better chances (around 40 percent) towards our
northern forecast area, which includes the US-10 corridor. That
said, the latest round of deterministic models agree quite nicely on
a secondary and more equatorward upper PV maximum that is advertised
to swing across Lower MI and even the OH Valley region. This
southern extension of upper forcing would boost PoPs for southern
Lower MI; however, similar QPF boosts this far south remain in
question.


- Cooler temperatures beginning Sunday with intermittent rain chances

Broad upper troughing over the eastern Contiguous U.S. will continue
through mid-week and with it will be multiple reinforcing shortwaves
passing through.

As noted previously, we could be looking at lake effect rain as
early as Sunday/Monday, especially given the presence of cyclonic
flow aloft augmented by the passage of upper PV maxima. Lake
effect rain is something that NBM guidance has little hope of
seeing right now given that it uses a 60 to 90 day look-back
period to bias correct QPF and PoPs; hence, this puts this
guidance at a serious disadvantage as we embark upon our first
lake effect related events of the year. PoPs will almost surely
need to be bumped upward for the Sunday/Monday timeframe and
possibly beyond that as we get closer to this weekend.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 731 AM EDT Wed Aug 20 2025

Many of our terminals are now reporting ceilings in the fuel
alternate range (1000 to 1900 feet AGL). We expect a gradual
increase in ceilings to at/above 2000 feet AGL by late morning.
What`s a little less certain is how quickly we go VFR this
afternoon/evening. It could be a slow process. In fact, some
guidance suggests that JXN could be keeping ceilings well into
tonight.

The good news is that after approximately 14-15Z this morning,
the remaining terminals that still are reporting ceilings below
1000 feet should be out of IFR restrictions. At that point, IFR
restrictions are not expected to occur again at any of our TAF
locations over the next couple of days.

&&

.MARINE...
Issued at 325 AM EDT Wed Aug 20 2025

Have issued a Beach Hazards Statement and Small Craft Advisory
(SCA) for the Sable Points region to our north starting this
afternoon. There is a lesser but certainly non-zero chance that
beaches farther south could be impacted later today, but given the
current uncertainty, have left this for now and will let the day
shift assess.

As noted previously we expect quieter conditions Thursday and
Friday, but could see our next SCA as early as Saturday. Things look
quite interesting by Sunday/Monday with a push of much cooler air
that, with a warm Lake Michigan, puts us in play for lake effect
rain and even waterspouts.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement from 4 PM EDT this afternoon through
     this evening for MIZ037-043.
MARINE...Small Craft Advisory from 4 PM this afternoon to 11 PM EDT this
     evening for LMZ848-849.

&&

$$

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