204
FXUS63 KGRR 200725
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
325 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 /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 139 AM EDT Wed Aug 20 2025

Drizzle, IFR ceilings, and MVFR visibility restrictions are being
reported at some terminals, including AZO and GRR. At GRR
precipitation has been heavy enough to wet the pavement. This is
associated with a wind convergence boundary at the leading edge
of a southwestward push of northeast winds. Observations west of
this boundary indicate winds from the north/northwest. We expect a
continuation of IFR restrictions at many of the terminals
overnight, thanks to this boundary as well as a reinforcement of
low ceilings originating from the Saginaw Bay region.

Terminals eventually will climb out of IFR into fuel alternate
MVFR ceilings by late morning/early afternoon, and finally into
VFR by afternoon/late evening. Expected timing for this remains quite
similar to what was advertised with the 00Z TAFs, with only minor
adjustments.

&&

.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 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