522
FXUS63 KGRR 212323
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
723 PM EDT Thu Aug 21 2025

.KEY MESSAGES...

- Next chance of rain Friday night and Saturday

- Much cooler Sunday-Tuesday with lake effect rain showers possible

- Moderating temperatures late next week with another chance of rain

&&

.DISCUSSION...
Issued at 312 PM EDT Thu Aug 21 2025

- Next chance of rain Friday night and Saturday

Stratocumulus clouds that developed over Lake Huron and spread to
the WSW last night have lifted into a cumulus deck this afternoon
with diurnal heating. This is likely due to the added moisture
advecting in from Lake Huron. We expect this cloud cover to
dissipate this evening. A weakening flow and residual moisture in
the boundary layer is setting the area up for some fog tonight.

Clouds will break up on Friday morning, and likely lift into a more
flat and shallow cumulus deck during the day. Then later in the day,
high clouds will start streaming in from the SW with the approach of
the next front late Friday night and Saturday.

We continue to expect that this is not going to be a big rain event
for the forecast area. There are multiple factors working against
widespread rain, and much if any convection. The first factor is the
upper wave supporting this front has a vast majority of the energy
with it north of the area. This keeps the deeper cooler air aloft
from making it in initially, and limits instability with a semblance
of a cap. In addition, the low level jet associated with the system
will be north of the area, especially the leading edge/nose of it.

Moisture is limited in the lower levels as the source region for
these air parcels is from our SW over land. Finally, the front will
actually be moving through very late Friday night, and mostly
Saturday morning. This will limit the additional boost of
diurnal instability that we could see.

- Much cooler Sunday-Tuesday with lake effect rain showers possible

Even though the initial upper wave will miss most of the area with
the front, we will see the long wave upper trough settle over the
area for much of the period from Sunday through Tuesday. This is the
result of a fairly amplified flow affecting much of the hemisphere.
This results in the cold pool aloft settling over the area, with
short waves riding over the area from the NW.

The cold pool aloft and short waves will be enough for some small
rain chances by themselves. Then, you factor in 850 mb temperatures
dropping down to around +6 C and you get some lake effect/
enhancement take place. 850 mb - lake sfc temp differentials will be
running around the mid teens C with lake sfc temps in the lower 20s
C. The low level flow will favor the NW favored areas from Sunday
through Tuesday.

- Moderating temperatures late next week with another chance of rain

We will see the coldest of the temperatures aloft and the cyclonic
flow lift out then after Tuesday. We will still be under NW flow
aloft with strong upper ridges over the Western U.S. and the middle
of the Atlantic. The colder air aloft moving out will allow for
temperatures to rebound over the area.

Short wave ridging will build in likely on Wednesday. This will end
any rain chances temporarily. This will be short lived however as
another fairly strong upper trough dives down into the area from the
NW by Thursday. The forcing is pretty good, and will provide a
decent chance of rain for the area then. Once again however, the
flow feeding into the system will be not one of a good moisture
source.

&&

.AVIATION /00Z TAFS THROUGH 00Z FRIDAY/...
Issued at 711 PM EDT Thu Aug 21 2025

No changes from the previous forecast. Latest visible loop shows
cu dissipating which will lead to clear/pt cldy skies overnight.
Light winds and low level moisture will conspire to create fog
after 08z tonight and it will likely become IFR at most of the
terminals. Visibilities will become unrestricted by mid morning.

&&

.MARINE...
Issued at 140 PM EDT Thu Aug 21 2025

Wind and waves will remain below advisory levels through Friday.
However, a cold front will move through Saturday and northwest
winds behind it will cause winds and waves to increase; a small
craft advisory will likely be needed by Saturday afternoon.

&&

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

&&

$$

DISCUSSION...NJJ
AVIATION...04
MARINE...04