086
FXUS63 KGRR 082318
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
718 PM EDT Fri Aug 8 2025

.KEY MESSAGES...

- Hot and humid with small chances for storms through Sunday

- Good chance of rain late Monday through Tuesday

- Cooler Wednesday and beyond with chances for rain

&&

.DISCUSSION...
Issued at 247 PM EDT Fri Aug 8 2025

- Hot and humid with small chances for storms through Sunday

The main impact through the weekend looks to be the heat once again,
with some smaller chances for showers and storms.

The showers and storms that brought localized heavy rainfall to
Newaygo and Mecosta counties this morning has long since moved out.
This complex was indirectly associated with a weak short wave and
associated low level jet that have now moved out of the area. Short
wave ridging behind it has built in over the area this afternoon.

This is providing some resistance in the form of a weak mid level
cap to convective growth trying to occur along some enhanced cu
fields. These cu fields extend from South Haven to Mt. Pleasant,
likely at the edge of the lake shadow, and another along U.S.-127.
We expect that a 20-30% chc will cover this well this afternoon.

The long wave upper ridge axis is then expected to move directly
over the forecast area for Saturday and into Sunday. This will
strengthen the mid level cap, and bring the hottest weather of this
stretch to the area. 850 mb temps are forecast to climb to 19-20C,
which easily supports highs near 90 away from Lake Michigan. Dew
points will be high and increase the heat index a couple of degrees
warmer than the air temp, but we are not expecting heat indices to
hit 100F or higher, so we will not be issuing a Heat Advisory at
this time.

The upper ridge slips a little east of the area by Sunday. Saturday
evening, we will see storms form in Wisconsin where the deeper
moisture/theta e axis will be located. This is where the low level
jet axis will also be to help fire convection with short waves. It
appears that the majority of this activity should stay WNW of the
area. It can not be ruled out that something may try to propagate
toward the western portions of the area Saturday evening/night. It
will be coming into a somewhat hostile environment for it to
survive as much better deeper instability remains west of the area.

Sunday may be a little uncertain with regards to heat and heat index
values with possible convection and convective debris from it. Temps
aloft cool slightly with the upper ridge axis moving east, but we
will be able to mix a little better. Bottom line though is we do not
expect Sunday to be much, if any warmer than Saturday at this time,
so thinking no Advisory at this time.

- Good chance of rain late Monday through Tuesday

We see the rain chances increase through Monday, and likely peaking
later Monday and into a good chunk of Tuesday. This is when we see
the theta e axis evident by PW`s of 2+" and potential conveyor of
weak short waves to move along this axis.

Severe potential can not be ruled out, but details are WAY too
uncertain at this time to even try to discern. One thing that could
become concerning would be hydrology concerns as this pattern has
the potential for training of showers/storms with heavy downpours to
affect localized areas. This axis of best moisture and short wave
activity will sink south of the state by Wednesday morning.

- Cooler Wednesday and beyond with chances for rain

There could be a few showers that linger Wednesday morning, but we
should gradually dry out and see cooler temperatures and lower dew
points move in.

Thursday has become a bit more interesting as an upper low that gets
left over the Central Plains by the long wave trough will slowly
lift toward the state by Thursday. This low/trough will have
sufficient forcing/lift, and looks to draw a good slug of Gulf
moisture northward toward the area. What looked to be another dry
period moving in, now looks like it could potentially be a nice
beneficial rain if the current trends of bringing the low this far
north hold.

Friday would see short wave ridging building in, and drying the area
out once again.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 715 PM EDT Fri Aug 8 2025

Late afternoon convection along a convergent boundary has begun to
wane as sunset approaches. We`ll see residual convective debris
aloft but low level cumulus will diminish this evening. VFR
expected through the period. South winds will increase after 12z
and become gusty.

&&

.MARINE...
Issued at 247 PM EDT Fri Aug 8 2025

A moderate to high swim risk will be present through the weekend
north of Grand Haven with south winds 15 to 25 knots. This will
build waves into at least the 2 to 4 ft range and possibly 3 to 5
feet. It`s possible that with the hot weather over the weekend, the
relatively cooler (70 degree) water temperatures may create a bit of
stability in the lower atmosphere which would subdue the higher end
of the wind forecast range and thus keep waves more so in the 2 to 4
foot range, which is still a moderate swim risk, as currents in the
water moving along-shore and out against south pier structures will
also exist.

Thunderstorms from Wisconsin could attempt to cross Lake Michigan
Saturday afternoon or evening. It`s likely they will diminish in
intensity as they do so, but could produce an area of gusty winds
and wind shifts in their wake.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement from Saturday morning through Saturday
     evening for MIZ037-043-050.
MARINE...Small Craft Advisory from 11 AM to 11 PM EDT Saturday for
     LMZ847>849.

&&

$$

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