239
FXUS63 KGRR 041211
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
811 AM EDT Fri Jul 4 2025

.KEY MESSAGES...

- Scattered Showers and Storms Possible Today

- Showers and Storms Possible Saturday Night into Sunday

- Hot Weekend, Then Slightly Cooler

&&

.DISCUSSION...
Issued at 400 AM EDT Fri Jul 4 2025

- Scattered Showers and Storms Possible Today

Despite upper-level ridging, there is about 500-1000 J/kg of
MUCAPE spreading into the area as warm air advection brings
pockets of higher moisture and a few clouds within the 850-800 mb
layer that is rather uncapped. Scattered showers and storms may
bubble up today in the vicinity of a stationary-turning-warm
front draped over West and Mid Michigan. HREF member models offer
fair confidence on a WSW-to-ESE moving threat area for storms this
morning through mid-late afternoon between western and south-
central portions of Lower Michigan, perhaps latching onto the
effect of mid-level wind and wave perturbations resulting from
the upstream convection over Wisconsin. By dusk this evening,
precip probability appears lower but still around 10 percent.

- Showers and Storms Possible Saturday Night into Sunday

Mid level wave and surface cold front arriving Sunday morning may
touch off scattered showers and storms. High PW would favor
locally heavier downpours where storms. Deep layer shear will be
lacking, though low level wind fields 30-35 knots and 0-3 km shear
potentially 20 knots would favor some potential for spots of gustier
winds.

- Hot Weekend, Then Slightly Cooler

Temperatures today may be kept below 90 if clouds and showers are
more prevalent, though dew points climbing to 65 to 70 would put
the heat index into the lower 90s. Saturday still looks hot and
humid, with temperatures in the lower 90s and a heat index in the
mid 90s. This does bring at least moderate concern for heat
stress for those vulnerable to heat, such as those active outdoors
or without air conditioning. Behind Sunday`s cold front, dew points
fall into the mid 50s to low 60s for Monday and Tuesday.

&&

.AVIATION /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 811 AM EDT Fri Jul 4 2025

The broken line of thunderstorms over Lake Michigan has become a
bit more cohesive and trackable, so MKG and GRR have a narrower
time window and higher confidence for vicinity thunder.
Maintained prob30 thunderstorm at AZO, BTL, LAN, JXN for later
this morning into mid afternoon. Mid-level instability is present
and this convection should persist in some form off its own
atmospheric perturbations. Each of the 00Z HREF member models
persist these showers and storms to various extents/intensity
through southwest/southern Lower Michigan today. The threats of
the day with any TS include lightning, IFR visibility with heavy
rain, LLWS and gusts around 25 knots.


&&

.MARINE...
Issued at 400 AM EDT Fri Jul 4 2025

Prevailing winds from the south today with 1-3 ft waves, exception
being if scattered thunderstorms develop over the lake later this
morning as some models have been apt to suggest. If storms occur,
they could produce wind shifts and gusts around 30 knots in their
vicinity.

Swim Risk becomes at least moderate for Grand Haven area and north
on Saturday as a breeze from the south strengthens and builds
waves to 2 to 4 feet. High Swim risk with 3 to 5 ft waves is
possible Saturday afternoon for some beaches in Oceana and Mason
counties.

Moderate to High Swim Risk is possible through Sunday morning at
many West Michigan beaches as winds veer more from the west.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Air Quality Alert until midnight EDT tonight for MIZ050-056-057-
     064-071.
MARINE...None.

&&

$$

DISCUSSION...CAS
AVIATION...CAS
MARINE...CAS