706
FXUS63 KGRR 031731
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
131 PM EDT Thu Jul 3 2025

.KEY MESSAGES...

- Slight chances for showers or isolated storm today and July 4

- Holiday weekend heat

- Showers and storms possible Saturday night through Sunday

&&

.DISCUSSION...
Issued at 348 AM EDT Thu Jul 3 2025

- Slight chances for showers or isolated storm today and July 4

A weak front becoming oriented NW-SE and flow-parallel is mostly
stalled out over Lower Michigan today. It may have a better
pronounced moisture difference on either side of it than a
temperature difference. Some afternoon surface convergence over
southern portions of Mid Michigan today may assist in popping up
isolated showers. The threat of storms will be tempered by midlevel
height rises and warming temperatures around 10-15 kft / around 0
Celsius which will limit instability at and above that level.

For July 4, all of the latest HREF member models show spotty precip
over Western/Mid Michigan, worth adding some 20 percent PoPs /
slight chance showers or storm during the day. Elevated
thunderstorms may be ongoing in the morning over Wisconsin, fed by
southwesterly warm air advection. Some models take storms across
Lake Michigan during the morning but those solutions may be too
aggressive. Additional surface-based development may occur over Mid
Michigan during the afternoon within a zone of low-level
convergence. The day-by-day increasing low-level heat and moisture
may just counter-balance the upper-level ridging, which yields
more modest lapse rates and skinny CAPE, offering some chances of
localized showers/storms but again with a tempered overall threat.

- Holiday weekend heat

A building upper-level ridge is expected over the Midwest and Great
Lakes Fri-Sat, July 4-5. Temperatures near 90 on July 4 with dew
points in the mid 60s will yield a heat index in the lower to mid
90s. On Saturday, temperatures are favored to reach in the lower 90s
in most locations away from the Lake Michigan shore, and heat index
values may reach the mid to upper 90s. Whether or not this reaches
the traditional "heat advisory" criteria, the impacts of heat and
humidity will be at least of moderate concern for people active
outdoors or without air conditioning.

- Showers and storms possible Saturday night through Sunday

An upper-level trough and surface cold front passing through late
Saturday night through Sunday may touch off areas of thunderstorms.
While rain is not a guarantee everywhere, the high PW environment
with increasing wind shear may produce some heavy downpours along
with a marginal severe wind threat.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 127 PM EDT Thu Jul 3 2025

A weak frontal boundary bisects the Lower Peninsula this afternoon
and may be enough to pop a shower or weak storm yet this
afternoon. However, chances are low enough at the terminals to
preclude a mention in the TAFs. Otherwise, diurnal cu will
dissipate with sunset. Given increasing dewpoints, it`s possible
that some patchy early morning fog may develop for an hour or two
but will burn off quickly after sunrise.

&&

.MARINE...
Issued at 127 PM EDT Thu Jul 3 2025

High pressure over Michigan will drift east Saturday. Southwest
flow will increase as this occurs and waves will build to 3 to 5
feet north of Holland by Saturday afternoon. Swim risk will rise
accordingly with lowest swim risk near South Haven and highest
north of Muskegon. Prior to Saturday afternoon waves will aob 2
feet.

&&

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

&&

$$

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