623
FXUS63 KGRR 181148
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
748 AM EDT Mon Aug 18 2025

.KEY MESSAGES...

- Showers and storms from this afternoon through Tuesday night

- Dry and less humid for Wednesday through Friday

- Cooler with a few showers possible next weekend

&&

.DISCUSSION...
Issued at 400 AM EDT Mon Aug 18 2025

- Showers and storms from this afternoon through Tuesday night

We will likely see a few showers try to sneak into Western Lower
Michigan off the big lake today, but the main push will come
tonight. The low level jet is very weak today near and upstream of
our area. Tonight it ramps up in speed and pushes into our area.
We have 20-30 pct chances for precipitation in the west today,
increasing to 60-80 pct tonight. PWAT values increase to in excess
of 2.00 inches tonight which is near the daily max via sounding
climo via SPC. Locally heavy rain is the main threat from storms
tonight. We are in a general thunder outlook from the SPC both
today and on Tuesday. Severe weather is not a big concern given
weaker shear and MUCAPE values that are rather modest by August
standards. The main surge of showers and storms will come through
tonight into Tuesday morning between 06z and 18z.

- Dry and less humid for Wednesday through Friday

We look to be drier and less humid for the latter half of the work
week as a high pressure settles into the area from the north. High
temperatures will mainly be back into the lower 80s.

- Cooler with a few showers possible next weekend

A bit of a pattern changes is in store for the weekend as a cold
front moves through Friday night into Saturday. We could see a few
showers associated with that frontal passage. Even cooler air will
flow in from the north out of Canada with highs slipping back into
the 70s for Sunday and next Monday. We are pushing into late
August and early fall cold fronts are certainly in the cards this
time of year. We have the +6C isotherm at 850mb near the northern
CWA next Monday night and a small area of 0C 850 air northeast of
Lake Superior at the same time.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 748 AM EDT Mon Aug 18 2025

VFR weather is expected today as cumulus clouds fill in. Bases
should around 5,000 feet. Showers and thunderstorms will move in
from the west tonight impacting all TAF sites by 12Z on Tuesday.
KMKG will be impacted first with activity becoming likely around
03Z, with showers/storms moving east the remainder of the night.
Winds today will be east to southeast at 5 to 10 knots.

&&

.MARINE...
Issued at 400 AM EDT Mon Aug 18 2025

Current Small Craft Advisory looks to be in good shape. The Port
Sheldon buoy is gusting to 25 knots as of 300am. The wind is off
shore, so the highest waves will be found at the far west portion
of the nearshore marine zones which is 5 miles out. Waves will
peak in the nearshore zone before daybreak around 2 feet out at
the 5 mile mark. The 800am end time looks fine as the core of wind
is nocturnal and will be fading as we approach that time.

Otherwise, a similar setup occurs tonight as a southerly nocturnal
low level jet occurs with a max speed around 25 knots. The 3km
NAM and the ARW are the most bullish on SCA criteria off shore
winds. At this point will hold off to avoid double headlines and
to see if the HRRR comes in line with the other two models because
as of now it is not.

Next item of note is the potential for our "advancing high" setup
on Tuesday afternoon and evening which could produce Small Craft
and Beach Hazards headlines potentially in a developing north
flow.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...Small Craft Advisory until 8 AM EDT this morning for LMZ845>848.

&&

$$

DISCUSSION...Duke
AVIATION...Duke
MARINE...Duke