976
FXUS63 KGRR 171157
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
757 AM EDT Fri Oct 17 2025

.KEY MESSAGES...

- Strong fall storm to affect the area this weekend

- Another system expected Monday into Tuesday

- Warm Today and Saturday then trending colder/more seasonal

&&

.DISCUSSION...
Issued at 405 AM EDT Fri Oct 17 2025

- Strong fall storm to affect the area this weekend

Some light rain is spreading across Central Lower Michigan this
morning associated with a push of warm air advection on the back
side of departing high pressure. This activity is very light and
will end this morning. Partly sunny skies are expected this
afternoon with filtered sunshine through high clouds.

Surface low pressure is moving into Northwest Ontario this morning
from the northern plains. The cold front associated with that low
will progress east slowly today into Saturday. The cold front will
move into Wisconsin Saturday. The front will be parallel to the
upper flow which will allow it to move slowly into the area.
Given the proximity to the approach of the front we expect to see
showers to spread into the area tonight and Saturday. We have
high pops to account for this on the order of 60-100 pct.

The more impactful weather looks to come Saturday night into
Sunday when deep low pressure forms in the Great Lakes. A complex
evolution of upper feature is set to take place which has led to
some model uncertainty the past few days. That uncertainty is
beginning to lower and we are zeroing in on a consensus. The main
players in this evolution are the upper trough over the northern
plains, mid level shortwave energy diving southeast on a strong
upper jet across the Northern Rockies and Plains and a wave moving
out of the desert southwest (CA) presently. These three mid/upper
level features phase over the Southern Great Lakes region Saturday
night into Sunday and cyclogenesis occurs along the cold front. On
Sunday the operational models deepen the low into the upper 980s
mb. This is certainly a strong fall storm. See the below hydro
sections for details on rainfall, but suffice it to say anomalous
moisture will be in place with PWATs near 1.5 inches, 850mb dew
points near +12C and surface dew points into the 60s.
Thunderstorms will be possible from Saturday into Sunday. With the
strongest LLJ off to our south and east severe weather is not a
concern.

- Another system expected Monday into Tuesday

Another strong system is forecast to affect the area from Monday
into Tuesday with a low traversing the Northern Great Lakes.
Additional rainfall is expected but it will be much lighter given
moisture levels well below the weekend system. We have the
potential to see some lake enhanced/lake effect rain given the
lake is around +17C right now. The GFS has -1C over our area on
Tuesday, so delta Ts will be in the upper teens C. Monday and
Tuesday will feel very fall like with showers and blustery
conditions.

- Warm Today and Saturday then trending colder/more seasonal

Today and Saturday will be the warmest days of the 7 day forecast
with temperatures into the 70s in spots across the CWA. We tumble
back to near normal values by the end of the period with highs in
the 50s mid next week.

&&

.AVIATION /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 756 AM EDT Fri Oct 17 2025

VFR weather is in place at the start of this TAF period and this
will continue to be the case through the day. Some light rain over
Central Lower Michigan will pivot off to the east this morning
leaving only mid and high clouds in its wake with bases above
10,000 feet. A cold front will approach from the west tonight and
spread rain into the area late. Rain will spread into Western
Lower Michigan in the 06z-09z time frame. VFR weather is even
expected with this developing rain through 12z. South winds today
will reach the 10-20 knot level with higher gusts at MKG
especially.

&&

.MARINE...
Issued at 405 AM EDT Fri Oct 17 2025

Looking into the details of the forecast for the next 7 days on
Lake Michigan it will surely look like fall on the Great Lakes.
The early autumn was fairly serene as compared to what we have
coming with a couple of upcoming systems. The gales of November
has its place in history, but October can bring it as well.

We start off with a Small Craft Advisory in effect from this
morning right through the day on Saturday. Much of the next 36
hours will feature a 20-30 knot wind out of the south. This wind
will be generated by a pressure gradient between a departing high
to the east and deepening low pressure moving into Northwest
Ontario from the northern plains. The largest waves will be
found from Holland to the north where 5-10 footers will be common
into Saturday afternoon.

There will be a brief lull in the wind and waves Saturday night as
the next much bigger event takes shape. Deep low pressure lifting
through the Great Lakes on Sunday will ramp winds up suddenly
Sunday morning. Northwest gales to 40 knots will slam into the
nearshore waters during the morning hours. Waves will
corresponding surge in height reaching 10-15 feet up and down the
lakeshore during the afternoon. Latest run of the WaveWatch3 is
showing waves peaking at 15+ feet in the afternoon between Grand
Haven and South Haven. The strongest winds and highest waves will
occur during the day on Sunday which is when the tightest
pressure gradient and strongest pressure rises sweep over Lake
Michigan. Coordinated with surrounding offices and the consensus
was to hold off on a Gale Watch at this point given its out in the
5th forecast period.

Another gale is likely Monday into Tuesday when another autumn low
sweeps through the Great Lakes. Waves will once again push to
double digit heights. Bottom line...much of the next 5 days the
big lake will not be a place for smaller craft. A couple of those
days even the lakers will be looking for an upwind shore.

&&

.HYDROLOGY...
Issued at 405 AM EDT Fri Oct 17 2025

We enter into a wet/rainy period in good shape with area rivers
well below normal. USGS streamflow conditions show most sites in
the 24th percentile or lower. So, we have room to put water into
the river systems. Most of the area is running at a precipitation
deficit over the past 30 days.

WPC QPF over the next 7 days is indicating the potential for a
widespread 1 to 2 inches of rain. The bulk of this rain will occur
on Saturday into Sunday and from Monday into Tuesday. The heaviest
may occur on Sunday in a deformation zone on the back side of a
deep low moving northeast through the Lower Peninsula. At this
time that swath of precipitation looks to occur in the GRR
forecast area. We will be watching Saturday night into Sunday
closely. We are outlooked for excessive rain via WPC.

In terms of the details we agree with the general 1 to 2 inch
widespread rain across the area. We will need to keep an eye on
the potential for totals exceeding 2 inches with the deformation
zone heavy precipitation band on Sunday. Looking at ensemble data
most of the rivers in the area are looking at substantial within
bank rises. Given the low flows at present we are not expecting
much if any in the way of mainstem river flooding. What we are
expecting is the potential for a few flashy sites like Holt on
Sycamore Creek and Jackson on the Grand to make runs at action
stage. Smaller creeks and streams have the potential to exceed
bankfull during the heaviest rain. Low lying area may also see
ponding of water Saturday night int Sunday especially.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...Small Craft Advisory until 8 PM EDT Saturday for LMZ844>849.

&&

$$

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