651
FXUS63 KGRR 181735
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
135 PM EDT Sat Oct 18 2025

.KEY MESSAGES...

- Periods of Showers/Storms Through Sunday with Locally Heavy Rain,
Windy Sunday

- Another Period of Rain and Wind Monday Night through Tuesday

- Unsettled Weather Wednesday through Friday

&&

.DISCUSSION...
Issued at 351 AM EDT Sat Oct 18 2025

- Periods of Showers/Storms Through Sunday with Locally Heavy Rain,
Windy Sunday

No noteworthy changes to the forecast are needed compared to
yesterday`s day shift forecast. A few light showers have made it to
the NW corner of our forecast area this morning. Much more
widespread showers and scattered thunderstorms are currently over
the SE half of Wisconsin as of 3 am EDT this morning. This area of
rain seems to be directly correlated to the 40+ knot low level jet
centered over that area.

These showers and storms are expected to move over our area toward
and just after daybreak as the low level jet axis shifts eastward
over the area. The showers and storms over IA are likely the area
that will move overhead. The storms with this should not initially
be capable of much beyond rain and lightning as these storms are
elevated in nature. They could strengthen a bit before moving out as
they may be able to tap some additional instability that may build
with some diurnal heating.

We expect additional showers and storms to develop this afternoon
with MU CAPEs building to 500+ J/kg. This will likely be across the
SE corner of the forecast area as some of those areas may just miss
out on the morning convection, and some better instability will
build. In addition to better instability, it will become more sfc
based, and it will have good deep layer effective shear values of 40-
50 knots, and some decent low level shear values with 30 knots of
the low level jet remaining over the SE corner of the area. Can not
rule out some localized wind with DCAPEs 700-800 J/kg, or some small
hail with mid level lapse rates around 6.5-7 C/km. The stronger deep
layer shear could help some of the storms rotate aloft if enough
instability can sync up with the stronger shear.

We will then watch the heaviest of the rain move in by late evening,
and continue through a good portion of the night. The sfc low will
be driven by the long wave trough coming in. At the same time, we
will see a short wave ride up along the front, and induce a nice
band of FGEN just NW of the sfc low due to the interaction. This
plethora of forcing will be acting on an atmosphere that will have
200-250% of Precipitable water values normal for this time of the
year. This looks to squeeze out a good 1-3 inches, with the heaviest
axis between GR and Lansing. Please see the Hydrology discussion
below for more details.

The heavy rain will move out by Sunday morning, but we are looking
at another full day of steady rainfall to occur over the area. This
will be the case with the long wave trough coming in and interacting
with the residual moisture..

- Another Period of Rain and Wind Monday Night through Tuesday

After a break Sunday night and much of Monday, we are looking at
another period of showers developing Monday night. This system will
be coming in from the NW, vs. the system that will be coming in from
the SW tonight. Initially moisture will be limited coming in from
the NW. A long trajectory of low level flow will begin tapping Gulf
moisture once again, and will be absorbed by low. The winds with
this system will be somewhat similar in magnitude with a strong
pressure gradient developing on the stronger sfc low.

The difference with this system is it will drag a bit more of colder
air in over the relatively warmer waters of Lake Michigan. 850 mb
temperatures will be dropping to as low at -2C, yielding delta t`s
in the mid to upper teens. This scenario will likely kick up  some
lake effect rain showers for the lakeshore counties with a flow from
the NNW.

- Unsettled Weather Wednesday through Friday

Wednesday will see some residual lake effect rain showers earlier,
before they should diminish through the day. Some weak ridging will
build in for Thursday. The long wave flow will feature more
troughing over the region lingering from the systems passing
through. We could see another short wave dive in for Friday,
bringing another threat of rain showers.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 135 PM EDT Sat Oct 18 2025

VFR conditions will prevail this afternoon except for the
potential for heavier showers and scattered thunderstorms bringing
briefly lower ceilings and visibilities in heavy rain late this
afternoon across southeast Lower Michigan from LAN to JXN. Then
steady rain will move in from the south tonight with conditions
falling to IFR after midnight and some patchy LIFR by 12Z. Winds
will be south into this evening around 10 knots them go northwest
on Sunday and gust over 20 knots by the afternoon with some gusts
over 30 knots possible.

&&

.MARINE...
Issued at 351 AM EDT Sat Oct 18 2025

We will be upgrading the Gale Watch to a Gale Warning for Sunday and
Sunday evening. Gales (34+ knots) look to be a certainty on the
backside of the low as it departs Sunday morning. The highest winds
will likely be Sunday morning as the colder air on the backside of
the system rushes in, and coinciding with the pressure fall/rise
couplet.

Winds and waves will subside a bit for Monday, before coming up
significantly on Tuesday with the next system. This system looks to
bring Gales to the nearshore waters also. Obviously a bit of time to
monitor that.

&&

.HYDROLOGY...
Issued at 318 PM EDT Fri Oct 17 2025

Significant rainfall is expected over the next several days across
much of Lower Michigan. The total amount of rain we are expecting
continues to slowly nudge upwards as forecast details come into
focus. The rain will be spread out over several rounds, primarily
spanning Saturday and Sunday, with the heaviest rain currently
expected before/during sunrise on Sunday. Overall averages across
much of the area will probably end up in the 1 to 2 inch range, with
some multi-county streaks of 2-3"+. The good news is that because
we`re in a drought, the rivers have plenty of room to put this
water. Some of the midsized rivers in the area like Sycamore Creek
(Holt), Buck Creek (Grandville), and the Looking Glass River (Eagle)
are the most likely to see rises close to bankfull, but no river
flooding is expected unless this storm seriously overperforms our
expectations. It would probably take 4"+ over a fairly large area to
begin flooding the midsized tributaries mentioned above.
Nevertheless, with water levels currently running lower than normal
due to the drought, notable within-bank rises are expected across
minor flooding develop in the classic low spots and other poor
drainage areas. It`s hard to pin down which urban areas would be
most at risk of this happening, but recent model trends suggest the
eastern edges of our area are most at-risk, which would include
Lansing and Jackson areas. Anyone with personal property near the
waters edge leftover from the summer should prepare for rising
water.

While river flooding isn`t a big concern, if some slow-moving heavy
rain bands park over some of our urban areas we could see some
localized minor flooding develop in the classic low spots and other
poor drainage areas. It`s hard to pin down which urban areas would
be most at risk of this happening, but recent model trends suggest
the eastern edges of our area are most at-risk, which would include
Lansing and Jackson areas.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...Small Craft Advisory until 8 PM EDT this evening for LMZ844>849.
     Gale Warning from 8 AM to 11 PM EDT Sunday for LMZ844>849.

&&

$$

DISCUSSION...NJJ
AVIATION...Ostuno
MARINE...NJJ
HYDROLOGY...AMD