448
FXUS63 KGRR 180752
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
351 AM 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,
ll 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 /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 142 AM EDT Sat Oct 18 2025

This forecast period is starting out with mainly high clouds in
place at all of the terminals. Showers are starting to form over
Lake Michigan and are ongoing over Wisconsin. This axis of shower
activity with some isolated storms will arrive at KMKG by 11z or
so, and will continue to move across most of the rest of the
forecast terminals, except maybe staying north of KJXN. MVFR
conditions are likely, and there may be some periods of IFR with
the heavier showers/storms. This first batch will move out by
about 18z.

A few showers may linger, but there will be a relative lull in
the more widespread shower activity. Later in the afternoon and
into the evening, the I-94 corridor is likely to see additional
scattered showers and isolated storms develop. Then beginning
around 03z, widespread rain will move over the I-94 corridor and
spread north through 05-06z. IFR conditions are forecast to
develop in this widespread rain. Some isolated storms will remain
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 midsize 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 midsize tributaries mentioned above.
Nevertheless, with water levels currently running lower than normal
due to the drought, notable within-bank rises are expected across
most of the area. 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 Sunday morning through Sunday evening for
LMZ844>849.

&&

$$

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