666
FXUS63 KGRR 181928
AFDGRR
Area Forecast Discussion
National Weather Service Grand Rapids MI
328 PM EDT Fri Sep 18 2026
.KEY MESSAGES...
- Heavy Rain Likely Across Southwest Lower Michigan This Weekend
- Mainly Dry Next Week After Weekend Rain
- Cooler Than Normal Weather Much of the 7 Day Forecast
&&
.DISCUSSION...
Issued at 310 PM EDT Fri Sep 18 2026
- Heavy Rain Likely Across Southwest Lower Michigan This Weekend
We have gone ahead an issued a Flood Watch for Allegan, Barry, Van
Buren, Kalamazoo and Calhoun Counties. These are the counties that
we have more confidence in seeing the higher amounts (2-4 inches)
by Sunday morning. 1 to 2 inch amounts are likely outside of those
5 counties.
Aloft, a zonal upper flow will be in place through Sunday with
numerous shortwaves working east through the flow over the Great
Lakes region. Down at the surface weak low pressure will move from
the Central Plains into the Ohio Valley. An overrunning situation
sets up as south/southwest flow rides northeast over cooler air at
the surface over Lower Michigan. This lift will result in
precipitation that lasts the better part of 24 hours from near
midnight tonight to midnight Saturday night. The repeated training
of echoes over Southwest Lower Michigan will result in heavy
rain. Forcing will also be provided by the right front quad of an
upper jet to our north that will lead to a low level jet response
that stays fairly steady state (aimed at our area) for the better
part of 24 hours. PWAT values over Southern Lower Michigan will be
in the 1.75 to 2.00 inch range which is very high for this time of
year. Surface dew points will not be as high (in the 60s F), but
this is still high for this time of year as well. Bottom line...we
are looking at rounds of heavy rain that will likely lead to
ponding of water in low lying areas and urban centers as well.
Rises on area rivers and streams will occur. At this time we are
not expecting widespread river flooding in terms of reaching flood
stage but we will be monitoring closely.
- Mainly Dry Next Week After Weekend Rain
We are looking at mainly dry weather next week for the bulk of the
area as Canadian high pressure sags in from the north. The
question mark will be Southern Lower Michigan which may remain
just close enough to low level moisture as we sit beneath an upper
trough.
- Cooler Than Normal Weather Much of the 7 Day Forecast
Cooler than normal weather is expected through the period with
highs in the 60s much of the time. Rain this weekend and then
cooler air moving in at 850mb will be the reason for the cooler
weather. 850mb temperatures in the teens C this weekend will cool
into the single digit C range next week. Normal highs are in the
lower 70s for the most part and we should be below that much of
the next 7 days. It will feel like fall.
&&
.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 156 PM EDT Fri Sep 18 2026
Flight restriction probabilities:
- Through 00Z: 90 percent chance VFR all terminals
- After 06Z: 30 percent chance MVFR fuel alt ceilings (below 2 kft
AGL) at all terminals except LAN/JXN
Main additions this update:
- extra detail regarding precipitation after 06Z tonight
- new PROB30 group for heavy rainfall for western terminals (MKG,
AZO, GRR, BTL) centered around 12Z
We are seeing growing chances for locally heavy rainfall over
parts of southwest Lower MI Saturday morning, so reflected this
in the terminals with PROB30 groups featuring +TSRA. As noted
previously, we will see a thickening and lowering of cloud bases
through the next 12 to 18 hours. It is possible that later updates
will need to feature wind gusts more prominently, but that remain
highly uncertain at this point as it will depend on exact
placement of the heaviest rain bands.
&&
.MARINE...
Issued at 310 PM EDT Fri Sep 18 2026
We are entering a period of easterly winds on Lake Michigan from
this evening right through weekend. The result will be lower wave
heights, 0-2 feet near shore and 1-3 feet out towards the 5 mile
mark. Tonight wind will be east at 10 to 20 knots becoming
northeast at 10 to 20 knots for Saturday through Sunday. We feel
we should remain below Small Craft Advisory wind criteria, but
we will be watching gusts at the lakeshore sites for values into
the 22+ knot range. It will not be a great weather weekend for
those trying to hang on to summer, with a bit of a wind, some chop
and plenty of rain.
&&
.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Flood Watch from 2 AM EDT Saturday through Sunday morning for
MIZ064-065-071>073.
MARINE...None.
&&
$$
DISCUSSION...Duke
AVIATION...TJT
MARINE...Duke
NWS GRR Office Area Forecast Discussion
