960
FXUS63 KGRR 161844
AFDGRR
Area Forecast Discussion
National Weather Service Grand Rapids MI
244 PM EDT Wed Sep 16 2026
.KEY MESSAGES...
- Showers and Slight Chance Thunderstorms into Thursday Morning
- Heavy Rain Possible Saturday
&&
.DISCUSSION...
Issued at 244 PM EDT Wed Sep 16 2026
- Showers and Slight Chance Thunderstorms into Thursday Morning
The chance for occasional showers and perhaps a thunderstorm
continues into Thursday morning, though the potential for heavy
rain should likely be contained south of southwest Michigan
tonight. Low and midlevel cloud cover and patches of light rain
stretch from southern Michigan into Iowa. A train of midlevel
mesoscale vorticity centers has been evident on radar and
satellite among these clouds along the effective midlevel front,
and will be the primary impetus for showers this afternoon. 850 mb
air parcels are modeled to be weakly unstable, though convective
tops may struggle to reach -10 C, which supports showers but with
a limited chance of lightning.
Tonight into Thursday morning, a southwesterly low-level jet
noses into Iowa and becomes more of a westerly LLJ over northern
Illinois and Indiana. Thunderstorm development will be focused
over this region along the effective midlevel front, as the
convection-allowing models have been suggesting. A few showers
may still develop over Lower Michigan from weakly unstable cloud
layers between 850 and 700 mb under a slight uptick in more
favorable upper- level jet dynamics. The ECMWF ensemble has been
most aggressive in providing a soaking rain to Lower Michigan (the
00 Z ensemble even gave GRR a median QPF of 1 inch, though the 06
Z run did back off some). However, the GEFS and CMC ensemble
system along with the CAMs have been keeping the soaking rains
south of Michigan, which I am more inclined to believe.
- Heavy Rain Possible Saturday
Late Thursday into Friday is favored to be dry as the front moves
farther south while surface high pressure scoots across northern
Ontario. Rain chances then become highly likely from late Friday
night into a portion of Saturday. The front well to our south and
southwest will surge back northeast as a warm front, approaching
southwest Michigan, with a 35 knot low level jet ascending up the
frontal surface. PWAT values around 2 inches and weak instability
will also be present.
The global ensemble systems have generally been producing median
QPF around 1 inch, with about 20 percent reaching 2 inches,
indicating a widespread soaking rain is likely along with swaths
of locally more than 2 inches where thunderstorms persist. There
will remain some question for now regarding where the axis of
heavier rain will fall, whether it be more so in central Michigan
or in southern Michigan, as it will depend on the latitude of the
low pressure development and its associated warm frontal surge,
and whether the thunderstorm complexes may preferentially
propagate farther south into the better instability compared to
where the global models place them.
&&
.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 116 PM EDT Wed Sep 16 2026
Conditions this afternoon range from IFR across southern Lower
Michigan to MVFR across central Lower Michigan. We expect a trend
towards IFR overnight as low clouds and some showers expand
northward. This is followed by slow improvement Thursday morning
as the bulk of the rain and low clouds moves off to the east.
&&
.MARINE...
Issued at 244 PM EDT Wed Sep 16 2026
Relatively light and generally northeast winds expected through
Friday will keep nearshore waves low. There is a slight chance of
a thunderstorm cell or two developing over the lake tonight. Small
craft advisory conditions are possible Friday night into Sunday
as winds strengthen with gusts to 25 knots. These winds would most
likely have an easterly directional component, keeping the larger
waves away from shore. There may also be thunderstorms over a
portion of the lake on Saturday, lightning being the primary
hazard.
&&
.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...None.
&&
$$
DISCUSSION...CAS
AVIATION...Ostuno
MARINE...CAS
NWS GRR Office Area Forecast Discussion
