684
FXUS63 KGRR 062318
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
718 PM EDT Mon Oct 6 2025

.KEY MESSAGES...

- Showers and chance of thunderstorms tonight

- Frost/Freeze conditions for mid-week

&&

.DISCUSSION...
Issued at 253 PM EDT Mon Oct 6 2025

- Showers and chance of thunderstorms tonight

The best chance of rain since Sep 23, or really for many, the
first rain in nearly two weeks is highly likely this evening
through early Tue morning. Increasing upper-level troughing and
jet dynamics, increasing low-level moisture, and a touch of
convective instability will help develop batches of rain showers
and a chance of a few thunderstorms along and behind a cold front.
This will end the late stretch of 80-degree weather and usher in
more seasonable or somewhat cooler-than-normal air, for the rest
of the week.

Initially 250 J/kg of MLCAPE will be present this evening with
potential cloud tops around -10 C and effective shear around 30
knots. If an isolated, more robust shower develops amid the
cumulus field south and east of Grand Rapids later today, it could
produce isolated gusty winds greater than 40 mph. More numerous
showers and a few thunderstorms are expected after dark as MUCAPE
in some models climbs closer to or above 500 J/kg and cloud tops
extend into air colder than -10 C. Wind shear in the lower levels
will still be present but lessening Downdraft CAPE will diminish
the potential for gusty winds going into the night. However,
decent effective shear through the convective layer and even some
clockwise-curved hodographs could support some embedded linear or
mesocyclonic (rotating) storm organization for a few hours after
nightfall that could pose a non-zero severe weather threat.

For rainfall, global model medians and the NBM deterministic are
generally in the ballpark of 0.25 to 0.50 inches for much of the
area, though the 00z and 12z HREF local probability-matched mean
looks more realistic. It shows a belt of higher rainfall favored
between South Haven to Holland, Hastings to Grand Rapids, and
Lansing to Alma, with narrow swaths of 1 to 2 inches possible.
Areas closer to Ludington and Jackson are more favored to end up
on the lower end, with amounts as low as 0.10 inches.

- Frost/Freeze conditions for mid-week

Wed and Thu are expected to bring the first appearance of a
cooler-than-normal air mass since the beginning third of
September. Frosty conditions Tue night / Wed morning are likely in
Clare, Osceola, Lake, and Mecosta counties but limited to the
locally colder locations elsewhere. Then with high pressure more
centered over Michigan Wed night / Thu morning, frost is a good
bet in most locations except for near Lake Michigan. Freezing
temperatures at 2 meters Wed night are greater than 70 percent
probability north of Big Rapids and Clare, and around 20-40
percent near and between M-66 and US-127 from Montcalm/Isabella
counties down to Calhoun/Jackson, per the NBM.

You can look up the average first fall freeze date for your
county at the Midwest Regional Climate Center...
mrcc.purdue.edu/freeze/freezedatetool

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 718 PM EDT Mon Oct 6 2025

A complicated aviation forecast is expected over the next 24
hours. Initial concerns revolve around shower and thunderstorm
development associated with a cold front crossing the area this
evening. Some initial convective activity will be moving on shore
and may clip the vicinity of GRR given its current path. The HRRR
seems to be handling thunderstorm activity best at the moment and
favors this. Additional showers, with isolated thunderstorms, will
then persist through the night into Wednesday. Additional
thunderstorm activity is possible through 09 UTC howeveer timing
and confidence is to low to include at any terminal. Locally
heavy rainfall and gusty winds are possible in any thunderstorms.
After 03-04 UTC post frontal stratus will cause ceilings to fall
to IFR areawide with a low chance of LIFR conditions south and
east of GRR. Ceilings will slowly improve Tuesday morning into
Tuesday afternoon.

&&

.MARINE...
Issued at 253 PM EDT Mon Oct 6 2025

Waves are beginning to subside this afternoon so the ongoing
Small Craft Advisory should be able to expire as scheduled late
this afternoon. Thunderstorms are possible over the waters this
evening, with locally higher winds and waves possible in any
thunderstorm. A period of enhanced northerly flow around 15-20
knots is expected tonight into Tuesday which may cause waves and
gusts hazardous to small craft south of Holland. Given the ongoing
advisory near and north of Grand Haven, and marginal nature of
any winds and waves, will defer any headline issuance until a
subsequent package. High pressure then moves in for Wednesday
which should preclude the need for headlines for the latter
portion of the week.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...None.

&&

$$

DISCUSSION...CAS
AVIATION...Thomas
MARINE...Thomas