212
FXUS63 KGRR 191736
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
136 PM EDT Sat Sep 19 2026

.KEY MESSAGES...

- Periods of rain through tonight, locally heavy at times

- Cooler period through the week with small chances of rain

&&

.DISCUSSION...
Issued at 326 AM EDT Sat Sep 19 2026

- Periods of rain through tonight, locally heavy at times

The rain event we have been focusing on for the last few days is now
underway. A long line of showers here, extends all of the way back
into Northern Iowa where storms are present. This line is the edge
of the elevated warm front where the low level jet is surging deep
moisture over it. Precipitable waters pushing 2 inches is at the
core of this moist air mass advecting in.

The rain here so far has been on lighter side with limited low level
moisture, and with the best forcing focused west of the area so far.
This rain is at the leading edge of the low level jet, and we will
see the more concentrated rain push in between now and sunrise.

There is plenty of thunder to our west, but we are not expecting
that to be as widespread as it is currently. Instability progs like
MU CAPE show the elevated instability gradient staying mostly SW of
the forecast area, except the far SW corner of the area. Severe
weather potential is quite low, however this area is most likely to
see the heavier convective bursts under the stronger updrafts and
highest pwats. We could end up needing some Flood Advisories with the
heavier bursts. This first batch should be pushing out of the area
by noon.

We will see the intensity of the rain diminish, but we do not expect
a complete break in the rain for the afternoon. The models are
indicating that some semblance of a low level jet will remain, just
not as strong of a core as this morning. This overrunning will
continue, and keep rain possible, even if it is just drizzle with
lower moisture becoming plentiful.

We expect the rain to increase in coverage and intensity once again
toward and after sunset as another low level jet core increases
ahead of the main sfc low. This lasts into the early overnight hours
before the elevated front/baroclinic zone, and low level jet move
south of the area. Again, not expecting much thunder with elevated
instability fairly weak. Heavy rain is once again the main threat,
especially after the rain from earlier priming the ground and
rivers/streams. The highest potential for this is where the Flood
Watch is across the SW corner of the forecast area. River forecast
points will also see good increases.

There has been some uncertainty whether Sunday sees residual rain or
not. It looks like the best chance of any rain beyond sunrise on
Sunday would be early, and would move out quickly as the front sinks
south of the area. Clouds are likely to hold in most of the day.

- Cooler period through the week with small chances of rain

This wet system for this weekend will move out, but we are not
looking at any extended dry period behind it. We will see a weak
short wave ridge pass overhead on Monday, which may keep all of the
area dry.

We quickly see an upper low complex over the Upper Midwest rotate
over the area, and merge with additional upper level energy to
become a bigger complex. This complex then looks to sit over the
area from Tuesday through the end of the week.

Timing and exact track of any weak short waves rotating around it
will be problematic as is the case always with upper lows. What we
do know is that is will keep a cooler air mass over the area, likely
keeping temperatures in the 60s. Timing of showers will be a little
more favorable in the afternoon hours when some diurnal instability
can help with the development. This looks to hold pretty much
through Friday, before the larger upper ridge can push this system
east of the area.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 136 PM EDT Sat Sep 19 2026

Low clouds and occasional drizzle to continue for the next 4-6
hours before more widespread precipitation begins to move in. IFR
and LIFR conditions will be common through the taf period. Some
thunder possible...especially KAZO/KBTL/KLAN/KJXN with KGRR and
KMKG less likely to see sustained thunderstorms. Some improvement
in conditions possible very late in the TAF period...but not
confident enough now to introduce it except for KMKG.


&&

.MARINE...
Issued at 326 AM EDT Sat Sep 19 2026

We continue to watch the winds and waves today through this evening,
but will continue with no headlines at this time for today. Offshore
flow will dominate with winds staying just below criteria for today.
Limited mechanical mixing today due to the clouds and rain will keep
winds in check on this side of the lake.

Sunday looks like a better shot of needed a Small Craft Advisory,
mainly due to the winds. The flow will remain offshore, but will
increase enough to likely need a Small Craft Advisory with winds up
around 30 knots. Waves will remain 1 to 4 feet with the limited
fetch.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Flood Watch through Sunday morning for MIZ064-065-071>073.
MARINE...None.

&&

$$

DISCUSSION...NJJ
AVIATION...Maczko
MARINE...NJJ