929
FXUS63 KGRR 270555
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
155 AM EDT Fri Sep 27 2024

.KEY MESSAGES...

- Gusty winds Friday

- Rain chances Saturday through Tuesday

- Drier and cooler mid-week next week

&&

.UPDATE...
Issued at 922 PM EDT Thu Sep 26 2024

Cirrostratus covers much Lower Michigan east and south of
Ludington, a little farther northwest than the 12z HREF was
suggesting. Some models including the HRRR and ARW have been
trying to saturate the near-surface layer late tonight around
Lansing to Mount Pleasant, though this may be overdone as winds
just above the surface will increase to 15-20 knots which should
lessen the fog threat along with the high clouds. The only patchy
fog mention will be in northern Clare county (and to the north)
where winds are weaker and cirrus thinner.

&&

.DISCUSSION...
Issued at 228 PM EDT Thu Sep 26 2024

- Gusty winds Friday

The low level pressure gradient increases considerably tonight
and more so on Friday. Looking at Bufkit and mixing heights for
tomorrow, we may see gusts into the 35 to 40 mph range, especially
south of I-96. These values are supported by the latest ensemble
gusts from the various models. Such values commonly lead to
isolated power outages. No headlines at this time but we will
mention the stronger gusts in social media posts. It`s worth
noting that if we do mix slightly deeper than forecast, gusts
could start to reach into the advisory level, which will need to
be monitored with future forecasts.

- Rain chances Saturday through Tuesday

Run to run consistency remains high with regards to the main
players in the weather. The main players are the upper low
centered currently near the Bootheel of Missouri, and what is left
of Hurricane Helene as it gets absorbed by the upper low.

The majority of the weather with the upper low and remnants of
Helene is expected to remain south of Michigan as the upper low
wraps Helene to the WNW, and the upper high tries to hold on over
the area. The systems will be close enough that Southern Lower
will see occasional rain shower chances Saturday through Monday
with additional moisture moving overhead, and impulses on the
northern periphery of the low rotating from east to west across
Lower Michigan. Can not rule out some thunder late Saturday. The
better chance of some thunder will be Sunday and Monday afternoons
during peak heating.

The upper low will get a kick then by late Monday by another
stronger upper jet and upper trough digging into the middle
portion of the country. This trough will bring a fairly decent
fall cold front through Monday night/Tuesday. A few showers/storms
will be possible along the front, but there will not be good
moisture to interact with it, as it will be swept out with the
weekend low.

- Drier and cooler mid-week next week

Once the front moves through earlier on Tuesday, a decent amount of
cool fall air will surge in from the NW. Max temperatures can be
expected to be 10 degrees cooler or so for Wednesday into Thursday.
Strong subsidence building in over the area will assure dry weather
for Wednesday and Thursday.

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 155 AM EDT Fri Sep 27 2024

LIFR conditions in fog/stratus remaining north of the TAF sites
overnight. Otherwise clouds mainly aoa 20K ft today and tonight
with thickening cirrus canopy north of Helene overhead. East winds
increasing this afternoon with gusts to 25 kts as pressure
gradient tightens due to tropical system lifting rapidly north to
Kentucky. It looks like eventually late tonight or Saturday some
some showers and MVFR or even IFR cigs will make it into srn Lwr
MI, especially south and east of GRR, but that should hold off
until after 06Z Sat.


&&

.MARINE...
Issued at 228 PM EDT Thu Sep 26 2024

The pressure gradient will be on the increase tonight and more so
on Friday. The winds up around 925 mb are shown to increase to 40
knots or higher over the southern parts of the nearshore area
Friday morning. While we are not forecast to mix to that level
over the water, the winds will increase considerably. At this
time, most of the guidance supports gusts to 30 knots for the
southern half of the marine area, where we will issue a small
craft advisory. The main period for higher wind speeds is from 12z
Fri to 12z Sat. For the north half, it`s not until Fri evening
that the winds may reach into hazardous levels, so we will hold
off on any headlines there. With an offshore component to the
wind, the waves will be limited in height until closer the the 5
nm mark.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...Small Craft Advisory from 8 AM this morning to 8 AM EDT Saturday
     for LMZ844>846.

&&

$$

UPDATE...CAS
DISCUSSION...MJS/NJJ
AVIATION...Meade
MARINE...MJS/Thielke