411
FXUS63 KGRR 271846
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
246 PM EDT Fri Sep 27 2024

.KEY MESSAGES...

- Showers Move in Saturday

- Lingering Chances for Light Rain Sunday-Tuesday

- Drying Out for Wednesday Through Friday


&&

.DISCUSSION...
Issued at 244 PM EDT Fri Sep 27 2024

- Showers Move in Saturday

Tropical Depression Helene is in the process of getting absorbed
into an upper low presently across KY/TN. The pressure gradient
force between this system and a high across the Upper Great Lakes
will lead to gusty winds on the order of 30-40 mph across the
region, especially in the 8pm-2am time frame tonight near and
south of M-46. The northern periphery of Helene`s rain bands will
likely make it as far north as southern Lower Michigan by
Saturday. GFS/ECMWF RH layers indicate saturating conditions in
the 925-700mb layer with PWAT values climbing to around 1.75",
quite high for late September. Rain showers will initially develop
in the early morning, moving from SE to NW, but as deep E/NE flow
takes hold, a more east to west banding is likely.

Model guidance is depicting narrowly forced rain bands, likely due
to the limited geographical breadth of deeper moisture overhead that
coincides with relatively weak frontogenetical forcing on the
northern periphery of a decaying Helene. All this to say that rain
amounts will be quite variable, with most areas less than 0.25" but
a few distinct, very narrow bands that could produce 0.50"-1.00" or
so, per the 12z HREF 24 hr LPMM through Saturday evening. The
northernmost extent of rain will likely halt somewhere near I-96,
so it is from that corridor to the south where the greatest
likelihood for showers will be. The highest amounts will likely
fall near and east of a AZO/LAN line. The Nam 3km and HRRR are
showing 100-300 J/kg of MUCAPE rooted above the 700mb layer where
a general lack of saturation is present, thus the threat for
thunderstorms is quite low.

- Lingering Chances for Light Rain Sunday-Tuesday

Low level moisture and precipitable water values around an
inch linger over the area into Tuesday. No one day will be a washout
but any slight amount of lift will result in drizzle or rain
showers. Monday`s chances are better in the afternoon as we see some
diurnal instability develop with mean CAPE values of 100 to 200
J/kg. Tuesday`s chances are better as the front moves through though
there is some uncertainty at the exact timing as seen in the
ensembles.

- Drying Out For Wednesday Through Friday

Behind the front Tuesday drier air filters in with northwest flow
becoming more geostrophic for the end of the work week. Temperatures
will be cooler as well with highs in the 60s and lows in the 40s.
With the cooler temperatures and partly cloudy to sunny skies,
weatherwise it will be a fitting start to October.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 128 PM EDT Fri Sep 27 2024

Gusty northeast winds around 25 to 30 knots are expected this
afternoon with gusts around 20 to 25 knots into Saturday. Ceilings
will gradually lower overnight and scattered showers work their
way into the area from the south. By 12Z expect MVFR ceilings
along and south of a BIV to MOP line with AZO to LAN and southward
IFR. Scattered showers are also more likely along and south of an
AZO to LAN line with BIV to GRR to AMN on the edge of seeing rain
or no rain.

&&

.MARINE...
Issued at 244 PM EDT Fri Sep 27 2024

Winds over the nearshore waters are expected to gust into the
25-30 kt range across the SCA area near and south of Grand Haven,
lasting into at least Saturday morning. A possible extension may
be needed into Saturday afternoon depending on the persistence of
the higher winds. Given the ENE flow, the highest wave action will
be south of South Haven. Conditions do look to improve later
Saturday into Sunday.

&&

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

&&

$$

DISCUSSION...Hoving/RAH
AVIATION...RAH
MARINE...Hoving