676
FXUS63 KGRR 050528
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
128 AM EDT Sat Oct 5 2024

.KEY MESSAGES...

- Areas of frost in central Michigan tonight

- Chance of shower or thunderstorm late Saturday night

- Mainly dry and cool next week

&&

.UPDATE...
Issued at 940 PM EDT Fri Oct 4 2024

A batch of thicker-looking cirrus clouds is currently impacting
northern and central lower Michigan. These clouds may slow/hold
back radiational cooling over the next few hours where the Frost
Advisory is in effect. Nevertheless our forecast low temps and the
Frost headline still look fine since high level RH progs show
these clouds exiting to the southeast after 06Z at which time the
more ideal radiational cooling will occur. That`s plenty of time
for temps to tumble into the mid 30s near/north of U.S. 10.

&&

.DISCUSSION...
Issued at 342 PM EDT Fri Oct 4 2024

- Areas of frost in central Michigan tonight

A center of high pressure at the surface is progressing west to east
and will be over Lower Michigan during the middle of the night.
Clear skies and calm winds particularly in interior central Michigan
will allow for good radiational cooling. The air in the north is
drying this afternoon also. MOS guidance is lower than raw short
term guidance but does suggest temperatures in the lower-mid 30s in
the typical cold spots near/north of US-10. This looks similar to
Wednesday morning when a few spots there fell to about 32.

- Chance of shower or thunderstorm late Saturday night

A robust trough and closed low at 500 mb will quickly translate
eastward over northwest Ontario Saturday night. The base of the
trough with its associated vorticity and upper-level jet stream will
be over Lower Michigan very late in the night. A strong pressure-
gradient driven low-level jet will develop overnight, the core of
which will be over northern Michigan. In the low levels, a surface
cold front will pass through early Sunday morning.

The main question will be whether elevated showers/storms will
develop as far south as I-96 or if they will be focused in northern
Michigan. Being mostly in the divergent part of the low-level jet
may limit our chances somewhat. However, lapse rates will be fairly
steep in the mid to upper levels with moderate RH, so any layers
where saturation occurs, most likely around 700 mb or 10,000 feet,
may be able to overcome weak inhibition and realize up to 1000 J/kg
of CAPE.

If storms do develop this far south, the bulk of the CAPE being in
the hail growth layer a decent amount of ambient RH may support hail
production, though shear in the cloud-bearing layer may be limited.
Closer to the ground, temperatures will be mostly stable, limiting
mixing and buffering weaker downdrafts. However, 40 to 50 knots at
1000 to 3000 feet means this will have to be watched closely if a
stronger thunderstorm core does develop. Even if no thunderstorms
develop this far south, gusts over 40 mph may mix down from 1000
feet during a few hour period during the night, off the lake marine
layer, affecting areas within a few miles of Lake Michigan between
Holland and Ludington.

- Mainly dry and cool next week

The weather pattern for the region will be dominated by NW flow
aloft for the entire period from Sunday through next Friday.

Initially, we will be under the influence of an upper trough Sunday
through Tuesday. The core of the cooler air aloft will stay just
north of the area. This should keep the cold air cumulus and rain
shower chances mainly up there. The models are hinting at a little
stronger short wave diving SE over the area centered around the
Monday night time frame. This wave will have little to no moisture
with it, so not expecting much from it. Highs will mainly be in the
60s with 850 mb temps in the lower single digits C.

We will see the upper trough hanging over the area early in the week
shift just a little east of the area by Wednesday. This will put the
area under the control of confluent flow aloft downstream of the
upper ridge axis. High pressure at the sfc will remain overhead
through early Thursday, before a warmer return flow sets up on the
backside of it by late Thursday and Friday. Highs are likely to warm
back into the 70s by week`s end with dry weather continuing.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 128 AM EDT Sat Oct 5 2024

VFR conditions will continue through Saturday evening. Southeast
winds around 10 knots today with gusts over 20 knots in the
afternoon. LLWS will increase to 45 to 55 knots late Saturday
evening.

&&

.MARINE...
Issued at 342 PM EDT Fri Oct 4 2024

Have issued a gale watch for the possibility of a few hours of 35 to
40 knot gusts in the nearshore waters north of Holland Saturday
night. Sustained winds 25 to 30 knots appear likely and 35 knot
winds won`t be too far aloft, though mixing may be somewhat limited
by a stable temperature lapse rates. That is to say nothing of the
chance of a passing thunderstorm which might develop around that
time. Regardless of any of that, small craft advisory conditions are
likely everywhere between Saturday night and Monday morning.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Frost Advisory until 8 AM EDT this morning for MIZ038>040.
MARINE...Gale Watch from this evening through late tonight for LMZ846>849.

&&

$$

UPDATE...Meade
DISCUSSION...NJJ/CAS
AVIATION...Ostuno
MARINE...CAS