263
FXUS63 KGRR 201555
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
1155 AM EDT Fri Sep 20 2024

.KEY MESSAGES...

- Showers/Storms Today, Sunny Saturday

- Likely showers and possible thunderstorms Sunday and Sun Night

- Unsettled next week, likely not as warm, some rain chances

&&

.UPDATE...
Issued at 1155 AM EDT Fri Sep 20 2024

A cold front is situated north/south over Eastern Wisconsin
extending southward into Northern Illinois. The front is
advancing our direction and will move out over Lake Michigan this
afternoon. Ahead of the front surface dew points have surged into
the middle 60s to around 70. A narrow ribbon of 850mb dew points
extends northward up Lake Michigan with values around +14C. The
ribbon of low level moisture (surface to 850mb) is narrow and
will advance into Western Lower Michigan this afternoon.

High resolution model data indicate that MUCAPE values will
increase into the 1000-1500 j/kg range in spots. DCAPE values are
already around 1000 j/kg which is fairly sizable for this time of
year. DCAPE to this level makes damaging winds the primary threat
today. There is enough low level curvature in model hodographs
that there will be some storm organization (0-1km SRH of 75
m2/s2). Deep layer shear (0-6km) is on the order of 30-40 knots
today. So, at max heating this afternoon, hail is also a threat
today, but a notch below the wind threat. We remain in a SPC
marginal and that seems like the correct category at this point.

HREF and machine learning model data suggest our western CWA, near
and west of highway 131 will be the main area in play today. This
is due to the fact that the moisture does not surge east until it
is well after sunset. Again, threat area is in the west and the
main time frame for strong to severe storms being from noon to
800pm. Within that time frame 300pm to 600pm will likely contain
our strongest storms. We are not expecting widespread severe
weather but some isolated to scattered wind/hail reports will
certainly be possible today.

&&

.DISCUSSION...
Issued at 340 AM EDT Fri Sep 20 2024

- Showers/Storms Today, Sunny Saturday

We finally have some rain in the near term forecast. Latest sfc
analysis shows a weak cold front over western WI slowly moving
east. Radar is showing a slow moving band of showers beginning to
move over the western part of Lake MI, then extending southwest
into northwest IL. Most of the lightning associated with this was
in far southeast WI and northern IL. As the front moves east today
into Lower MI, we`ll see rain chances increase quite a bit.

Moisture is progd to increase substantially just ahead of the
front this afternoon. Precipitable water values jump up to nearly
2 inches and surface dewpoints climb into the upper 60s as a
diurnally strong LLJ noses into the southwest cwa. Concurrently,
the left exit region of the upper jet will nose into the southwest
cwa this afternoon which will provide decent lift within the
column. More lift will be created by the short wave moving across
the lake this afternoon. Sfc-3km lapse rates are
impressive too...8c/km. SBCAPE values in the 1200-1700 j/kg range
coupled with bulk shear values 35-40 kts suggest a potential
severe threat as cells organize.

Forecast soundings show a very impressive inverted V below 750 mb.
Corresponding DCAPE values aoa 1200 point toward a wind threat.
SPC has expanded the marginal risk to include the western three
rows of counties along/south of I-96 and includes Grand Rapids,
Kalamazoo and Battle Creek.

The main threat window is from roughly 18-00z. Storms will
diminish as the front moves through.

Partly cloudy skies are expected tonight followed by sunny weather
Saturday. Little, if any, cooler air is expected behind this
front. Dewpoints in the mid 60s will result in humid conditions
Saturday.

- Likely showers and possible thunderstorms Sunday and Sun Night

Ensembles are consolidating around likely showers with some
thunderstorms centered between the latter part of Sunday through
Sunday night. An upper-level low will migrate from the Desert
Southwest to the Central Plains on Sunday. Ahead of it will be a
surface frontal zone from the Central Plains to the Western Great
Lakes, connecting to a northern-stream low over Northwest Ontario. A
weak surface triple-point low over Michigan on Sunday with
perturbations in the upper-level flow ahead of the Central Plains
low should serve as the focus for precipitation here as warm and
moist air in the low-mid levels is advected toward Michigan on the
back side of a southeast US high.

There is a large spread in the high temperature solutions for Sunday
depending on when the showers start, with a range between mid 70s
and mid 80s being plausible around Grand Rapids, more likely on the
warmer side farther southeast toward Jackson. Deep-layer moisture
will be present late Sunday and PWAT climbs to 1.5 to 2.0 inches,
making locally heavier rain amounts over 1 inch possible especially
if any instability can be realized. Area-averaged rain amounts look
to be around 0.5 inches.

- Unsettled next week, likely not as warm, some rain chances

Additional chances for rain may linger Monday night into Tuesday as
the main upper-level disturbance over the Central Plains on Sunday
moves toward the Great Lakes, though shearing out and weakening as
it gets incorporated into the northern stream. The ECMWF ensemble
offers more rainy solutions than the ensembles of the GFS and CMC.

From there, predictability in the day-to-day weather mid-late week
is rather low as a chaotic interaction between northern and southern
stream wave patterns unfolds over North America. The median
temperature solutions among the ensembles offers highs generally in
the lower 70s and lows in the 50s, which is closer to normal for the
time of year, perhaps slightly warmer than normal. The ECMWF
ensembles are tending cooler than the GFS and CMC ensembles, many of
them creating another blocking pattern with a large upper-level low
sitting over the Great Lakes. Some solutions among the GFS and CMC
ensembles offer more ridging and a warming trend in temperatures
later in the week. No matter what happens, there is quite a variety
of QPF solutions out there, anything from staying completely dry, to
having a day of heavier rain, to having multiple days of lighter
showers. So, unsettled is the word, but at least it`s more like
autumn.

&&

.AVIATION /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 725 AM EDT Fri Sep 20 2024

Patchy shallow fog in the vicinity of JXN and LAN should be less
of a factor with winds increasing this morning. VFR is likely
through the morning (16 Z). Light rain presently over Lake
Michigan will diminish as it heads east into drier air. After 16
Z, renewed shower development is likely with a fair bet of
scattered thunderstorms affecting most aerodromes for a few hours
at a time, starting earliest at MKG, and the time frame being
later in the afternoon the farther southeast you go. While
cumulonimbus cloud bases may be VFR today, the thunderstorms will
come with their typical hazards of IFR visibility within rain,
and some may also produce hail aloft and to the surface, and
possibly LLWS from isolated microbursts. LIFR ceilings/vis are
likely to develop later tonight from either fog or clouds a few
hundred feet above ground.

&&

.MARINE...
Issued at 340 AM EDT Fri Sep 20 2024

South winds will increase ahead of the cold front today. Waves
will respond by increasing to around 2 feet. It`s possible that we
could see waves up to 3 feet, but likely only briefly. The front
isn`t strong and winds on the back side of the front will be
fairly light.

Thunderstorms are expected along the front today, which will pose
a threat to mariners and beach goers.

&&

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

&&

$$

UPDATE...Duke
DISCUSSION...04/CAS
AVIATION...CAS
MARINE...04