982
FXUS63 KGRR 021743
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
143 PM EDT Tue Jul 2 2024

.KEY MESSAGES...

- Chance of Showers and Scattered Thunderstorms Through Wednesday

- Risk for Stronger Storms Friday

&&

.UPDATE...
Issued at 1100 AM EDT Tue Jul 2 2024

No major changes to the forecast. Showers continue to move into
the region from the west with them moving over a dry air mass
which should limit most rain to north of the I 96 corridor. Latest
CAMS do have some increase POPS possible along and just to the
south of the I 96 corridor this afternoon. Waves along the lake
are currently low along and south of Muskegon. Winds and waves
are expected to increase north of Muskegon this afternoon into the
evening hours.

&&

.DISCUSSION...
Issued at 335 AM EDT Tue Jul 2 2024

- Chance of Showers and Scattered Thunderstorms Through Wednesday

Warm advective pattern over Lower Michigan resumes on the west
side of the departing high pressure area, while a weak surface low
associated with an upper-level shortwave trough will organize
over Iowa and move toward Northern Michigan tonight. As midlevel
moisture streams in today, altostratus clouds will thicken and
occasionally produce precip amid isentropic lift and perhaps some
weak and shallow instability layers. The lower atmosphere is still
fairly dry so will tend to reduce the precip to sprinkles south
of I-96. North of Muskegon to Mt Pleasant has the better chance of
measurable rainfall during the day.

Continued 850-700 mb moisture advection tonight and slightly
cooler air above may yield instability deep enough for scattered
thunderstorms, especially NW of Grand Rapids. HREF member models
overall are not producing much convection given the limited
instability and lifting mechanisms. If storms do occur, forecast
soundings are still fairly dry below 5,000 feet, and with low-
level winds just above the stable surface layer increasing to 40+
knots, there is potential for locally gusty conditions early Wed.

On Wednesday, the surface trough / weakening cold front will
progress east of most of the CWA, leaving the chance of afternoon
convection development constrained to near and south/east of
Jackson.

- Risk for Stronger Storms Friday

The ECMWF continues to advertise a stronger southwesterly upper
level jet streak arriving here in MI during the day on Friday. A
digging 500 mb trough forms into a closed low as is tracks
northeastward through the state. The surface pattern shows a weak
warm front extending eastward through the CWA midday Friday before
lifting northeast of the CWA later in the afternoon. Deep layer
shear and surface based instability both show favorable setup for
at least a period of organized storms as the instability axis
shifts northeast through the region during the early to mid
afternoon hours on Friday. The GFS is similar to the ECMWF with
the setup, although it is several hours slower with the warm front
and instability axis shifting through the region and the Canadian
is even slower than the GFS. Thus, uncertainty exists with the
timing, which will be important when determining how much if any
impact the region see`s from the storms. The various model
ensemble qpf output shows considerable spread in the values
amongst the members so that adds the the uncertainty. For now, we
will continue to feature higher POPs for Friday throughout the CWA
as most ensemble members are showing measurable values.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 143 PM EDT Tue Jul 2 2024

Our focus of this forecast period will be on trends of shower
activity, and any associated low clouds/fog that will be
associated with them.

The first main batch of rain is affecting mainly KMKG at press
time, with lots of rain not reaching the sfc elsewhere further
south and east. Dry air below 8-12k ft agl is holding on at all
of the other sites where the rain is not as heavy. We expect a few
light showers/sprinkles at the other sites, and will account for
this with some VCSH/p6sm -shra wording.

There will be a lull in the widespread activity then this evening
and early overnight. A core of winds from the SW around 40 knots
at 2k ft at the northern terminals and lighter winds at the sfc
will produce enough low level wind shear to mention. The southern
terminals will not see as much wind aloft.

Additional rain showers will move in later tonight from west to
east ahead of a front. The lower levels are expected to moisten a
bit, and allow more rain to reach the ground everywhere. We do
anticipate that all sites should see MVFR conditions late tonight
and Wednesday morning. Clouds will lift toward and just after the
end of this forecast period Wed afternoon.

&&

.MARINE...
Issued at 335 AM EDT Tue Jul 2 2024

The small craft / beach hazards threat today and Wednesday will
come in two parts. Will issue advisory/statement for the first
part today and adjust them later on if needed for Wednesday.

South-southeast winds over Lake Michigan strengthen late this
morning to mid afternoon as a pressure gradient briefly tightens
between relatively higher pressure ridging over southern Michigan
and lower pressure troughing over the UP. Frequent gusts 25 to 30
knots are most favored north of Whitehall, according to the varied
solutions of the short-term models. There may be a few hours when
winds become hazardous to small craft between Whitehall and Grand
Haven but this is less certain. Given the southeast component of
winds and thus limited fetch, the high risk of hazardous waves and
currents for swimmers may stay focused around Whitehall to Little
Sable Point and also Ludington State Park and Big Sable Point,
while a moderate risk extends south from Whitehall to Grand Haven.
The risk may coincide with the arrival of rain showers.

Winds veer southwest early Wednesday morning in the vicinity of a
cold front with scattered showers and thunderstorms that may
sporadically increase the gust potential. Hazardous conditions for
swimmers and small craft may develop along much of the West
Michigan shoreline pre-dawn Wednesday into mid day.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement until 8 PM EDT this evening for MIZ037-
     043.
MARINE...Small Craft Advisory until 8 PM EDT this evening for LMZ847>849.

&&

$$

UPDATE...Ceru
DISCUSSION...MJS/CAS
AVIATION...NJJ
MARINE...CAS