325
FXUS63 KGRR 241722
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
122 PM EDT Mon Jun 24 2024

.KEY MESSAGES...

- Mostly sunny today, stormy late tonight/Tuesday morning

- Medium/High confidence in fair weather Wednesday and Thursday

- Medium confidence in showers and storms late this week

&&

.DISCUSSION...
Issued at 322 AM EDT Mon Jun 24 2024

- Mostly sunny today, stormy late tonight/Tuesday morning

Every model run of the GFS since last Wednesday has had some
flavor of an MCS developing over MN/northwest WI tonight and
moving southeast across WI and then over Lower MI. The CAMs caught
on to that scenario too over the weekend with some taking the MCS
south across WI and into northern IL. However, a few others
continue to take the MCS across MI...either northern Lower or
across the GRR cwa. The model consistency alone gives some
confidence that storms are going to form tonight to our northwest.
The question is where are they going to go.

Model thermodynamics/kinematics point toward a classic MCS
scenario in which an upper short wave induces low pressure over
the northern Plains in concert with a northward moving warm front.
A strong nocturnal LLJ then pushes moisture toward the front like
a fire hose with storms developing near the triple point of the
warm front/cold front. Models tonight show a 55 kt LLJ pointed at
the warm frontal boundary and moving east with time so that the
LLJ is over the cwa by 12z. Meanwhile, a 60-70 kt mid level jet
pushes the storms east along the warm front following mid level
thickness contours to the ESE. The key to whether we see storms
over the cwa tonight will be how far north they develop. If they
develop over southern MN, then they should be more inclined to
take a southeast turn across southern WI/northern IL. However, if
they develop farther north, chances are greater that they will
move across northern WI and cross the lake and move southeast
across the cwa after 08z.

There`s also some uncertainty surrounding how strong the storms
will be. Shear values are aoa 40 kts which suggests organization
is possible. Given the strong mid level winds, damaging wind is
the main threat. There is some mid level capping that would likely
be overcome by a mature MCS.

The entire CWA is in a marginal risk for severe storms tonight
and Tuesday. The southwest cwa is in a slight risk for severe
storms Tuesday due to the potential for storms to fire on the
trailing cold front that models suggest will be in the vicinity of
the WI/IL border east toward the I-94 corridor. There`s a lot of
uncertainty around whether storms will develop over the southern
cwa Tuesday afternoon, because it depends on what happens with the
Tuesday morning convection. It`s possible that storms Tuesday
morning essentially push the cold front south of the cwa and we
don`t get any redevelopment Tuesday afternoon. Less convective
activity late tonight would likely result in higher chances for
storms Tuesday afternoon.

Storms that develop over the southern cwa Tuesday afternoon will
have the potential to train as they move east which would result
in a heavy rain threat.

- Medium/High confidence in fair weather Wednesday and Thursday

The flow regime across the Great Lakes this week is dominated by two
shortwave troughs in quasi-zonal flow between subtropical ridging
across the southern CONUS and an upper low across central Canada.
THe first trough is moving through by Wednesday with showers ending
early and sfc high pressure building in with fine summer weather for
Thursday into early Friday.

- Medium confidence in showers and storms late this week

The surface high moves east with warm advection pattern developing
which could bring showers by Friday morning and more likely Friday
night and Saturday as the next trough moves through. The usual
ensemble spread at that time range makes the timing of the showers
and storms hard to pin down, but the highest chances appear to be on
Friday night. A severe weather threat is possible at that time as
Lower Michigan is briefly in the warm sector and model soundings
show 30 to 35 knots of shear ahead of the cold front.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 103 PM EDT Mon Jun 24 2024

A complicated aviation forecast is expected over last 12 hours of
the TAF window. High confidence in VFR through 06-08z as just a
few dirunal cumuluss, a few cirrus, and light winds dominate the
forecast.

After 08z a line of showers and storms apoproaches the Lake
Michigan shoreline and moves across West Michigan. However, exact
placement and timing of this line is uncertian given dependence on
how storms develop to our northwest. For the three sites with the
best chance of seeing TSRA (MKG/AZO/GRR) have included a PROB30
group with current favored timing of thunder and prevailing SHRA
for more widespread showers. For all other TAF sites confidence in
thunder is to low to include in TAFs so only included SHRA. This
will be updated as needed as upstream trends become more apparent.
While certainty is low in potential and placement, the heart of
the line could see gusts of 35-50 knots. Potential showers and
thunderstorms should exit the terminals by 15z.

Outside of thunder concerns, a strong low-level jet will bring
LLWS to all terminals with SW 40-45 knot winds at 2kft for a few
hours, along with southwest winds gusting to 25 knots through the
end of the TAF window. There are also weak signals for MVFR
stratus to develop at the I96 terminals mid-Tuesday morning,
however this is likely dependent on where the heaviest
precipitation falls from the line of storms tonight. Given that,
will include a mention of low stratus but not include it as a
ceiling with this package.

&&

.MARINE...
Issued at 322 AM EDT Mon Jun 24 2024

Southwest winds will increase tonight behind a warm front and that
will result in wind and waves that will become hazardous to small
craft. We will issue a Small Craft Advisory and Beach Hazards
Statement for this. Additionally, thunderstorms may develop over
the lake after midnight tonight.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement from late tonight through Tuesday
     evening for MIZ037-043-050-056-064-071.
MARINE...Small Craft Advisory from 2 AM to 8 PM EDT Tuesday for
     LMZ844>849.

&&

$$

DISCUSSION...Ostuno/04
AVIATION...Thomas
MARINE...04