283
FXUS63 KGRR 290538
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
138 AM EDT Sat Jun 29 2024

.KEY MESSAGES...

- Showers and storms tonight into Saturday

- Dry and comfortable Sunday through Monday

- Unsettled and warmer much of the holiday week

&&

.UPDATE...
Issued at 942 PM EDT Fri Jun 28 2024

The 00Z run of the HRRR is quite startling in that it paints a
narrow swath of 4+ inches of rain overnight along the I-94
corridor between Kalamazoo and Jackson. There is a decent theta-e
surge and a low-level wind confluence axis that makes localized
heavy rain seem plausible. The 00Z SPC HREF Prob Matched Mean
(PMM) and Local PMM guidance should lend a bit more insight once
it becomes available in the next hour or two.

Not going to completely bite on a single deterministic run at
this point, but certainly there is some concern since this could
cause pronounced localized flooding should this come to pass.
Otherwise, no changes to the forecast planned in terms of overall
coverage and timing of anticipated thunderstorms.

&&

.DISCUSSION...
Issued at 337 PM EDT Fri Jun 28 2024

- Showers and storms tonight into Saturday

Some light rain is falling in places this afternoon across
Southwest Michigan. This rain is falling from high cloud bases
that from the most part are above 5,000 feet. This precipitation
is associated with a weak shortwave moving through the area as
well as the initial surge of moisture advection.

The most substantial rainfall will occur tonight into Saturday as
an upper trough digs into the northern Great Lakes driving a cold
front through the area on Saturday. Instability tonight is not all
that substantial, but on the order of 500-1000 j/kg. The HREF
would suggest some embedded storms within a broader area of rain
showers. Not expecting anything close to severe tonight given the
limited instability. We are likely looking at 0.25 to 0.50 inch
rainfall amounts in many areas tonight with local amounts towards
an inch in the heavier swaths.

On Saturday the cold front will bisect the area at daybreak with
storms mainly expected southeast of a Holland to Clare line. As we
work through the day, the best chance for storms in the afternoon
and early evening will be near and south of I-96. Showers and
storms on Saturday will be a bit more transient so rainfall totals
will be lower, ranging from trace amounts to 0.30 to 0.40. We are
in a SPC marginal risk for severe weather, but we expect
instability to be muted by clouds and morning precipitation.
Chance for any severe weather look low.

HREF tries to indicate some scattered showers and storms moving
back into Central Lower Michigan during the late afternoon and
evening ahead of the main upper shortwave. This activity will be
scattered and given the drier air moving in we have a lower
confidence in this occurring. There are hints at some lower clouds
developing Saturday night in the wake of the precipitation. We
have brought some of these clouds into Central Lower Michigan at
this time.

- Dry and comfortable Sunday through Monday

After the unsettled weather in the first couple of periods of the
forecast, the area will see dry weather and cooler temperatures make
their way over the area for Sunday and Monday. This will be the
result of a fairly decent upper ridge and Canadian sfc high moving
over the area. This will provide for strong subsidence to cap any
clouds that try to grow at all in the vertical. 850 mb temperatures
on Sunday around 7 to 9C will support max temperatures in the low to
mid 70s. Return flow setting up on Monday will support highs in the
upper 70s.

- Unsettled and warmer much of the holiday week

The upper ridge responsible for the dry weather Sunday and monday
will slip east of the area by Tuesday. This will allow a long wave
trough over the Pacific NW to move over the upper midwest. Deeper
flow from the SW will bring warmer and more moist air to the state.
Embedded short waves in the flow will bring multiple chances of
showers and a few storms to the area for Tuesday and Wednesday.
Details at this point are obviously a bit up in the air, but we can
expect temperatures to warm back into the 80s with more humid air,
and occasional chances for rain.

There has been some decent agreement that much of the wet weather
may settle just south of the area on the Holiday. The short waves
moving through Tuesday and Wednesday flatten out the upper jet a
bit. They will also lay out a front that looks to stall out over the
southern portion of our area, or just south of the state. This will
take the better chances of rain further south. However, this could
end up being a scenario where waves ride along the front, and MCS
activity just north of the front could be found. For this reason,
small rain chances will be maintained for the holiday.

The uncertainty by next Friday with respect to frontal and wave
positions will obviously be fairly high due to the sequence events
that takes place leading up to then. There is some hint that maybe a
short wave ridge may build in on Friday in the wake of the early
week trough, and the next trough reloading over the western portion
of the country.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 128 AM EDT Sat Jun 29 2024

Based on radar trends, newer model data and lightning plots, I
added more widespread shower and thunderstorm activity to the TAF
forecasts for the remainder of tonight. Convective cells with
occasional lightning strikes were already approaching the KJXN TAF
site. Further west and northwest, the cells crossing Lake MI had a
few lightning strikes in the as well and they were rapidly moving
onshore. Given the very moist airmass moving, it has not take much
of a cell to send VSBY`s down into the IFR category as evidenced
by the brief IFR conditions as KAZO and KBTL in the past hour.
Through the remainder of the night, the atmosphere only becomes
more unstable and the latest model data shows the convection
increasing. However, between 10 and 13z the strongest cells
dissipate from west to east.

IFR is likely to overspread the area as the strongest cells shift
east of the TAF sites. It`s a very moist airmass moving in and
with a cold front approaching from the northwest, that will keep
low level convergence going which will support low ceilings and
even some drizzle. However the low cloud conditions are expected
to lift into the MVFR/VFR range mid to late afternoon. An isolated
shower/storm is possible then, but the coverage at this time
looks to thin to warrant adding them to the TAF forecast.


&&

.MARINE...
Issued at 337 PM EDT Fri Jun 28 2024

We have extended the current Small Craft Advisory (SCA) through
800am on Saturday from Grand Haven northwards. South winds of 15
to 25 knots will produce marginal SCA conditions of 3-5 feet
tonight. The highest waves per the WaveWatch3 model look to remain
offshore which is why we do not have a corresponding Beach Hazards
Statement (BHS) out.

After tonight, the focus will shift to our next wave event which
develops behind a cold front Saturday night and Sunday. 4 footers
develop just after midnight Saturday night and will continue
through the day on Sunday. Waves will peak on Sunday morning in
the 4 to 6 foot range. A SCA and BHS will be likely on Sunday.
That said, weather conditions will not be ideal for a beach day
with water temperatures in the 60s (and falling). Air temperatures
on the beach will likely hold in the 60s as well, so we are not
expecting a high beach population and even less so in the water.

A north wind is the direction that are biggest upwelling occurs
with. As a north wind commences, each layer below the surface
turns a bit to the right due to Coriolis. Therefore the general
flow below the water surface (in a north flow above the water) is
offshore and upwelling occurs. So, with air temps in the 60s and
falling water temperatures we are not expecting lots of people in
the water. We will message it with full force on Saturday a day
ahead of the actual event.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...Small Craft Advisory until 8 AM EDT this morning for LMZ847>849.

&&

$$

UPDATE...TJT
DISCUSSION...Duke/NJJ
AVIATION...MJS
MARINE...Duke