509
FXUS63 KGRR 011125
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
725 AM EDT Sat Jun 1 2024

.KEY MESSAGES...

- Rain and slight chance thunderstorm this afternoon-tonight

- Lingering humidity and cloud patches on Sunday

- Medium/High Confidence in evolution of upper troughing next week

&&

.DISCUSSION...
Issued at 319 AM EDT Sat Jun 1 2024

- Rain and slight chance thunderstorm this afternoon-tonight

Midlevel trough and surface low over Missouri will move into
Illinois today. Ahead of it, dry air will be in place through early
afternoon in Michigan with dew points at or below 50. High and mid
altitude clouds will thicken as precipitation nudges closer,
eventually moistening the low levels and cooling off temperatures
once rain reaches the surface this afternoon-evening.

HREF and global model ensembles have come into much better alignment
compared to 24 hours ago regarding rain chances, and now the
scenario for little to no rain seems to be remote, at least for
areas south of US-10. The arrival of rain is likely in West Michigan
during mid-late afternoon (3-5 pm or so) and near US-127 during the
evening (8-9 pm). Rain should move out late in the night or early in
the morning.

Forecast soundings during the middle of the event show moisture and
lift through much of the troposphere. Lapse rates in the low-mid
levels that are neutrally buoyant and possibly slightly unstable for
moist parcels may also support a period of more efficient rainfall
production in some spots. The chance of lightning is slight but not
zero. Broadly, 0.25 to 0.75 inch rain totals are most favored but
there could be embedded spots or small swaths of 1 to 1.5 inch
totals, per ensemble guidance.

- Lingering humidity and cloud patches on Sunday

Moisture in the low levels will linger on Sunday as the trough
departs, and there may be drizzle early in the day. Temperatures
should warm back up to seasonable levels but dew points near 60 will
make it feel a little more sticky. The HRRR is less enthusiastic
about ridging and warming in the midlevels, so it does touch off
some afternoon showers south/east of Grand Rapids. Will go with a 10
percent chance of measurable rainfall.

- Medium/High Confidence in evolution of upper troughing next week

Ensemble spread is lower than usual and good run to run consistency
is indicating higher confidence in the details for sensible weather
next week.

There continues to be a signal for rain Monday and Tuesday as a
shortwave trough approaches and minors out...then a stronger
signal on Wednesday as a well-defined shortwave cuts off as it
approaches from the Northern Plains. This low figures in the
evolution of persistent troughing across the Great Lakes by the
end of the week with lower 500 mb heights and low level northwest
flow indicating cooler than normal temperatures into next weekend.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 725 AM EDT Sat Jun 1 2024

VFR conditions through this morning then MVFR conditions
overspread Lower Michigan from the south during the afternoon.
Conditions will lower to IFR across southern Lower Michigan on
Saturday evening as rain becomes more steady, then LIFR after
midnight.

Winds will be generally southeast less than 10 knots into this
evening then light and variable overnight.

&&

.MARINE...
Issued at 319 AM EDT Sat Jun 1 2024

Fairly light winds and 1-2 foot waves continue through the weekend.
Patches of fog may develop over the lake Saturday night into Sunday
morning, in the wake of the rain that will arrive this afternoon. A
moderate swim risk is possible near/north of Whitehall on Monday due
to waves building amid a south breeze.

&&

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

&&

$$

DISCUSSION...Ostuno/CAS
AVIATION...Ostuno
MARINE...CAS