268
FXUS63 KGRR 111132
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
732 AM EDT Sat May 11 2024

.KEY MESSAGES...

- Rain exits Saturday, Warming Trend Begins Sunday

- Rain Chances Remain for Sunday Night Through Early Tuesday

- Quiet Midweek, Rain Chances Return Late Week

&&

.DISCUSSION...
Issued at 250 AM EDT Sat May 11 2024

- Rain exits Saturday, Warming Trend Begins Sunday

Light rain moves off to the east this morning as surface low moves
slowly through northern Michigan into Ontario. Northwest flow could
bring a few sprinkles through this afternoon, but most of the area
should remain dry into the day on Sunday. Northwest flow will also
keep temperatures on the cool side today and tonight before warmer
air begins to build in Sunday. Sunday should be mostly dry, though
precipitation chances begin to increase late in the day.

- Rain Chances Remain for Sunday Night Through Early Tuesday

An approaching cold front Sunday night is expected to stall out
overhead and become stationary into Monday. As this feature
approaches, MUCAPE values of 500 J/kg or a bit better are shown as
elevated instability moves over Lake Michigan and western Lower
Michigan Sunday night. PWAT values are shown to increase to over
1.00" during this time as well. However, LLJ divergence is indicated
over the region, leading to some question marks about extent of rain
coverage and thunderstorm chances. Nevertheless, enough of a signal
is there to warrant likely POPs for showers and a few storms
especially near and north of I-96 Sunday night.

During the day Monday, the surface front may help provide enough
lift to support some showers and a few storms, though upper level
forcing is not really there. By Monday night, better LLJ forcing is
shown especially by the GFS near and south of I-96, indicating a
threat for wider coverage of showers but probably little in the way
of thunderstorms as any instability looks to fizzle out. The frontal
boundary looks to slide south of the state by Tuesday morning or
early afternoon, taking the threat for rain with it. Overall, the
highest rain totals from Monday into early Tuesday look to be near
and south of I-96.

Given the cooler air advecting in behind the front Tuesday, we may
see highs slip back into the mid 60s. Highs should bounce back to
around 70 on Wednesday.

- Quiet Midweek, Rain Chances Return Late Week

Mid level height rises are shown for midweek with a good probability
for dry conditions Wednesday into at least part of Thursday.
Ensemble guidance has diverged a bit from 24 hours ago regarding the
late week synoptic pattern. The GEFS remains resolute in upper
troughing arriving by Friday (mean 500 mb heights), but the ECE has
steadily weakened this feature with the last couple runs. This leads
to some uncertainty for rain chances late in the week, but a fair
amount of ensemble members support a risk for showers and perhaps
some thunderstorms by Friday or so. NBM guidance is broadly
indicating 30-40% risk for rain Thursday into Saturday, but this is
likely due to some timing differences in the medium range guidance.
Once ensemble guidance has a better handling on the potential upper
trough (or lack thereof), we should get more fine tuned
probabilities for when the next risk for rain will be.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 724 AM EDT Sat May 11 2024

The terminals are expected to escape IFR though it has been a
close call for GRR at times this morning. Will keep GRR at MVFR
though can`t rule out some brief dips just below 1000 ft mainly
through 15z. The risk at MKG is even lower, but not zero.
Otherwise, expect a gradual lifting of the ceilings throughout the
day. Breezy conditions are expected at all terminals with steady
WNW to NW winds gusting in the 25-30 kts range. Winds won`t
appreciably diminish until after 00z Sunday.

&&

.MARINE...
Issued at 250 AM EDT Sat May 11 2024

Sustained winds over 20kts should be in place through much of the
morning today with waves of around 4 feet. Small Craft already
out through this afternoon to cover this. Winds may persist in the
evening tonight with somewhat lighter but still breezy conditions
expected Sunday. Still good potential for a Small Craft Advisory
on Sunday.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...Small Craft Advisory until 5 PM EDT this afternoon for
     LMZ844>847.

&&

$$

DISCUSSION...Maczko/Hoving
AVIATION...Hoving
MARINE...Maczko