-- Discussion containing changed information from previous version are highlighted. --
944
FXUS63 KGRR 111703
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
103 PM 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

&&

.UPDATE...
Issued at 1107 AM EDT Sat May 11 2024

Forecast looks on track today with no major changes made in the
morning update. A compact low can clearly be seen in the mid
levels spinning over Northeast Lower Michigan late this morning,
centered near Oscoda. At the surface, the low is not far off in
locations from its mid level center, located in much the same
spot. The stacked low is forecast to slowly pivot to the east and
southeast this afternoon from its current location to Southwest
Ontario towards sundown. This will keep the chances for showers
going in our eastern CWA towards Highway 127 much of this
afternoon. The greatest chances will be over our Northeast CWA
towards Mt. Pleasant in the next few hours. Showers should dwindle
in coverage through the afternoon and early evening and end
towards sundown. Clouds will clear out from west to east across
Southwest Lower Michigan, with the last clouds exiting the Highway
127 corridor in the 800p-900p hour. So, with more aurora expected
tonight a better sky condition is setup for tonight for sure.

If wanting to monitor current aurora conditions the Space Weather
Prediction Center is the website to go to. Once there monitor the
planetary kP index which measures disturbances in the earths
magnetic field. When values get to 8 or 9 (which we had last night
and which do not occur very often at all) the aurora can be seen
all across the area. Current conditions as of mid to late morning
remain at levels seen last night.

&&

.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 /18Z TAFS THROUGH 18Z SUNDAY/...
-- Changed Discussion --
Issued at 1258 PM EDT Sat May 11 2024

Widespread MVFR cigs are present across West Michigan this
afternoon and will lift to VFR over the next few hours as an
upper-level disturbance pulls away. Cigs erode this evening
leaving FEW cirrus to SKC conditions for all TAF sites. A cloud
deck at around 10kft then enters the area with a push of mid-level
moisture late Sunday morning. West-northwest winds of 15-20 knots
gusting to around 25 knots are expected through this evening
before becoming southwesterly at 5-10 knots tonight into tomorrow
as high pressure slides into the region.
-- End Changed Discussion --

&&

.MARINE...
Issued at 1048 AM EDT Sat May 11 2024

Given ongoing conditions we have expanded the Small Craft Advisory
northward through Big and Little Sable Points. Big Sable has been
gusting consistently into SCA range this morning. Wave heights at
the nearshore buoys are all around 4 feet. We expect SCA
conditions to continue through the afternoon. We pushed the SCA
end time just a bit as well, ending it now at 600pm.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...Small Craft Advisory until 6 PM EDT this evening for LMZ844>849.

&&

$$

UPDATE...Duke
DISCUSSION...Maczko/Hoving
AVIATION...Thomas
MARINE...Duke