587
FXUS63 KGRR 111945
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
345 PM EDT Sat May 11 2024

.KEY MESSAGES...

- Cloud / Aurora forecast for tonight

- Showers and storms expected Sunday evening and Sunday night

- Showers and a Chance of Storms Monday and Tuesday

- Dry Mid Week with Rain Chances Late Week

&&

.DISCUSSION...
Issued at 345 PM EDT Sat May 11 2024

- Cloud / Aurora forecast for tonight

Vertically stacked low is crossing southeast from the Thumb at the
present time out onto Southern Lake Huron. Essentially, the low
and its affects are moving away from the forecast area. A few
showers may linger for another couple hours in the southeast
portions of the forecast area towards Jackson, but overall we
should be dry. Clouds in the center of the state along Highway 127
are a bit thicker and are of the stratocumulus variety. Again,
these clouds are pulling east with the low. Elsewhere, clouds are
of the diurnal cumulus variety and should dissipate rapidly with
the setting sun. There is an area of showers that has developed
over Northeast Wisconsin, but these clouds/showers are also
diurnally driven and should fade as well with sunset. They will
also be moving over the cold waters of the big lake and that
should lead to a diminishing trend as well. So, the cloud forecast
is a good one for aurora viewing...one of clearing.

As for the aurora, the kP index which is monitored for
disturbances in the earths magnetic field has maintained high
levels right into today. The scale goes from 0 to 9 and we have
maintained 8 (severe) or 9 (extreme) values all of last night and
through the daylight hours of today so far. The forecast is for
these levels to continue as solar flares have continued to emanate
off the sun. It is the flares interacting with the earths magnetic
field that produces the aurora. Forecasts from the Space Weather
Prediction Center (NOAA) continue to show high levels of aurora
activity tonight. Keep in mind that conditions can ebb and flow a
bit within a solar storm so continue to check the sky if you are
interested. Also, your phone will be able to pick up aurora easier
than your eye, so open your camera and take a look if you are
unsure as to what you are looking at. A 3 second exposure on your
camera will produce more vivid photographs.

Sunset across the area is generally between 850pm and 900pm.
Nautical twilight when it will be easier to see aurora does not
occur until 1000p to 1010p. So, after 10pm is the time to take a
look skyward.

- Showers and storms expected Sunday evening and Sunday night

Quiet conditions are expected overnight and through much of Sunday
as high pressure drifts through the forecast area. A cold front
will approach from the northwest Sunday afternoon with chances for
showers and thunderstorms from roughly 600p Sunday evening into
Sunday night. The best chances for showers and storms will come
during the evening across Central Lower Michigan. A 30-40 knot low
level jet will bring in marginal moisture at 850mb of +8C. Surface
dew points only make it into the middle 50s F. MUCAPE values
increase to around 1000 j/kg for a short time. So, instability is
not all that impressive. We expect showers and storms to nudge
into the U.S. 10 corridor Sunday evening with a marginal chance at
being severe. An isolated warning cannot be ruled out for some
hail/wind, but overall this does not look to be a significant
threat of severe weather. Showers and storms will sag south
through Sunday night weakening with time as the instability wanes.

- Showers and a Chance of Storms Monday and Tuesday

Rain chances continue into Monday as a frontal boundary stalls
across the CWA in a zonal mid-level pattern. MUCAPE values near and
north of 1000  J/kg will be supportive of thunder chances. However,
given the lack of upper-level support unsure on the extent of
convective coverage during the day. In addition, the lack of any
significant low or mid-level jet means deep layer shear is week
making any storms garden variety thunderstorms.

Better upper-level forcing arrives later Monday in the form of an
approaching shortwave and placement in the right entrance region of
the upper-level jet. However this occurs as daytime heating is lost
meaning thunder chances will be more limited. The frontal boundary
then is driven south of the state by the aforementioned shortwave
Tuesday morning causing rain chances to end.

Highs in the 70s Monday fall into the 60s for Tuesday given
northerly flow. Highs then rebound into the 70s Wednesday given
ridging in place.

- Dry Mid Week with Rain Chances Late Week

The mid-week period is expected to be dry across West Michigan as
guidance is consistent in surface and upper-level ridging across the
region. Wednesday in particular looks to see plenty of sunshine
given the lack of deep moisture across the area.

Uncertainty increases by late week as predictability in the upper-
level pattern weakens. A review of the cluster analysis phase space
suggests that guidance is generally locked into individual solutions
due to questions in the presence of troughing across our area. One
set of clusters, primarily leaning towards the GFS/GEFS/GEPS
supports robust troughing arriving later Thursday into Friday and by
extension increasing rain chances into the weekend. In contrast, the
EPS/Euro leaning clusters support zonal to slightly ridged mid-level
flow and lower rain chances late next week. Will cut PoPs from the
NBM for Thursday given the lingering effects of surface based
ridging and will leave the chance PoPs beyond that given the notable
uncertainty remaining in the forecast.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
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.

&&

.MARINE...
Issued at 345 PM EDT Sat May 11 2024

Plenty of whitecaps continue on area webcams as of 300pm. Waves at
all of the nearshore buoys are at 5 feet with shoreline
observation sites indicating 20-30 knot gusts. Given current
conditions not confident that conditions will be below SCA
criteria by 600pm so we have extended the SCA through 900pm.
Conditions should improve later this evening as the winds subside.
The lull will be short lived however as southwest winds ramp up
for Sunday and especially Sunday night into Monday. Most likely
area for higher winds and waves will be north of Muskegon Sunday
night into Monday. It will be a warm air advection wind so all the
usual caveats apply. Most importantly will the winds make it to
the lake surface given the likely inversion. We will let the
current headline play out before looking into the next one.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...Small Craft Advisory until 9 PM EDT this evening for
LMZ844>849.

&&

$$

DISCUSSION...Duke/Thomas
AVIATION...Thomas
MARINE...Duke