139
FXUS63 KGRR 211947
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
347 PM EDT Sat Sep 21 2024

.KEY MESSAGES...

- Rain moves in Sunday with Thunderstorms in the afternoon

- Chance of Showers and Storms Tuesday, More Uncertain Late Week

&&

.DISCUSSION...
Issued at 347 PM EDT Sat Sep 21 2024

- Rain moves in Sunday with Thunderstorms in the afternoon

Some much needed rain is expected on Sunday as a negatively-
tilted upper trough axis across southern Canada moves east and a
trailing cold front pushes through Lower Michigan. We expect
showers and thunderstorms to form ahead of the cold front and
reach western zones later tonight then expand east across the
forecast area on Sunday.

Model sounding suggest enough instability for convection during
the late morning through afternoon and wind profiles show about 25
to 30 knots of deep layer shear. So some of the convection could
be persistent and organized.

The storms move east as the front pushes through Sunday night with
fair and cooler weather moving in.

- Chance of Showers and Storms Tuesday, More Uncertain Late Week

As the wave from Sunday pulls away, Monday will be dry as surface
high pressure and upper-level shortwave ridging sit over the Great
Lakes. These exit Tuesday as a mid-level southern stream wave ejects
from the Plains into the central Great Lakes driving an associated
surface low with it. This will provide our best chance of rain for
the week, however many ensemble members are unenthusiastic about
rain chances given questions in evolution of the mid-level pattern.
Most solutions keep any rain, if it occurs, under 0.5 inches but
there are a few that make a run at 24 hour totals of 1-1.5 inches.

Uncertainty only increases mid to late next week as ensembles show a
cutoff low developing between the northern and southern stream jets.
However where this low forms and how it interacts with northern
stream ridging remains highly uncertain at this range. Latest
cluster analysis shows the most likely outcome is a dry Thursday
However, there`s still about 40 percent of members that keeps more
persistent troughing and associated shower chances. Outgoing
forecast leans to the more likely outcome. While the where of the
cutoff low is uncertain, better cluster agreement exists in this low
moving closer to the Great Lakes shower chances so have introduced
low PoPs, albeit with still plenty of support for a dry Friday in
the ensembles. This flavor of uncertainty is quite common with
medium range upper-low setups.

Temperature wise, there`s still a 4-8 degree spread in the 25th-75th
percentile of high temps given the uncertainty in the upper level
pattern. The median solution at present favors a near normal
temperature pattern with highs in the low to mid 70s.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 141 PM EDT Sat Sep 21 2024

Skies are clear this afternoon as morning fog has dissipated. VFR
continues into tonight with mid to high clouds moving in after 04z
ahead of our next system. Westerly winds around 10 knots today
become light tonight and increase from the south Saturday morning.

The main aviation concern, widespread rain showers approach MKG
after 12z, and may reach GRR/BTL/AZO through 18z Sat.However,
with low-level dry air it is uncertain the extent these showers
will bring impacts until after more moisture arrives near 18z.
Therefore except at MKG where SHRA to be prevailing have added
PROB30 groups for SHRA at BTL/AZO/GRR. At MKG, signs exist that
heavier showers could cause MVFR or lower conditions after 15z.
Have elected to add this as a PROB30 with this package as while
the chance exists, aforementioned dry air may prevent these
conditions from prevailing through 18z.

&&

.MARINE...
Issued at 347 PM EDT Sat Sep 21 2024

Winds and waves will be relatively light through Sunday then
increase behind a cold front on Sunday night. A small craft
advisory may be needed Sunday night and Monday.

&&

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

&&

$$

DISCUSSION...Ostuno/Thomas
AVIATION...Thomas
MARINE...Ostuno