409
FXUS63 KGRR 120659
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
259 AM EDT Sun May 12 2024

.KEY MESSAGES...

- Warmer Today, Chance of Storms this Evening and Tonight

- Showers With a Slight Chance for Storms Monday Night/Tuesday

- Dry Midweek, Then Rain Chances Return Late

&&

.DISCUSSION...
Issued at 254 AM EDT Sun May 12 2024

- Warmer Today, Chance of Storms this Evening and Tonight

Warmer day in store today as high pressure and abundant sunshine
build over lower Michigan. Temperatures should climb into the 70s
today before a cold front works into lower Michigan this evening.
A low level jet and heating of the day should help to provide
some moisture and instability to aid in the possibility of some
thunderstorms this evening and tonight. The greatest risk appears
to be up north where hail and gusty winds will be possible in any
more organized storm activity. Severe potential continues to be
limited, though considering the cold temperatures aloft a couple
reports of severe hail cannot be ruled out. Cold front stalls out
over the lower peninsula where it will aid in the potential for
showers and storms late Monday.

- Showers With a Slight Chance for Storms Monday Night/Tuesday

Model guidance continues to favor showers and perhaps a few storms
Monday night into approximately midday Tuesday. Upper divergence is
shown to move over southern Lower Michigan by the 00z GFS and 03z
RAP, and to a lesser extent the 00z ECMWF. LLJ convergence is also
indicated by the GFS and RAP near and south of I-96. PWAT values are
forecast to be 1.25"-1.50". A surface cold front will be sagging
south through the region as this occurs. Thus, an elevated risk for
showers and a few storms is indicated with POPs around 80% along and
south of I-96. Rain amounts look variable, with general areal
averages in the 0.25"-0.50" for most of the region (mainly less than
0.25" north of M-20), though the NAM 3km and GFS both indicate
potential for pockets of an inch or so especially near and south of
I-96. The ECMWF favors the highest amounts near and south of I-94. I
think it just depends on how far north the LLJ gets, as weak as it
may be. Nevertheless, showers are likely and with a small amount of
elevated instability (maybe a couple hundred J/kg) there could be a
few storms, but those will likely be the exception to the rule.
Expect the precipitation to slowly erode from N to S during the day
Tuesday, with areas along and south of I-94 the last to dry out.

- Dry Midweek, Then Rain Chances Return Late

Good model agreement exists in giving us a dry midweek period (as
early as Tuesday for central Lower MI, but for all areas Wednesday).
This occurs given N/NE low to mid level flow and a dry airmass in
place. Seasonable temps are expected Wednesday with highs around 70F
for most. The latest ECE mean 500 mb height pattern is starting to
reverse course and trend toward the GEFS mean, which favors upper
troughing moving into the upper Midwest by Thursday into Friday.
Ensemble mean PWATs are climbing a bit higher with the 00z guidance
for Thursday night into Friday, and ECMWF/GFS deterministic guidance
suggests upper jet divergence may move into the region with some
lower level jet reflection potentially as well. As a result, rain
chances are warranted especially later Thursday into Friday. NBM
POPs are not especially high yet with 40-50% chance, which seems
reasonable this far out, though POPs could certainly trend higher if
ensemble agreement increases further.



&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 118 AM EDT Sun May 12 2024

Fairly quiet conditions at the terminals for most of this period,
but two main items to watch will be a risk for some showers and a
thunderstorm after 00z, along with an introduction of low level
wind shear. A front sinking south late tonight may bring some
showers into MKG and GRR primarily closer to 06z, but that is far
from certain. There is potential for a thunderstorm to form, but
gusty winds may not quite be able to make it to the surface as
any storms will likely be elevated. Low level wind shear could
reach 40 kts from the surface up to 2000 ft in the 00z-06z window
as a low level jet increases overhead.

&&

.MARINE...
Issued at 254 AM EDT Sun May 12 2024

Some potential late in the day for small craft conditions on Lake
Michigan, especially near and north of Muskegon. However, winds
will have a hard time mixing to the surface outside of any storms
that develop late in the day. Will continue to hold off on
headline at this time.

&&

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

&&

$$

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