134
FXUS63 KGRR 091930
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
330 PM EDT Thu May 9 2024

.KEY MESSAGES...

- Showers end tonight as system moves away

- Another round of showers Friday night into Saturday

- Frost Potential Saturday night

- Showers and storms return Sunday/Sunday night, remaining
  unsettled

&&

.DISCUSSION...
Issued at 330 PM EDT Thu May 9 2024

- Showers end tonight as system moves away

Precipitation over Southern Lower Michigan has become a bit
elongated as the system begins to shear out and move to the south.
HREF 3 hour precipitation shows rain tapering rather quickly in
the next 3 to 6 hours and ending completely shortly after
midnight. As the overnight discussion mentioned, there has been a
diurnal flare up of showers over Northern Lower and this will
concentrate towards Ludington this evening for a time before
ending. Several hundred j/kg of CAPE noted over Northern Lower at
this time and this may be just enough for an isolated thunderstorm
this afternoon/evening over West Central Lower MI.

- Another round of showers Friday night into Saturday

Another system will move through during the Friday night and
Saturday time frame. A compact vort max will dive through the area
during this time frame. The surface low will dive directly through
Lower Michigan from northwest to southeast bringing rain and
possible a few thunderstorms. This system will have more
instability than today`s system so slightly better chances for
thunder. 400-800 j/kg of MUCAPE looks to be in the forecast area
on Saturday. More than enough for a chance of thunder. 0.25 to
0.50 rainfall amounts will be common.

- Frost Potential Saturday night

Low temperatures are forecast to drop into the upper 30s across
Central Lower Michigan Saturday night and around 40 near I-94.
Some colder readings are possible which makes frost a distinct
possibility. There are concerns that there may be lingering clouds
though from the departing system so confidence is not high yet. We
will be watching this time frame for possible headlines.

- Showers and storms return Sunday/Sunday night, remaining unsettled

No changes to previous forecast reasoning. Broadly cyclonic and
slightly confluent flow aloft over the Canadian border will help to
force a cold front southeastward into the southern Great Lakes.
Sunday into Sunday night is when we have the most certainty about
precipitation occurring with an increasing risk for thunderstorms
moving into Sunday night.

Afterwards, the front becomes nearly stationary, still across the
southern Great Lakes, thanks to substantially weaker and unphased
CONUS upper flow. Whenever the words "weak" and "unphased" get used
in relation to the upper flow pattern, that tends to be associated
with poor predictability. Moreover, the lingering surface front adds
more uncertainty; while we do expect steady frontolysis in general
during the coming work week, the front nonetheless should remain
capable of providing a source of forcing for deep moist convection
in the foreseeable future. Temperatures during this time should
remain near to slightly above normal.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 149 PM EDT Thu May 9 2024

There is a low probability for IFR at the terminals overall, but
should it occur, it would most likely be at AZO and possibly BTL today
before 22Z. The chances for VFR at all terminals goes up
significantly after 00Z tonight.

There is a better chance for brief fuel alternate ceilings (cigs)
below 2000 feet at the I-94 terminals (AZO, BTL, JXN) as
suggested by even lower cigs just south of I-94, which we don`t
expect to encroach much farther north. Not expecting visibility to
drop below 3SM at this time. The rainband that`s oriented roughly
along a BIV to DTW line is beginning to pivot counterclockwise as
it shifts eastward, meaning that GRR should start to see
improvement relatively soon, whereas BTL and JXN could be seeing
the longest duration restrictions this afternoon into early
tonight.

&&

.MARINE...
Issued at 330 PM EDT Thu May 9 2024

North to northeast winds late this afternoon and evening will push
wave heights into the 2 to 4 foot range up near Big and Little
Sable Points. Otherwise, waves will be on a downward trend into
and through Friday. We are not expecting to need Small Craft
Advisories through Friday evening.

A compact area of low pressure will move through the area Friday
night which will boost winds into the 15 to 25 knot range veering
from southwest to northwest. There will be a few hour period near
the frontal passage where winds may be briefly higher around 06Z.
Late Friday night and into Saturday morning there is potential for
a Small Craft Advisory but will hold off for now.

&&

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

&&

$$

UPDATE...Duke
DISCUSSION...Duke/TJT
AVIATION...TJT
MARINE...Duke