619
FXUS63 KGRR 130802
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
402 AM EDT Sat Jul 13 2024

.KEY MESSAGES...

- Occasional thunderstorms possible tonight into Monday

- Pattern change to cooler weather occurs mid next week

- Temperatures a bit up and down this week into next week

&&

.DISCUSSION...
Issued at 402 AM EDT Sat Jul 13 2024

- Occasional thunderstorms possible tonight into Monday

We start off this forecast period with a bit of fog this morning.
It is likely to fill in some through daybreak as a moist air mass
is in place due in large part to recent rains. Grand Rapids and
Kalamazoo stand the best chances for patchy dense fog as both
locations received rain yesterday evening towards sunset.

Occasional storms look to be possible over the coming days. The
upper pattern is dominated by a strong ridge over the Southern
Rockies centered near the Four Corners. North of this feature the
westerlies are situated from Southern Canada into the Great Lakes
and this will be the effective storm track from Tonight into
Monday. Significant instability is in place even at this hour
across the Northern Plains. The most likely convective evolution
scenario is for storms to continue to work to the southeast
towards our area today driven by the west-northwest flow aloft.
The HREF 4 hour max reflectivity would suggest storms move out
over Lake Michigan this evening and make inroads into Western
Lower towards midnight. This thinking is also supported by the low
level jet bending into our area overnight. Deep layer shear is on
the weaker side (around 20 knots) tonight and the storms will be
moving away from the stronger instability reservoir to our west.
So, storms will likely not be severe overnight with wind being the
main threat. Highest chances at severe weather would be towards
the Lake Michigan shoreline.

The storms will likely work off to the east on Sunday during the
day, which will likely inhibit strong instability developing in
our area due to clouds and precipitation.

The next chance for storms will likely come Sunday night into
Monday in a similar fashion. A relatively better chance for severe
weather and storms in general will come Sunday evening into Sunday
night as we will be closer to the impetus. In fact, the potential
is there that storms may form just upstream over the lake or
directly overhead. The low level just is situated in the area
Sunday evening. Outflow boundaries from Saturday night/Sunday
morning convection over the Southern Great Lakes will likely
become the focus for new convection. Deep layer shear and
instability will be greater, so there is a better chance for
severe weather during this time.

- Pattern change to cooler weather occurs mid next week

A pattern change from a hot and humid one with upper ridging will
switch to cooler weather with a deep upper trough pivoting through
the area. A surface front driven by the upper trough will push
through the region between Monday night and Tuesday night.
Additional shower and thunderstorm chances are expected during
these three forecast periods. All in all a very active weather
period from tonight through Tuesday night with periodic chances
for showers/storms.

- Temperatures a bit up and down this week into next week

A very up and down temperature pattern is expected as we warm into
Sunday and Monday. The warmest day may potentially be Monday when
evening 850mb temperatures overhead reach the +21C to +24C range
in the ECWMF and GFS respectively. These are quite warm readings
for us in any given summer and usually parlay into highs in the
90s. We will see what kind of debris clouds and precipitation we
have around which would inhibit heating. Temperatures fall to the
+7C to +9C range Wednesday night into Thursday with the upper
trough directly overhead. Highs will likely hold in the 70s Wed
and Thurs.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 149 AM EDT Sat Jul 13 2024

The main and really only aviation concern in the 24 hour TAF
period of 06Z Saturday to 06Z Sunday is fog and possibly stratus
this morning.

Isolated showers occurred this past evening, two of which hit the
GRR and AZO airports. Given that fresh moisture on the ground,
clear skies and light winds we expect fog to form at both of those
sites. In fact given the amount of rain that has fallen over the
bulk of the area in the last week, we see fog formation at all
sites this morning with the exception being MKG. MVFR visibilities
should be in place at most TAF sites by 09Z with IFR likely
settling in during the 10Z to 13Z time frame. A rapid clearing
should occur given the shallow nature of tonight`s fog. Stratus
formation is difficult to say, but it is certainly possible this
morning.

Otherwise, VFR weather is expected from mid morning through the
rest of the TAF period. More active weather, showers and
thunderstorms are expected to develop late tonight just beyond
this TAF period (06Z to 12Z Sunday).

Winds will be light and variable this morning becoming southwest
at 5-10 knots today.

&&

.MARINE...
Issued at 402 AM EDT Sat Jul 13 2024

The wind field across Lake Michigan this morning is very light
therefore we start the forecast period with no concerns. Winds
will begin to increase some today out of the south as high
pressure slides away to the east. Winds do not look to be
significant enough (south at 5-15 knots) to cause any issues for
boaters or swimmers (waves 1 foot or less).

Tonight a low level jet moves across the lake overnight which by
itself usually kicks up a wave field. Beyond that feature, a
thunderstorm complex is looking more likely overnight which will
build waves as well. In general we expect waves to build from
around 1 foot at sunset to 1-2 or 1-3 feet at daybreak on Sunday.
These values are still below advisory levels.

Waves will likely peak on Sunday around midday in the 2-3 or 2-4
foot range from Holland to the north. Winds and waves slacken off
again for Sunday night in a weak gradient.

&&

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

&&

$$

DISCUSSION...Duke
AVIATION...Duke
MARINE...Duke