021
FXUS63 KGRR 151930
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
330 PM EDT Sat Jun 15 2024

.KEY MESSAGES...

- Low confidence for showers late tonight and Sunday

- Likely Hot and Humid Much of Next Week With Some Chances for
Thunderstorms

&&

.DISCUSSION...
Issued at 330 PM EDT Sat Jun 15 2024

- Low confidence for showers late tonight and Sunday

Convective complex currently across Minnesota and Iowa is lifting
northeast in area of warm advection/isentropic ascent. We will
have to watch the eastward development of a warm advection "wing"
currently approaching the western coast of Lake Michigan, which
could graze the far northwest zones this evening. The progress of
a line of showers and storms extending south into Missouri will
also have to be monitored as some of the CAMs bring these showers
into the northwest forecast area late tonight into Sunday
morning, primarily near Ludington.

Chances for additional showers and thunderstorms appear minimal on
Sunday, but cannot be totally ruled out as warm advection pattern
combines with increasing deep layer moisture advection. There are
some indications that increasing moisture and development of a
LLJ could bring at least isolated showers and storms Sunday night.
Slight chance POPs were continued for Sunday night to account for
this.

- Likely Hot and Humid Much of Next Week With Some Chances for
Thunderstorms

High temperatures in the 90s are likely on Monday as 850 mb air
around 20 C from the midcontinent advects over Michigan amid sunny
skies while the upper level ridge strengthens. The ensemble means of
the ECMWF and CMC have consistently produced highs in the mid 90s
(away from Lake Michigan) with low spread while the GFS ensemble
mean has been aiming for highs at or slightly below 90. Increasing
humidity with dew points between the mid 60s and 70 would contribute
to the heat index climbing to around 100.

Confidence remains lower than usual from Tuesday onward regarding
impacts from prolonged heat or mitigation via occasional
thunderstorms. The mean of the ensembles of the ECMWF, GFS, and CMC
each show a 500 mb high centered over Penn/NJ peaking on Wednesday
then slowly flattening out, but the CMC mean high is strongest with
an expansive 597 dam height contour while the GFS is relatively
weakest. For our area, the temperature spread among each ensemble
system grows from Tuesday onward along with differences in when
precipitation may occur. This is a common challenge along the
periphery of a heat dome where convective capping is weaker. The
forecast will feature a slight chance or chance of thunderstorms
each day, with chances being relatively higher in northern locations.

Tuesday`s temperatures may also be slightly cooler (but still hot)
as our source of low-level air becomes more from a rainy Gulf of
Mexico, which could increase our cloud cover but still maintain a
tropical feel to the air. Even if showers or thunderstorms develop
on Tuesday, local downpours could be heavy but not provide much
relief from the warmth and humidity. The cumulative effects of
multiple days of heat and humidity, including lows in the 70s, will
also be felt if heat persists beyond Tuesday.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1248 PM EDT Sat Jun 15 2024

VFR is likely over the next 18-24 hours, with the main exception
being the possibility for showers or thunderstorms near and north
of MKG after 15 Z Sunday. Confidence is not high enough to
include that in the TAFs at this time. If showers/storms do
approach that area, cloud bases would be around 7,000 feet, and
visibility would be restricted temporarily by rain. 25 knots of
LLWS is likely around 10 to 12 Z. Winds will mostly be from the
southeast, becoming gustier late Sunday morning.

&&

.MARINE...
Issued at 330 PM EDT Sat Jun 15 2024

South winds will be increasing overnight with waves building to
four feet or above after midnight, so a Small Craft Advisory and
Beach Hazards Statement have been issued for north of Grand Haven.
These are set to run through Sunday evening. Although winds should
diminish a bit by Monday, the advisory may need to be extended for
Big and Little Sable Points for Monday and beyond.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement from 2 AM EDT Sunday through late Sunday
     night for MIZ037-043-050-056.
MARINE...Small Craft Advisory from 2 AM Sunday to 2 AM EDT Monday for
     LMZ847>849.

&&

$$

DISCUSSION...Ostuno/CAS
AVIATION...CAS
MARINE...Ostuno