930
FXUS63 KGRR 301045
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
645 AM EDT Sun Jun 30 2024

.KEY MESSAGES...

- Cooler Today and Monday With Much Less Humidity

- Potential for Thunderstorms Late Tuesday into Early Wednesday

&&

.DISCUSSION...
Issued at 315 AM EDT Sun Jun 30 2024

- Cooler Today and Monday With Much Less Humidity

Today and Monday`s weather will be governed by synoptic scale
ridging and surface high pressure that will translate eastward
through the Midwest and Great Lakes. Following Saturday night`s cold
frontal passage, cold air advection this morning is ushering in 850
mb temperatures by 18Z that are at least 10 Celsius cooler than 24
hours ago. The feel of humidity will be drastically lower today
compared to yesterday, with dew points no longer around 70 F but
instead around 50 F. Partly cloudy skies this morning with
stratocumulus should give way to plenty of sunshine by afternoon as
much drier midlevel air infiltrates into the diurnal PBL.

- Potential for Thunderstorms Late Tuesday into Early Wednesday

A southwesterly low level jet moves into the CWA during the day on
Tuesday.  This feature will advect in a moist and unstable airmass
by the afternoon.  PWAT`s are progged to climb up to around 2 inches
Tuesday night.  A weakening cold front is shown to push down from
the northwest later Tuesday night or early Wednesday morning with
scattered to numerous showers/storms forming out ahead of it. Models
are in relative agreement on this scenario. Ensemble QPF trends
still support higher POP`s during this period as most members are
showing measurable QPF. Ensemble wind gusts from the GFS and ECMWF
have a signal for gusty conditions as well. Values over 25 knots are
looking possible.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 638 AM EDT Sun Jun 30 2024

A large area of high pressure over the upper MS Valley will build
east today and into tonight. Cold air advection over the TAF sites
here southwest Lower MI will lead to some lake effect clouds.
However strong subsidence moves in and dries the atmosphere out,
especially this afternoon. Thus we will see any MVFR conditions,
improving to VFR then. Gusty winds will persist into the early
afternoon hours but by late afternoon the winds will diminish as
the mixing decreases and the pressure gradient starts to relax.

&&

.MARINE...
Issued at 315 AM EDT Sun Jun 30 2024

Small Craft Advisory and Beach Hazards Statement for dangerous
swimming conditions along Lake Michigan today as north winds 20 to
25 mph build waves 4 to 7 feet. The hazards are likely to persist
till around sunset today, though may end a little sooner around
Ludington and a little later around South Haven to St Joseph. The
strong north winds may also cause upwelling of colder water at some
beaches that would linger into Monday. (This upwelling is due to a
net westward transport of water during strong north winds due to the
Ekman Spiral, a consequence of the Coriolis effect.)

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement through this evening for MIZ037-043-050-
     056-064-071.
MARINE...Small Craft Advisory until 10 PM EDT this evening for LMZ844>849.

&&

$$

DISCUSSION...MJS/CAS
AVIATION...MJS
MARINE...CAS