371
FXUS63 KGRR 300607
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
207 AM EDT Mon Jun 30 2025

.KEY MESSAGES...

- Storm chances through Monday

- Heat and humidity through Monday, some relief midweek

- Heat, humidity, and storm chances return for holiday weekend

&&

.DISCUSSION...
Issued at 255 PM EDT Sun Jun 29 2025

- Storm chances through Monday

Sunday morning thunderstorms around Ludington left behind weak
surface outflow and/or 850 mb perturbation that has mostly washed
out between Muskegon and Mount Pleasant. A few but not all
convection-allowing models have been popping up isolated showers
or storm cells between Grand Rapids and Clare for late this
afternoon into evening, focused along a weak surface wind
convergence zone. Will go with a slight chance of precip/thunder
at this time. Any storm cell may produce an isolated 30-40 mph
gust.

Additional thunderstorm complex developing from SW to Central
Wisconsin this afternoon should attempt a trek across Lake
Michigan after sunset, and may reach parts of West Michigan around
or after midnight (Muskegon-Ludington most favored). A weakening
trend is expected as 850 mb moisture transport convergence is
quite weak and may actually be slightly divergent at that level.
Still, outflow from any weakening storms over the lake could
outrun the rain and cause wind shifts and gusts over 30 mph.

Scattered storm redevelopment is more likely by midday Monday
over inland areas away from Lake Michigan as a weak cold front
moves through. Isolated severe wind gusts are possible. Despite
some better upper-level wind shear, mid/upper level lapse rates
may not be very steep and this would result in weaker instability
through the hail growth layer.

- Heat and humidity through Monday, some relief midweek

Moderate heat-related impacts are expected this afternoon through
Monday. Highs near 90 today, lows near 70 tonight, and dew points
around 70, will contribute to max daytime heat index values in
the mid 90s. This is just slightly more bearable than last week`s
heat wave, which had heat index values topping 100.

Relief from the heat and humidity begins Tuesday into the middle
of the week with more climatologically average temperatures and
dew points.

- Heat, humidity, and storm chances return for holiday weekend

The majority of global model ensemble members are building an
upper- level ridge over the Great Lakes on Fri-Sat July 4-5, with
low-level flow from the deep southern US that will bring higher
dew points and temperatures returning to around 90. At least
moderate heat-related impacts will be a concern for outdoor
activities. A cold front in the region Saturday or Saturday night
may bring a threat of thunderstorms at some point over the
weekend. For now, model spread is too high regarding precipitation
timing/amounts and also next Sunday`s temperatures for there to
be much confidence in the details.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 207 AM EDT Mon Jun 30 2025

Light southerly flow early this morning is aiding in the
production of a few showers and thunderstorms, mainly out over
Lake Michigan. It appears the activity is beginning to weaken and
we expect this to remain the case through 12Z. Little to no
additional showers and storms are expected tonight. If we do see
any new development it would be near MKG.

VFR conditions are in place at 06Z and we expect this to remain
the case through 12Z. The next period of concern at the TAF sites
will be from 15Z through 22Z as a round of showers and storms
works from west to east through all of the TAF sites. MVFR
conditions are possible briefly as the showers and storms work
through, but at this point maintained VFR conditions and used VCTS
wording.

There will be a lull in the activity between 22Z and 01Z or so, at
which time the final round of precipitation may work through with
the cold front. It will be an active day with lightning in the
area. Southwest winds around 10 knots are expected today.

&&

.MARINE...
Issued at 154 PM EDT Sun Jun 29 2025

The pressure gradient will remain weak for the next few days.
Thus, waves will generally remain aob 2 feet. A cold front
approaching from the west Monday may generate a few thunderstorm,
which would be hazardous to boaters and beach goers.

&&

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

&&

$$

DISCUSSION...CAS
AVIATION...Duke
MARINE...04