480
FXUS63 KGRR 301147
AFDGRR

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

.KEY MESSAGES...

- Fairly high concentration of showers and storms this afternoon

- Mainly dry the rest of the work week

- Chances for showers and storms next weekend

&&

.DISCUSSION...
Issued at 348 AM EDT Mon Jun 30 2025

- Fairly high concentration of showers and storms this afternoon

Showers and storms have dissipated over Lake Michigan this morning
with the bulk of the area seeing dry conditions as of 330am. We do
not expect much of an uptick in activity early this morning, but
we cannot rule out some scattered showers and storms over Western
Lower Michigan and Lake Michigan. The SREF guidance does indicate
at least some activity will redevelop.

The HREF 4 hour max reflectivity has been consistent from run to
run and we will therefore not waiver in our forecast of showers
and storms developing in the late morning and becoming numerous
this afternoon. We expect showers and storms to develop on a lake
breeze boundary that will extend from southwest to northeast
across the area from South Haven to Big Rapids. The storms should
go up between 11am and noon and then fill in/increase in coverage
as they move east. Highest chances for showers and storms today
will be from Highway 131 to the east.

We will see solid instability today with MUCAPE values between
1500-2500 j/kg. Wind fields are mid summer like, specifically
weak. There is no low level jet to speak of with winds of 15
knots. The mid level jet is also weak around 25 knots. Deep layer
shear is on the order of 20 knots. We expect a wide coverage of
storms this afternoon that will largely be sub severe given the
weaker wind fields. That said we are in a SPC marginal risk which
is due to the fact that some wet microbursts will be possible.
Storms will be pulse type and multicellular at times. The threat
today will be from crashing cores that will be forced via the cold
front moving into an unstable air mass driven by an upper level
shortwave trough. So, while we are not expecting widespread severe
storms due to weak wind fields a few pulse severe wet microbursts
cannot be ruled out.

- Mainly dry the rest of the work week

The rest of the work week after today should be dry. Surface
ridging should be in place the bulk of the time. We will see a
slow warming trend through the week after near normal highs on
Tuesday in the lower 80s. By Friday into Saturday 850mb
temperatures surge once again to around +20C. So, some 90s will be
possible again Friday and Saturday.

- Chances for showers and storms next weekend

Models show a weakening cold front moving through the area on
Saturday into Saturday night. Our highest chances for
precipitation with this front comes Saturday night when we have 60
pct chances over much of the area.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 746 AM EDT Mon Jun 30 2025

A round of thunderstorms is expected today sweeping eastward
through the TAF sites between roughly 16z and 22z. The storms
will first form on a line from near LWA northeast to RQB. From
there, the showers and storms will develop eastward. We have
prevailing VFR weather in the TAFs at this time, but there will be
periods of MVFR and potentially lower at times. Timing that level
of detail through the TAFs is a bit uncertain at this time. VFR
weather should return tonight. Southwest winds around 10 knots are
expected today.

&&

.MARINE...
Issued at 348 AM EDT Mon Jun 30 2025

Conditions on the big lake are expected to remain below advisory
levels from today and through Tuesday. Winds today will be
relatively light from the southwest at 5 to 15 knots. Tonight a
cold front will move east, but winds over the lake and behind the
front are not expected to be strong. We may briefly (3-6 hours) reach
10 to 20 knots out of the northwest, but then winds taper off
again on Tuesday. So, areas south of Holland may see some 2-4 foot
waves observed towards daybreak on Tuesday. Otherwise conditions
look fairly benign into mid week with weaker winds and waves 2
feet or less.

&&

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

&&

$$

DISCUSSION...DUKE
AVIATION...DUKE
MARINE...DUKE