402
FXUS63 KGRR 291740
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
140 PM EDT Sun Jun 29 2025

.KEY MESSAGES...

- Main thunderstorm threat through the period will be Monday

- Warm weather back in full swing today and Monday

&&

.DISCUSSION...
Issued at 341 AM EDT Sun Jun 29 2025

- Main thunderstorm threat through the period will be Monday

The main thunderstorm threat through the 7 day forecast will come
on Monday. A cold front will push into Southwest Lower Michigan
from the west on Monday pushing into a warm and humid airmass. The
front is being driven by a upper shortwave trough which will swing
through the Great Lakes Monday into Tuesday. Most unstable CAPE
values are forecast to reach or exceed 2000 j/kg, but the wind
fields look a bit weaker in tonight`s model runs. The low level
jet is hard to find with all models showing 20 knots or less. The
mid level jet at the time of the storms is on the order of 25-35
knots. So, we will have instability, but organizing deep layer
shear may be lacking. Pulse storms and multicellular storms driven
by the front is the likely morphology. The HREF does show much of
the area impacted by at least some thunderstorm activity, but
lightning and locally heavy rain may be the main threats.

Otherwise, there may be some thunderstorm activity that tries to
initiate just east of GRR this evening and additional activity
that may spread into Central Lower tonight. All in all low to no
chances until we get to Monday. After the Monday storms slide east
Monday night, there are two main additional chances. They are
Wednesday night into Thursday as a weak boundary sags south
through the area and late in the week into next weekend. There is
some differing solutions on the lake week potential, so low
confidence at this point.

- Warm weather back in full swing today and Monday

As for temperatures the main story is warm. 850mb temps will stay
between +14C and +20C the next 7 days so mixing those numbers down
dry adiabatically we are looking at highs in the 80s much of the
time. Today will be the warmest day as southwest flow and plenty
of sunshine drive high temperatures to around 90. Humidity also
creeps up today with surface dew points rising to around 70. So,
after a brief reprieve yesterday it is back to deep summer today.
Agree with previous discussion that we should remain shy of Heat
Advisory criteria.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 138 PM EDT Sun Jun 29 2025

Scattered cu this afternoon will dissipate with sunset leaving
high clouds that are streaming in from the west. As an upper
trough approaches from the west Monday, we`ll see a mid cloud deck
develop. Isolated showers or storms are possible across the region
late in the period, but chances were too low to include in the
TAFs.

&&

.MARINE...
Issued at 341 AM EDT Sun Jun 29 2025

High pressure responsible for the light winds we currently have
across Lake Michigan will slide away to the east today. Southwest
flow will set up in its wake, but the gradient is not strong
enough to produce significant winds or waves. As we head into
Monday a cold front will approach and move through the region, but
once again, winds should remain below advisory criteria. Waves
should remain at or below 2 feet through Tuesday.

&&

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

&&

$$

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