632
FXUS63 KGRR 171943
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
343 PM EDT Tue Jun 17 2025

.KEY MESSAGES...

- Periodic Showers and Storms Tonight into Wednesday Morning

- Round of Severe Weather Expected Wednesday Afternoon/Evening

- Storms Possible Friday/Saturday

- Heat Wave Likely This Weekend Into Early Next Week

&&

.DISCUSSION...
Issued at 342 PM EDT Tue Jun 17 2025

- Periodic Showers and Storms Tonight into Wednesday Morning

Southwest Lower Michigan is squarely situated in the warm sector
of a developing system at this point with southwest winds pushing
temperatures in the 80s to around 90. Dew points are mixing out a
bit but it is still more humid than it has been in awhile. A cold
front is poised upstream of the area stretching from near the
Mackinac Bridge southwest across Wisconsin and into Iowa. The
front will sag south into the area tonight resulting in the
potential of some scattered convective activity late this
afternoon and early evening, with a better chance in the mid
evening hours as storms move out of Wisconsin and Illinois. These
storms will be spreading in as instability is beginning to wane,
but a few storms could be on the strong side with some wind gusts
to around 50 mph possible towards Lake Michigan. Downdraft CAPE is
in excess of 1000 j/kg owing to steep low level lapse rates and
some vertical mixing resulting in some drier air below cloud base.
Strongest cores this evening will have this stronger wind
potential.

A fade in activity is expected overnight as we lose the deeper
instability. Some scattered activity may persist through the night
however.

- Round of Severe Weather Expected Wednesday Afternoon/Evening

The main focus is clearly on Wednesday as a potent shortwave in
the mid levels works out of the Central Plains states tonight,
plowing into Southwest Lower in the afternoon hours. As the
associated surface low lifts our direction and through the
forecast area tomorrow it will deepen as it moves northeast. The
NAM has it below 1000 mb Wednesday afternoon. HREF parameters show
surface dew points near 70, MUCAPE values in excess of 2000 j/kg
and deep layer shear on the order of 40 knots. All which are very
much lined up for a severe episode.

The severe threat may be bimodal with rotating updrafts
interacting with a warm front that the HREF has positioned along
I-96 resulting in a tornado threat. Backed low level southeasterly
flow north of the warm front will increase SRH in the low levels.
In the open warm sector to the south of I-96 a supercellular
tornado threat also exists, but there is a higher likelihood of
damaging winds given strong unidirectional flow in this area 30-50
knots at both 850mb and 500mb out of the southwest. Mid level
lapse rates for hail are poor, but this could be made up for by
the strong shear. Bottom line, all hazards are in play Wednesday...
strong damaging winds, tornadoes and the potential in the
strongest storms for some near severe hail. Models seems to be
converging on a low lifting east-northeeast across the I-96
corridor. It is in this locations (I-96) and points to the south
where the best chances for severe weather exist. This is also
seen in the updraft helicity swaths that are maximized near and
just south of I-96. Time frame looks to be in the 200pm to 900pm
time frame for the main threat. Machine learning nadocast output
also supports the above thinking in terms of all hazards and
location.

As for a flooding threat, the HREF output supports localized short
term flooding Wednesday afternoon and evening. PWAT values are
around 2.0 inches which is about as good as it gets outside of a
true tropical connection. We will be monitoring heavy rain swaths
tomorrow as well for possible advisories and warnings. The
showers and storms sweep east Wednesday evening with the cold
front.

- Storms Possible Friday/Saturday

The forecast for Thursday has trended drier compared to 24 hours ago
as we mainly will be observing subsidence behind the mid-level wave.

Conditions become more unsettled Friday into Saturday as a building
ridge over the Great Plains leaves us on the eastward edge of the
best heating. ECMWF and GFS are in decent agreement that the favored
track for any resulting MCS would be north of the CWA, however a
developed MCS can propagate further south than expected meaning
thunderstorms are still possible, particularly Friday Night. Right
now, 30-50% chance PoPs are limited to near and north of M20.

Conditions then dry out later Saturday into early next week as the
ridge moves eastward, thoroughly capping the atmosphere.

- Heat Wave Likely This Weekend Into Early Next Week

The aforementioned ridge will be anomalously strong for the region
with 500mb heights exceeding EPS climatology. The Saturday into
Tuesday period will be hot, with the warmest days being Sunday and
Monday. Both days feature 60-70 percent probabilities of highs
exceeding 95F. High dewpoints (in the upper 60s to low 70s) will
only compound the issue with heat index values over 100 possible.
While there are still uncertainties given we are several days out,
heat headlines may be needed for at least part of the period.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 124 PM EDT Tue Jun 17 2025

MVFR cigs have broken but SCT 2-4kft clouds are expected over the
next few hours. VFR conditions persist through the night and into
the day Wednesday, with one key caveat.

Scattered showers and storms are possible between 02z-06z near I94
and LAN from a stalled front and have continued the PROB30
accordingly. After 6z and particularly after 12z scattered
showers and storms are possible across Central Lower MI including
LAN, MKG, and GRR. MVFR and lower conditions are possible in any
storms. Finally, Gusts to 20 knots are likely through this
evening.

&&

.MARINE...
Issued at 342 PM EDT Tue Jun 17 2025

Winds and waves will stay below levels hazardous to small craft
through at least Wednesday afternoon. Ahead of a cold front, 2-4
foot waves and isolated gusts up to 25 knots are possible. Winds
behind the front increase from the northwest to around 20 knots. A
period of winds and waves approaching levels hazardous to small
craft are likely through Thursday morning.

Outside of winds and waves, the primary concern is the potential for
a few rounds of thunderstorms over the next 48 hours. In particular,
thunderstorms will likely develop Wednesday afternoon with storms
near and south of Muskegon having a decent chance of being severe.
Wind gusts in excess of 40 knots are the main hazard with any storms
and hail and isolated waterspouts cannot be ruled out.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Air Quality Alert until midnight EDT tonight for MIZ050-056-057-
     064-071.
MARINE...None.

&&

$$

DISCUSSION...Duke
AVIATION...Thomas
MARINE...Duke