593
FXUS63 KGRR 170743
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
342 AM EDT Tue Jun 17 2025

.KEY MESSAGES...

- Small Chances for showers/storms through Wednesday morning

- Severe weather potential increasing for Wed afternoon/evening

- Storm chances Friday through Saturday; then Heat Builds

&&

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

- Small Chances for showers/storms through Wednesday morning

We are going to see mainly dry conditions dominate over the area
over the next 30 hours or so, but there will be multiple small
chances of a shower or storm popping through Wednesday morning.
These chances will be mainly focused along a weak cold front that is
currently over Western Wisconsin and far SE Minnesota.

There are a few showers and isolated storms over Eastern Wisconsin
currently as of 0730z/3:30 am EDT this morning. These showers and
storms are associated with warm and moist advection associated with
the low level jet ahead of the front. These showers and isolated
storms are falling apart, and will continue to do so. They may reach
the lakeshore before dissipating completely by mid morning.

The front will slowly push through the forecast area this afternoon,
which normally would be somewhat favorable to fire a few
showers/storms. We will have a less than favorable set up this
afternoon, barely to justify 20 pops. As the front drops in, the
flow will become more generally divergent downwind of Lake Michigan
with a flow off of the lake.

The front will sink south to near or just south of the I-94 corridor
tonight. The chance of a few showers and storms popping tonight will
be a little more favorable as a low level just out to our west will
focus a warmer and more moist air mass to be advected up and over
the front. This shower/storm activity does not look to feature any
severe weather as it will be mainly elevated north of the front.

- Severe weather potential increasing for Wed afternoon/evening

We will likely see some of the shower activity linger into the
morning hours of Wednesday as we see the low level jet approach the
area. The showers and storms should then push north and east as the
strengthening sfc low to our SW will drive the warm front north
across the forecast area. This will bring the very warm and humid
air mass in the warm sector up and over the area.

This warm and moist air mass will then drive instability up with
CAPEs forecast to increase to over 2,000 J/kg by late Wednesday
afternoon for a good chunk of the area. It will be late Wednesday
afternoon and evening that the strong short wave supporting the sfc
low will be pushing right up over the area.

The stronger and organized fashion of the short wave will likely
develop an arc of storms out ahead of it that will quite supportive
of damaging winds with it. Deep layer shear values are forecast to
rise to around 40-50 knots at this time also. The CAMs and even
synoptic models are all latching on to this idea with some small
variations. Large hail will be possible, but mid level lapse rates
are not that impressive with values around 6-6.5 C/km. The threat of
at least a tornado or two will be there, especially closer to the
sfc low where a backed wind flow in the column will support that.
Any tornadoes will likely by the QLCS type with the line of storms
expected. All of these factors definitely support the upgrade to an
enhanced risk by the SPC.

- Storm chances Friday through Saturday; then Heat Builds

It is looking more likely that Thursday may just end up being
cooler, less humid, and dry for the area. The upper trough does not
support another short wave moving in later Thursday as it once did.
It is more ridging then anything else in the wake of the stronger
sfc low moving through on Wednesday.

We are still looking at somewhat of an unsettled period for Friday
into Saturday, then resulting in heat and humidity building for
Sunday and Monday.

We will see the upper ridge start to build over the region on
Friday. We will be on the edge of the building warm dome that will
be building northward. Being on or near the edge will put us in the
transition zone with plenty of fuel/instability just south, and
short wave energy with the upper jet. This scenario will result in
the potential for a couple of MCS` that would ride along the jet.
Confidence is starting to increase that the favored zone may be just
north of the area. However, these MCS` can propagate a little
further south than models try to indicate.

There is good agreement however that the ridge builds sufficiently
to cap our atmosphere, and heat us up. We will have a decent
potential of touching 90F with this ridge.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 138 AM EDT Tue Jun 17 2025

We are looking at mainly VFR conditions continuing to dominate
through this forecast period at all of the terminals. The only
exception to this will be a small chance of showers and isolated
storm at the I-94 terminals late in the period.

We are seeing mainly high clouds overhead first thing this
morning, with some mid clouds scattered around. These conditions
will continue, with maybe an increase of mid clouds at KMKG after
10z with even a chance of a rain shower as a line weakens as it
approaches the site.

Most of the day will stay dry at most locations. Can`t rule out an
isolated shower popping up. We will mainly see an increase in
diurnal cumulus clouds, once again all VFR. Some wind gusts to 20
knots will be possible at the inland sites. We will see a few
showers pop up after 02z tonight along a frontal boundary that is
expected to be down along I-94. The certainty with regards to
placement remains somewhat low, so will go no more than a PROB30
group to cover this for now. Also, can`t rule out an isolated
storm with this, but that threat is too low at this juncture to
include for now.

&&

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

The latest trends for winds and resulting waves are for the
nearshore to stay below any Small Craft/Beach Hazards criteria until
Wednesday afternoon ahead of the incoming front. Small Craft
criteria is expected Wednesday afternoon ahead of the front, and
Gales are possible in the wake of the front on the backside of the
stronger low moving through.

&&

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

&&

$$

DISCUSSION...NJJ
AVIATION...NJJ
MARINE...NJJ