733
FXUS63 KGRR 170538
AFDGRR

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

.KEY MESSAGES...

- Shower and Thunderstorm Chances Increase into Wednesday

- Potential for Severe Weather Including All Hazards on Wednesday

- Periods of Showers and Thunderstorms Thursday Through Saturday

- Increasing Heat and Humidity Into The Weekend

&&

.DISCUSSION...
Issued at 339 PM EDT Mon Jun 16 2025


- Shower and Thunderstorm Chances Increase into Wednesday

Shower and thunderstorm chances exist in every forecast period
into Wednesday night with the peak in coverage and intensity
occurring on Wednesday. Tonight, a cold front will remain upstream
in Wisconsin, but some showers and isolated storms may move into
the northwest portion of the forecast area especially up towards
Ludington. 20-30 pct chances for showers/storms is in the forecast
at this point. HREF shows some weakening showers late.

Tuesday instability begins to build as surface dew points
increase. A chance at showers and storms exists on Tuesday. These
storms look to be non severe and almost air mass pulse variety
convection given weak shear. 30 pct chances for storms looks
appropriate centered in the afternoon hours.

Tuesday night (in the evening) a concentrated area of showers and
storms works into Southwest Lower Michigan around and after 00z.
This looks to be a bit more organized as deep layer sheer
increases to around 30 knots. This activity looks to be marginally
severe. Could see a few warnings being issued, limited mainly to
the southwest portion of our CWA, south and west of GRR.

- Potential for Severe Weather Including All Hazards on Wednesday

Focus remains on the Wednesday and Wednesday evening forecast with
the potential for severe weather including, damaging winds, large
hail, tornadoes and flooding.

There is still some disparity in timing of the key features like
the surface low and associated warm and cold front. That said,
models are beginning to close in on a solution. Namely, we like
the ECWMF as it seems to be the most coherent run to run. The
ECMWF has a deepening low (roughly 10mb over 24hrs from 12z Wed to
12z Thu) moving right through the forecast area. This will result
in a warm front lifting north through the area in the afternoon
and a cold front sweeping east in the evening. We are concerned
about both time frames given deep layer shear of 40-50 knots.
Looking at NAM soundings on the warm front over Central Lower
significant low level shear is in place with 0-1km values around
35 knots. 0-3 SRH values are north of 500 m2/s2. Large looping
hodographs are seen in the lowest 2-3 km. Obviously given these
parameters all hazards are in play. MUCAPE values look to surge to
around 3,000 j/kg which is above average even for June. We will be
monitoring this time frame going forward especially tonight when
the event comes into both the HREF and Nadocast windows.

PWAT values surge to around 2.0 inches in the warm sector so very
efficient heavy rain producers are expected. Short term flood
advisories and warnings are certainly possible. As far as the
river systems we are quite low at this time so we should be able
to absorb the rain without major issues. Within bank rises will
occur.

- Periods of Showers and Thunderstorms Thursday Through Saturday

Thunderstorm potential continues into the late part of the week
behind the Wednesday system. Initially Thursday, scattered showers
and thunderstorms are possible Thursday as the longwave trof
traverses the area. However given the lack of mesoscale driven
forcing, coverage is expected to be lower than Wednesday.

Attention then turns to the late week timeframe where a building
ridge out west places us into a "ring of fire" pattern on the ridge
of the greater heating. Synoptically, this would be favorable for an
MCS to track into the region, with the greatest chance of
thunderstorm activity Friday Night. Where a potentially developing
MCS would track remains unclear at this range given mesoscale
forcing is more nebulous, but given the ample moisture and
instability in place this warrants monitoring.

Going into the late part of the weekend and early next week, the
upper-level ridge moves overhead capping the atmosphere and limiting
precipitation potential.

- Increasing Heat and Humidity Into The Weekend

The aforementioned ridge is expected to bring significant heat and
moisture into the region. 850mb temps warm into the low to mid 20s
C Saturday into Monday bringing the potential for one or more days
with highs in excess of 90F. This will not be a dry heat with grand
ensemble probabilities of dewpoints of 65F or greater in excess of
90% Saturday into Monday and 70F or greater in the 70-90 percent
range. This may lead to apparent temperatures climbing well into the
90s across the region.

&&

.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 339 PM EDT Mon Jun 16 2025

Benign marine conditions expected through at least Tuesday morning.
South-southwesterly winds increase, particularly across the
northern half of the lake, Tuesday afternoon and evening. Current
thinking is this brings the potential for 2-4 ft waves and gusts
around 20 knots north of Grand Haven. A second round of increased
winds is possible behind a low pressure system Wednesday Night into
Thursday morning. Winds and waves may approach levels hazardous to
small craft.

Several rounds of showers and thunderstorms are expected between
Tuesday and Thursday. Locally higher winds and waves are possible in
any thunderstorms. The best chance for strong to severe
thunderstorms at this time is later Wednesday with damaging winds in
excess of 34 knots the greatest threat.

&&

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

&&

$$

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