299
FXUS63 KGRR 161149
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
749 AM EDT Sat Aug 16 2025

.KEY MESSAGES...

- Shower and Thunderstorm Chances Today through Tuesday

- Dry with Temperatures Towards Normal Wednesday through Friday

- Warm and Humid Through Monday

&&

.DISCUSSION...
Issued at 359 AM EDT Sat Aug 16 2025

- Shower and Thunderstorm Chances Today through Tuesday

There will be a chance for showers and thunderstorms in the next 7
(12 hour) forecast periods from Today through Tuesday.
Synoptically, we will see a front sag into the area from the north
today. Currently the front is situated across Lake Superior as of
07Z. The front will make inroads into our area up across Central
Lower Michigan in the late afternoon and evening. It is likely that
the front will be the spark for convection. There is a signal in the
models that we will see a line of storms sweep in from the northwest
during this time frame. The convection allowing models have slightly
different timing, but multiple models show a bowing linear system
moving southeast into our area off the lake and from Wisconsin. We
will be sitting on 2,000+ j/kg of MUCAPE so the storms will be
robust. Deep layer shear ahead of the line is weak so the severe
threat is overall lower, but agree with the SPC Marginal Risk given
what we are seeing in the hi-res models. In the fire tab of the
HREF, the 10m max gust field shows multiple models producing 60mph
winds over the lake and potentially into far Western Lower. For
today that is our threat, wind, from this afternoon into the
evening, with the lakeshore being the highest threat zone.

Showers and storms become a bit more nebulous to nail down beyond
today, but in general, the threat looks to sweep south of the area
tonight as the front pushes south. A chance will exist in the
south especially on Sunday as that area will be in closest
proximity to the front. Monday into Monday night a low forms out
to our west moving into Wisconsin on Monday and through our area
Monday night. On Tuesday into Tuesday night the entire system
sags south of the area. Bottom line is we cannot rule out periodic
showers and storms in any forecast period in this time frame. Best
chances look to be Sunday night into Monday night as those chances
will have synoptic support via an upper shortwave and associated low
level jet.

Isolated torrential rain at times is likely due to the fact we
will have PWAT values near and in excess of 2 inches. Rainfall
rates in the stronger storms will be 1-2 inches per hour at times.
So, while we are not expecting widespread flooding, there is an
isolated/short term flooding threat.

- Dry with Temperatures Towards Normal Wednesday through Friday

Very much the tail of two forecasts when comparing the first few
days of the 7 day forecast to the last few days. Surface high
pressure will dominate the mid to late work week with dry weather
and slightly cooler temperatures. Highs Wednesday and Thursday will
be closer to normal in the lower 80s.

- Warm and Humid Through Monday

850mb temperatures will be in the +18C to +20C range from today
through Monday. So, high temperatures will push towards 90F
outside of thunderstorm activity. The humidity returns as well as
dew points surge into the 70s. Heat index values today especially
will push well into the 90s.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 749 AM EDT Sat Aug 16 2025

VFR weather is in place to start the day. We expect those
conditions to continue through the remainder of the morning hours
as cumulus development commences. Scattered to broken cumulus
clouds should fill in by early afternoon with bases around 5,000
feet.

The main focus of the aviation forecast is the threat of
thunderstorms this afternoon and evening. Some models are showing
the threat of storms between 16Z and 20Z developing in the heat of
the day. We feel a better chance for storms exists between 21Z
and 02Z and have confined the thunder threat to within those
hours. There is not much of a trigger for storms in the early to
mid afternoon, with the more likely scenario of storms rolling
east out of Wisconsin and across the lake after 400pm. Confidence
is not high though when it comes to the evolution of storms
today.

Tonight showers and storms should wind down as we lose the
heating of the day. Winds will be southwest at 5 to 15 knots
becoming west.

&&

.MARINE...
Issued at 359 AM EDT Sat Aug 16 2025

Overall a pattern is in place that does not support significant
wind and waves the next few days. That said there are a couple
time frames we will need to watch. The first is this afternoon and
evening as there is a signal in the convection allowing models
that we may see a squall line sweep the lake from NW to SE. The
phrase...wind and waves briefly higher in and near
thunderstorms...certainly applies. So, while the WaveWatch3 is not
showing higher waves it is possible that we see a push into the 2-4
foot range depending on the strength of the line of storms and the
winds associated with it.

The second time frame to take note of is on Sunday afternoon when
our "advancing high" setup comes to fruition. The characteristic
pressure gradient is seen in the GFS where the isobars mimic Lake
Michigan. The lake helps accentuate the high and a thermal low over
the landmass aides in the push of winds down our shoreline at and
after peak heating. The higher winds can very much be seen in the
high resolution models up towards the point in the afternoon. At
least in that location...north of Whitehall...on Sunday we will have
to keep a close eye on things. At this point, no marine headlines,
but again we will be watching.

&&

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

&&

$$

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