924
FXUS63 KGRR 091051
AFDGRR
Area Forecast Discussion
National Weather Service Grand Rapids MI
651 AM EDT Sun Aug 9 2026
.KEY MESSAGES...
- Thunderstorms this afternoon and tonight with heavy rain
- Chances for showers/storms Mon-Tue but focus may be south
- Drying out mid to late week
&&
.DISCUSSION...
Issued at 402 AM EDT Sun Aug 9 2026
- Thunderstorms this afternoon and tonight with heavy rain
High pressure is in place this morning bringing quiet and dry
weather to the Great Lakes region. Upstream showers and
thunderstorms have formed over portions of Minnesota. The
expectation is that these storms will congeal and move our
direction through the course of the morning. These showers and
storms should move into the area this afternoon, after 200pm.
Feel there is more of a heavy rain threat this afternoon and
tonight than a severe threat, but cannot rule out severe weather
either. Machine learning schemes show all severe weather hazards
in play today as a frontal boundary will be draped across our
southern forecast area near I-94. Wind looks to be the primary
hazard near and south of I-96. MUCAPE values will be in the
1000-2000 j/kg range with deep layer shear on the order of 30-40
knots. We will be monitoring the state of the atmosphere as the
activity moves in during the afternoon. There is enough of a
moisture surge that comes in though which should keep instability
are a moderate level. PWAT values rise from near 1.00 inch to
start the day, surging towards 1.7 inches late in the day. A surge
of that magnitude in the mid summer can bring both severe weather
and hydro/flooding concerns. We are in a day 1 slight risk for
both severe weather and flooding potential. The HREF continues to
show some rather impressive 48 hour max rainfall totals. It is
indicating the potential for some rainfall amounts that may
locally push towards 5 inches through the day on Monday.
Models continue to show redevelopment/training potential into this
evening and tonight at the low level jet provides steady lift over
a cold pool that will be continually reinforced. Short term
flooding headlines are very possible this afternoon and evening.
- Chances for showers/storms Mon-Tue but focus may be south
Chances for showers and storms continue into Monday and Tuesday,
but the potential does not look as good as it did yesterday. The
focus of the low level jet is more off to our south, but the GFS
does bring it back in on Tuesday. There is model disparity with
regard to the LLJ in this time frame so confidence in forecast
particulars is not high. Expecting at least some showers and
storms in the south on the northern periphery of thunderstorm
complexes rolling from IL to IN/OH. Any additional rainfall during
this time could add to issues associated with what falls today.
The day 2 (Monday) slight risk is generally south of our area and
cannot argue with that given that is where the LLJ is focused.
- Drying out mid to late week
Surface high pressure builds in for mid to late week which should
bring a respite from the active weather of earlier in the week. We
actually look to dip back into the 70s for highs Thursday, Friday
and Saturday.
&&
.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 650 AM EDT Sun Aug 9 2026
Moisture is expected to surge back into Southwest Lower Michigan
this afternoon. This surge in moisture will result in building
instability. The bottom line for aviation interests is that
shower and thunderstorm activity should be on the increase. At
this point upstream thunderstorm activity is not impressive, but
we do expect an expansion in areal extent and an increase in
intensity. A band of showers and storms should work through the
TAF sites this afternoon between 19z and 23z with additional
activity develop in the wake of those storms lasting into tonight.
VFR weather is expected through 18z, with a slow downward trend
into tonight. We expect at least MVFR visibilities later this
afternoon and evening, with IFR potential late tonight. Southwest
winds of 10 to 20 knots are expected this afternoon.
&&
.MARINE...
Issued at 402 AM EDT Sun Aug 9 2026
Not expecting much in the way of marine issues the next couple of
days. Waves will reach the 1-3 foot range today with south winds
of 10-20 knots. Waves are forecast to be 2 feet or less from
tonight through Monday and into Monday night. The reason for the
lack of waves is the fact that we will be situated in a fairly
slack pressure gradient in a Col generally.
The time frame to keep an eye on though is today. We have
the potential to see higher waves on a short term basis as
thunderstorms move through the region. Winds and waves are usually
higher in and near thunderstorms and that should be the case today
as well. The 3km NamNest shows the potential for a period of 30-40
knot winds centered around 5pm. So, we will need to keep an eye on
this potential given its a mid summer weekend and boating/swimming
activity should be high.
&&
.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...None.
&&
$$
DISCUSSION...Duke
AVIATION...Duke
MARINE...Duke
NWS GRR Office Area Forecast Discussion
