150
FXUS63 KGRR 091737
AFDGRR
Area Forecast Discussion
National Weather Service Grand Rapids MI
137 PM EDT Sun Aug 9 2026
.KEY MESSAGES...
- Strong to severe storms most likely through this evening
- Residual small shower/storm chances Monday through Wednesday
- Mainly dry and seasonable Thursday and into next weekend
&&
.DISCUSSION...
Issued at 137 PM EDT Sun Aug 9 2026
- Strong to severe storms most likely through this evening
After above average uncertainty over the last couple of days, it
seems that the atmosphere is finally showing its hand more now. A
line of strong to marginally severe storms has organized over
Wisconsin, and is starting to move over the lake just before 1 pm
EDT.
This line of storms is associated with a MCV from earlier storms,
that is riding the instability gradient aloft ahead of a stalled out
sfc front over Northern portions of Illinois and Indiana as of early
this afternoon. The favored area for the strongest storms with this
line will be over the far SW corner of the forecast area toward Van
Buren and Kalamazoo counties. This is where the best instability
resides over the area where 2000 J/kg of MUCAPE sneaks up. Deep
layer effective shear is also sufficient for some organized storms
sitting at around 30 knots this afternoon.
Mid level lapse rates and CAPE thicknesses are not too exciting for
large hail potential. Gusty winds look like the biggest threat with
unidirectional wind profiles and the linear nature of the line.
Areas across the far SW corner of the forecast area do have a little
better chance of an isolated tornado being closer to the warm front
and the better low level wind shear.
What boundaries that are left behind from this afternoon are likely
to dictate the evolution of the storms for tonight. The reason this
is important is that the larger scale model solutions all have a low
level jet ramping up to our SW by mid evening, helping to fire more
convection along the stalled out front/outflow boundary. We are
expecting that most of any severe threat with the next round of
storms will likely stay south of our forecast area.
We do remain concerned however that heavy rainfall will remain
possible. There are signals in the various model solutions and
suites that there could end up being a heavier localized band
somewhere over our far southern counties, or potentially into
Northern Indiana. If the confidence goes up it will be in our area,
we may try to issue a Flood Watch.
- Residual small shower/storm chances Monday through Wednesday
We do expect that most of the widespread convection after anything
tonight will likely stay farther south across Indiana and Ohio. That
is where the effective front and baroclinic boundary ends up. We
do not see the degree of heat or instability south of the
boundary, but enough moisture and weak short waves in the steady
mid level flow will keep shower/storm chances present for the area
for mainly afternoon and evenings Monday through Wednesday. Lake
breezes and any other mesoscale boundaries will be the main focus
for any diurnal development.
- Mainly dry and seasonable Thursday and into next weekend
The models have been showing that there is a good chance we dry out
for at least Thursday and Friday, and maybe even into next weekend.
We will see a upper ridge across Canada to our NW, that will support
sfc ridging to our north. This will bring in a drier air mass from
Canada with seasonable temperatures.
Eventually a SW flow aloft tries to build over the country, which
would push the warmer air mass toward the area, and maybe
convection along the periphery around it. There is too much
uncertainty at this time to try and get too detailed with that at
this time.
&&
.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 137 PM EDT Sun Aug 9 2026
Main change this update was to be a bit more specific about
thunderstorm timing prior to 22Z now that we see the line of
storms bearing down. Main concern is strong/marginally severe
convective wind gusts, which have been mentioned in a PROB30
group. Model guidance shows another round of showers and storms
possible after this line of storms. Thus, there remain multiple a
PROB30 groups in the TAFs due to significant uncertainty about
future storms because their characteristics depend on what this
initial round of storms does.
Many of the TAFs end at 10/1200Z IFR/LIFR ceiling restrictions.
However, this is for the sake of brevity; in reality, ceilings
should gradually improve from 12Z to 18Z on Monday.
&&
.MARINE...
Issued at 137 PM EDT Sun Aug 9 2026
The best chance for marine headlines is actually early in the
forecast period from now through tonight. This is the case as we
have a low level jet aloft moving overhead. This wind is not
expected to be realized at the surface with it being warm air
advection over the relatively cooler waters of Lake Michigan. We
have some 3 ft waves being mentioned in the northern zones, which
should suffice.
We expect winds then to stay below 20 knots through most of, if not
all of this coming week. There looks to be some wind cores around 20
knots at night, associated with a weak low level jet. These winds
should not make it to the sfc either.
&&
.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...None.
&&
$$
DISCUSSION...NJJ
AVIATION...TJT
MARINE...NJJ
NWS GRR Office Area Forecast Discussion
