235
FXUS63 KGRR 011838
AFDGRR
Area Forecast Discussion
National Weather Service Grand Rapids MI
238 PM EDT Tue Sep 1 2026
.KEY MESSAGES...
- Increased chances for showers and thunderstorms today
- Thunderstorms, some severe, expected overnight
- Severe thunderstorm chances Wednesday and Thursday
- Mostly afternoon thunderstorm chances Friday into Labor Day weekend
&&
.DISCUSSION...
Issued at 238 PM EDT Tue Sep 1 2026
- Increased chances for showers and thunderstorms today and tonight.
Have raised precipitation chances significantly during this time.
Thunderstorms overnight have a chance of being severe with damaging
winds and large hail being the main threat.
Latest radar mosaic shows elevated showers with a few thunderstorms
extending from the OH Valley, through SW Lower MI, and as far west
as IA. The models are not handling this well at all. This activity
appears to be associated with a subtle mid/upper level shortwave
trough noted in water vapor imagery and local VWPs. Given the unbroken
chain of convection extending well upstream, PoPs look in order through
this afternoon for at least our southwest forecast area. The rest of
the forecast area will need to be watched for an escalating heat
risk this afternoon.
- Thunderstorms, some severe, expected overnight
Many of the 12Z Convection Allowing Models (CAMs) such as the HRRR,
NAMNest, and RRFS have come into much better agreement regarding
convective evolution tonight and this consequently has appeared in
the 12Z HREF. We expect a greater than 50 chance of thunderstorms to
develop or move into the forecast area around or shortly before
midnight tonight. The parameter space is impressive, with effective
shear greater than 40 knots and MUCAPE in excess of 3000 J/kg. A
long and generally straight hodograph would support splitting
supercells capable of producing large hail. Not surprisingly, there
are some SARS sounding analogs associated with significant hail to
back this up. This all being said, the convective mode could end up
leaning more towards multicell clusters instead of uncontaminated
supercells which would reduce the large hail threat somewhat.
Regarding the wind threat, ML guidance such as NadoCast tends to favor
this much more than other hazard types. Damaging wind is a plausible
concern, because although there will be a nocturnal/marine boundary
layer induced inversion, some models generate substantial DCAPE, and
when it comes to supercells with deep mesocyclones (as this environment
supports), there can be an added vertical non- linear dynamic component
to assist with downward momentum transfer.
As noted in the WPC outlooks, there is a marginal threat for excessive
rainfall. Precipitable water certainly is impressive but rainfall amounts
will likely be curtailed by a combination of progressive cell motion,
the presence of at least some hail, and reduced sub-cloud RH.
Models suggest the focus for showers and thunderstorms will shift
northward towards sunrise Wednesday morning.
- Severe thunderstorm chances Wednesday and Thursday
Predictability regarding thunderstorm specifics drops off rapidly early
in the forecast, but confidence is high that we will remain in an environment
supportive of strong to severe thunderstorms. Persistent strong conditional
instability during this time is noted in the ECMWF ensemble mean, which
shows MUCAPE around 3000 J/kg Wednesday dropping to a still respectable
2000 J/kg by Thursday. Effective shear also consistently remains in
40 knot territory during this time.
Both SPC SWODY2 and GEFS ML guidance show the greatest chance (Slight)
of severe convection over southeastern Lower MI and touching our forecast
area with a continued westward marginal risk area covering most of the
forecast area. The main threats in play would be wind and hail and
this would be most likely to occur in the afternoon. A marginal risk
for excessive rainfall will continue as well. The other thing to watch
will be Heat Indices exceeding 100F Wednesday afternoon towards I-94.
- Mostly afternoon thunderstorm chances Friday into Labor Day weekend
The upper pattern transitions to a sharp ridge just upstream, which
normally would be a dry pattern. However, in this particular case, conditional
instability supportive of showers and thunderstorms is expected to remain
into Sunday at least. The best chances for precipitation will be in
the afternoons/evenings as diurnal destabilization becomes a more
critical ingredient than before and favorable timing with transient
upper PV maxima will be an added plus.
&&
.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 135 PM EDT Tue Sep 1 2026
Radar shows a band of showers/storms moving ESE across southern
Lake MI and far southwest Lower MI. This precipitation is close
enough to AZO and BTL to include in the TAFs.
There is some concern for strong storms developing mid lake near
MKG around midnight and moving ESE across LAN/JXN during the
overnight hours. A few models show this, but not all. We included
a prob30 for this potential except at AZO/BTL which should be far
enough south to preclude mention.
Given high dewpoints and residual moisture from recent rains, some
fog/haze is possible Wednesday morning.
&&
.MARINE...
Issued at 238 PM EDT Tue Sep 1 2026
Winds and waves are expected to remain below advisory criteria
into Friday thanks a loose pressure gradient associated with a
persistent front in the area. The chief hazards will be cloud to
water lightning and locally elevated winds and waves associated
with scattered to numerous thunderstorms.
&&
.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...None.
&&
$$
DISCUSSION...TJT
AVIATION...04
MARINE...TJT
NWS GRR Office Area Forecast Discussion
