textproduct: Grand Rapids

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 /00Z TAFS THROUGH 00Z THURSDAY/

Issued at 724 PM EDT Tue Sep 1 2026

Light rain showers and isolated thunder continue to move ESE across far southwest Lower MI. Kept the mention of light showers in the AZO/BTL TAFs.

A model signal for storms late this evening and overnight continues to exist, although as before, not all models produce precipitation. So we also kept a Prob30 in the TAFS for the potential for storms during the early overnight period. Otherwise, some MVFR fog is possible toward sunrise due to high dewpoints and residual low level moisture.

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.