textproduct: Grand Rapids

KEY MESSAGES

- Areas of Fog Tuesday Morning

- Thunderstorms Possible Wednesday, Less Likely Thursday

- Active Pattern Could Develop By This Weekend

DISCUSSION

Issued at 310 AM EDT Mon Aug 24 2026

- Areas of Fog Tuesday Morning

With a surface high building into the region later today into tonight, winds will gradually tail off after dark and likely go calm for many areas. GFS MOS guidance along with NAM soundings do support areas of fog developing after 06z Tuesday as dew point depressions approach or reach zero in some spots and an inversion above the surface develops. Scattered clouds could be present, however, so a widespread fog event does not appear likely. Still, there is enough of a signal to put areas of fog in the forecast. Fog may develop especially in low lying areas, which could have some localized impacts to the Tuesday morning commute.

- Thunderstorms Possible Wednesday, Less Likely Thursday

Midweek rain chances remain on track as upper troughing is set to move through the Great Lakes during peak heating on Wednesday. Low level moisture advection ahead of this trough is shown by model guidance, with surface dew points starting the day (12z) in the upper 50s, then climbing to the mid to perhaps upper 60s by late afternoon. SBCAPE values shown by the RRFS are forecast to reach 1500 J/kg. Some model guidance supports up to 2000 J/kg, though this would likely be the higher end of what materializes. While deep layer shear looks to be about 25 kts, thus leading to quasi-organized convection, one supporting factor to some locally stronger storms is RRFS modeled DCAPE values of 800-1000 J/kg. We're expecting at least some scattered showers/storms to develop primarily after 18z and lasting to about 06z. Better coverage may get going after 21z. A scattered damaging wind gust is not out of the question Wednesday afternoon and evening, so will monitor trends.

The GEM and ECMWF support a slight chance for a few afternoon showers and thunderstorms Thursday mainly east of US 131 as residual surface based instability could remain underneath upper troughing. The GFS dries out the low levels quicker and would largely end the threat. The NBM POPs are quite low on Thursday, so rain is not a high threat at this point.

- Active Pattern Could Develop By This Weekend

Both ensemble and deterministic model guidance is signaling a potential shift in the synoptic pattern by this weekend into early next week. In fact, both the 00z ECE and CMC ensemble mean 500 mb heights are in the upper 580s dm across Lower MI by Sunday. This could potentially place the Great Lakes in stronger mid level flow with mid level jet cores moving overhead. The positioning of the upper ridge will be key to the location of convective activity. At this point we may be in an active corridor or close to it. PWAT values of 1.50"-2.00"+ are supported by ensemble guidance. So, these are some interesting trends to keep watching. We may round out the month of August and enter September with some periods of storminess, especially Sunday and beyond.

AVIATION /12Z TAFS THROUGH 12Z TUESDAY/

Issued at 640 AM EDT Mon Aug 24 2026

SCT to even BKN cloud bases around 3k ft have developed across portions of western and northwestern Lower MI, poorly handled by model guidance. With daytime mixing this morning, these cloud bases should lift toward 5k ft or so. Either way, not a major impact to aviation. Focus turns to the 08z-12z Tuesday time frame with possible fog development occurring. This is shown by NAM soundings with an inversion setting up above the surface and temp/dp depressions near zero at times. MAV/LAV guidance does suggest 2sm or below for the terminals with the exception of MKG, possibly dense fog at times for locations like JXN. Will continue assessing model guidance today to better pin down this threat.

MARINE

Issued at 310 AM EDT Mon Aug 24 2026

Waves should generally stay in the 2 to 4 foot range today as NW flow will be a bit weaker than Sunday, and also decreasing later in the day. Winds and waves should by and large stay below Small Craft Advisory criteria, though some 3 to 5 footers can't be ruled out early afternoon mainly south of Pentwater. A moderate swim risk will be in place given these waves. Will keep an eye on buoy trends today to see if waves overperform, and if so a Beach Hazards Statement and Small Craft Advisory may be needed for some areas. At this point we will hold off on any headlines.

GRR WATCHES/WARNINGS/ADVISORIES

MI...None. MARINE...None.