textproduct: Grand Rapids

KEY MESSAGES

- Patchy Fog into Mid Morning

- Chance for Thunderstorms Wednesday, Some Strong

- Warmer & More Humid Sunday into Early Next Week; Storms Possible

DISCUSSION

Issued at 248 AM EDT Tue Aug 25 2026

- Patchy Fog into Mid Morning

NBM hourly guidance as well as GFS MOS guidance continues to support a few hours of patchy fog mainly from 5am-9am this morning. Some could be locally dense. Low or zero dew point depressions, calm winds, and clear low/mid levels (just some scattered high clouds) should allow for some patchy fog. Will assess observation trends into mid morning, but confidence is not high that any headlines will be needed.

- Chance for Thunderstorms Wednesday, Some Strong

Model guidance continues to support scattered thunderstorms across the region Wednesday. Since we are now within 36 hours of the event, the CAMs are helping fine tune what the threat could be. 00z HREF guidance indicates a surface cold front will be located across western Lake Michigan by 18z Wednesday. Ahead of the front, the HREF mean is showing a plume of 1500 J/kg SBCAPE with maximum members indicating over 2000 J/kg. HRRR soundings for 18z at LAN show a bit more decent shear than previously indicated with 0-6km shear around 30 kts with decent directional shear in the 0-3km layer. Some of the Nam3km soundings look more ominous for low level shear ahead of the front. Overall, there appears to be enough shear and instability along with some low level curvature in the model hodographs to support some severe potential, including perhaps some clusters of storms with embedded strong winds or even some mini-supercells capable of a short duration tornado.

Some questions exist about midday shower activity prior to peak daytime heating. Some CAMs do indicate scattered morning showers moving into western Lower MI, which could complicate pre-frontal storm development in this area. Indeed, models such as the HRRR push showers through GRR around the lunch hour and effectively end the storm threat for the day there. We'll see how this pans out. The Nam3km would delay storms a bit, which could bring more of a severe weather threat to the area. If we had to highlight a particular region for strong storm development, it seems east of US 131 may have the best threat from 18z-00z Wednesday.

As previously mentioned, a few diurnally driven showers and a thunderstorms may develop on Thursday mainly near and east of US 127 given residual instability and moisture left in the area underneath upper troughing. Currently NBM isn't buying it, with POPs too low to mention a threat for rain.

- Warmer & More Humid Sunday into Early Next Week; Storms Possible

After we've gone through some fallish weather lately, it appears we'll reverse course a bit late weekend into early next week as anomalous upper heights move in per ensemble guidance (500 mb heights in the upper 580s dm) as well as 850mb temps that could even touch 20C across southern Lower MI. This also occurs with climbing dew points and PWAT values, as well as an active low level jet pattern somewhere across the Midwest. Depending on where we sit with relation to the upper ridge and low level jet, periods of thunderstorms are certainly possible.

AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/

Issued at 127 AM EDT Tue Aug 25 2026

Winds have gone calm at many locations and temperatures are continuing to fall, heading toward the dew point. Expecting some light fog development at most terminals around or shortly after 09z. NBM hourly guidance along with GFS MOS guidance suggests visibilities may dip to 1sm or lower at LAN and JXN. Given JXN typically overperforms in fog setups, have gone with TEMPO 1/4 mile there to handle the threat. If by 09z-10z it is apparent that fog is struggling to form, may be able to remove that mention. Otherwise, expecting mainly light fog to burn off in the 12z-13z time frame, making way for a mix of clouds and sun. LCLs suggest we may get some few/sct clouds around 4k ft during the day, so may add that back in to the next TAF issuance.

MARINE

Issued at 248 AM EDT Tue Aug 25 2026

Wednesday will be the next hazardous day for mariners and also lead to a high swim risk for many areas, especially Holland to the north. This occurs ahead of a cold front likely situated across western Lake Michigan around 18z Wednesday. Mariners will face increasing wind and wave activity, likely necessitating a Small Craft Advisory for Wednesday with 3 to 6 footers possible and SW winds 10-20 kts and gusts to 25 kts.

GRR WATCHES/WARNINGS/ADVISORIES

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