textproduct: Grand Rapids

KEY MESSAGES

- Showers and storms this afternoon and overnight

- Precipitation chances return Friday

DISCUSSION

Issued at 324 AM EDT Tue Aug 18 2026

- Showers and storms this afternoon and overnight

Now that we are at a closer forecast projection time to better leverage convection-allowing models (CAMs), a couple of differences are observed compared to this time yesterday. First, we see a greater amount of forecast conditional instability, which is now sufficient for thunderstorms across most, if not all, of the forecast area. The 18/00Z HRRR, for example, indicates a ribbon of MUCAPE approaching 2000 J/kg immediately ahead of the cold front that will traverse the forecast area tonight.

The second difference is that precipitation is now expected to exit the forecast earlier than previously thought, thanks to a more progressive cold front. Rain may even be totally out of the forecast area by sunrise Wednesday.

Both model guidance and satellite suggest stronger forcing aloft to hasten progress of the cold front. GOES full disk imagery shows a seasonably strong upper low entering the northern Ontario/southern Hudson Bay region from Manitoba. A strong upper jet is diving southward over Saskatchewan on the back side of this low and this likely will be the feature that that emerges as strong PV lobe rotating around the southern periphery of the low and crossing Upper MI and northern Lower MI early tonight.

Greater conditional instability naturally raises the question of severe potential with these thunderstorms. Several CAMs, both within the SPC HREF suite and also external members such as RRFS and MPAS- NSSL, depict discrete cells with notable mid-level updraft helicity moving across the northern forecast area early tonight; this speaks to the expected presence of marginally favorable deep layer vertical shear. Low level updraft helicity is much less pronounced, which is not surprising given that the best low level shear should be restricted to the poleward side of the front and away from the convection.

Estimated DCAPE will be respectable at 800+ J/kg, thanks in part to moderately steep mid-level lapse rates approaching 8C/km. Thus, it is no surprise that wind will be the most plausible severe weather type, as noted by SPC guidance across the west-northwest CWA. Still, with the potential for discrete supercell structures depicted by the CAMs, we can't rule out some marginally severe hail given the potential for mesocyclones in this steep mid-level lapse rate environment. We are a bit less concerned about heavy rain given the anticipated more progressive nature of the front, but do note that WPC continues a marginal risk for excessive rainfall across far southwest Lower MI.

- Precipitation chances return Friday

Northwest flow aloft will yield dry conditions Thursday. Broad upper troughing across the northeastern U.S., including the Great Lakes, will continue. Deterministic solutions show upper PV maxima working their way through this pattern during this time. The first one arrives Friday, bringing a chance for showers and thunderstorms by Friday night. The weekend continues to look potentially unsettled with a significant upper PV max diving southward and potentially evolving into a closed low through a deep layer of the troposphere somewhere over the western Great Lakes.

AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/

Issued at 645 AM EDT Tue Aug 18 2026

Through 18Z, VFR conditions are highly likely (95 percent chance).

Main change this update was to provide extra details regarding expected evolution of showers and thunderstorms tonight, specifically through and after 06Z. Terminals, especially MKG, could see convective wind gusts exceeding 30 knots at some point early tonight but it was too low probability to include even in a PROB30 group. Once storms develop, it will be possible to add more specific detail, but this likely won't really be feasible until well after 18Z.

MARINE

Issued at 324 AM EDT Tue Aug 18 2026

Based on latest wind and wave guidance, decided to issue a Small Craft Advisory for this afternoon into early tonight chiefly for our northern marine zones. There will be a brief window of brisk west-southwest winds ahead of the front which will grow waves into the 3-5 foot range.

Given that the hazard will begin late in the day and likely will be accompanied by precipitation, this is not a situation likely to involve many beachgoers. So, will hold off on a beach hazards statement for now, but the day shift may elect to issue this if the hazard begins earlier or precipitation begins later.

GRR WATCHES/WARNINGS/ADVISORIES

MI...None. MARINE...Small Craft Advisory from 5 PM this afternoon to 11 PM EDT this evening for LMZ846>849.