textproduct: Grand Rapids

KEY MESSAGES

- Scattered showers/thunderstorms today into Saturday

- Cooler and dry Sunday through Tuesday, midweek rain chances

DISCUSSION

Issued at 317 AM EDT Fri Aug 21 2026

- Scattered showers/thunderstorms today into Saturday

The only change in thinking from 24 hours ago is that Convection Allowing Models (CAMs) show widely dispersed convective cells developing this afternoon over much of southwest Lower Michigan. Please see the AVIATION section of this forecast discussion for more specific details.

We expect precipitation coverage to be a bit more widespread starting early tonight with additional convection approaching from Lake MI. The latest SPC SWODY1 even has a marginal risk for wind along our lakeshore. This would be predicated on already existing organized multicell clusters making it into the area. Some Machine Learning guidance, such as the NCAR suite, supports this idea.

Neither the thermodynamic environment nor the kinematics over western Lower MI look particularly impressive for convective maintenance, so would expect a fairly quick demise of any organized convection that makes it this far east.

Models have been fairly consistent with a gradual west to east shift of precipitation Saturday that mostly exits the area by evening. It appears that the severe threat will remain east of our area during this time.

- Cooler and dry Sunday through Tuesday, midweek rain chances

By late Sunday, the forecast area will be under deep, almost due northerly flow with surface ridging. This is a dry pattern, but as noted previously, can't completely rule out isolated pop up diurnal showers due to colder air aloft.

From Tuesday onward, flow aloft becomes more northwesterly and an upper PV max in this flow will provide enough forcing for upward vertical motion to reintroduce precipitation chances Tuesday night into Wednesday. After this, mean troughing aloft continues with unsettled conditions the rest of the work week. At this point, there will be limited predictability regarding the timing of subsequent precipitation chances.

AVIATION /12Z TAFS THROUGH 12Z SATURDAY/

Issued at 648 AM EDT Fri Aug 21 2026

Probability for VFR conditions at all terminals through 18Z is 90 percent. Main changes this update were to:

- delay onset of precipitation slightly - reduce PROB30 group convective wind gusts - add an extra PROB30 group for the overnight hours

These changes were based largely on most recent guidance, which happens to show less overall coverage of showers and storms this afternoon and tonight. Decided to trend this TAF package gently in that direction with the understanding that future guidance and radar/satellite trends could nudge things right back in the opposite direction.

MARINE

Issued at 317 AM EDT Fri Aug 21 2026

Although conditions will admittedly be marginally, we still opted to issue a Small Craft Advisory and Beach Hazards Statement for our northern two marine zones this afternoon into early evening. There will be a risk for longshore currents and locally higher waves, even though the overall wave field may be better described by sub-advisory criteria. Nonetheless, this messaging lines up well with APX's immediately to the north of these zones.

There is another reason for erring on the side of caution here that is not meteorologically based. We are looking at a Friday in the summertime that may be the last Friday before many people return to work or school. Consequently, there could be greater than normal willingness to be exposed to a marginal risk; this gives a net result of significant vulnerability.

GRR WATCHES/WARNINGS/ADVISORIES

MI...Beach Hazards Statement from 2 PM EDT this afternoon through this evening for MIZ037-043. MARINE...Small Craft Advisory from 2 PM this afternoon to 8 PM EDT this evening for LMZ848-849.