textproduct: Grand Rapids

KEY MESSAGES

- Becoming Hot and Humid

- Rain Chances Increase Tonight into Sunday

- Additional Chances for Rain and Thunderstorms Next Week

DISCUSSION

Issued at 226 AM EDT Sat Aug 29 2026

- Becoming Hot and Humid

As high pressure shifts east today, southwest flow behind the high will advect warmer temperatures and higher dew points. Highs today around 80 will increase into the mid 80s to low 90s by Tuesday. Dew points will increase into the mid to upper 60s with the potential for dew points in the low 70s on Tuesday and Wednesday. The combination of these temperatures and dew points will result heat index values in the 90s starting Monday. The hottest heat index values will be Tuesday and Wednesday with most of the area in the mid 90s. Areas toward south central Michigan may see heat index values approach 100 Tuesday.

- Rain Chances Increase Tonight into Sunday

The low level jet causing showers and thunderstorms to develop in eastern South Dakota and western Minnesota will gradually shift east today. A weak 500mb shortwave associated with this convection will move in during the late afternoon and early evening paired with the 850mb low level jet convergence. Instability will be limited over the area therefore expected a weakening trend as it approaches western Michigan. Showers will expand eastward overnight into Sunday as warm air advection and low level convergence set up over the area. The thunder potential will remain low into Sunday morning with the best chance for a rumble or two toward the lakeshore. Precipitable water values will approach 2 inches which is around the 90th percentile climatologically for this time of year. There is the potential for some pockets of heavier rainfall (around 2 inches) as seen in the localized probability matched mean of the HREF. The probability looks higher toward the lakeshore as there will be a little more instability to tap into (around 500 J/kg).

Most of the rainfall is expected Sunday morning with a lull during the afternoon. Areas toward US-10 could see rain all day. An additional round of rain and thunderstorms is expected to develop Sunday night into Monday morning with the low level jet convergence and warm air advection.

- Additional Chances for Rain and Thunderstorms Next Week

Forecast confidence decreases into next week as details get a little murky. Slight differences in the strength and placement of the upper level ridge and shortwave troughs moving through will impact our chances. With the mentioned high dew points and precipitable water values, it won't take much to spark off showers and thunderstorms. Therefore there are chances each day, although the best chances will be during the overnight timeframes matching with the best low level jet convergence and warm air advection.

AVIATION /12Z TAFS THROUGH 12Z SUNDAY/

Issued at 640 AM EDT Sat Aug 29 2026

VFR conditions are expected over the next 24 hours with winds becoming more southwesterly. Showers will slowly move in from the west late this afternoon into Sunday morning. MKG and GRR have the best chances to see showers between 6-12Z with elsewhere holding off until after 12Z Sunday. There is a low chance for thunder toward MKG, but not high enough to include in the PROB30 group.

MARINE

Issued at 226 AM EDT Sat Aug 29 2026

Southwest winds of 10 to 20 knots are expected today and will build to 15 to 25 knots Sunday. This will cause wave heights of 1 to 3 feet today to increase to 2 to 5 feet for Sunday with the highest waves north of Grand Haven. Winds may end up being lower as there is some uncertainty if higher gusts will be able to mix down with the warmer air moving over the waters. Highest gusts may be right along the lakeshore vs over the open waters. Southwest winds continue into next week with wave heights remaining largely in the 1 to 3 foot range until Tuesday.

A few thunderstorms are possible this late this afternoon into Sunday with better chances Sunday night into Monday morning north of Grand Haven. Daily chances for showers and storms exist through next week, most during the overnight timeframe.

GRR WATCHES/WARNINGS/ADVISORIES

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