textproduct: Grand Rapids

KEY MESSAGES

- Heat Wave Coming

- Dry Much of the Week, But Some Thunderstorms May Develop

DISCUSSION

Issued at 219 PM EDT Sat Jun 27 2026

- Heat Wave Coming

A cooler than normal June will close out on a hot note as a pattern change gets underway. A well-positioned building upper ridge will set up across the Ohio Valley this upcoming week. This will push upper heights into the mid 590s dm across Lower Michigan and send 850 mb temperatures into the 20-25C range. The kicker will be high dew point values reaching into the mid 70s, so it will be humid and uncomfortable.

Ensemble guidance tells the story regarding temperatures. Combining the ECE/GEFS/CMC into a probabilistic dataset shows there is a high likelihood (80%) for highs in the 90s to last at least 4 days starting Monday and a 70% likelihood for at least 5 days. Some areas away from the lake and especially east of US 131 may exceed 95 degrees at times and upper 90s are possible especially near and east of Lansing and Jackson. These are rare temperatures for the region and could lead to health hazards for outdoor workers and those without air conditioning. Max heat index readings are very likely to get into the 100-105 range with 105-110 possible. Heat Advisory criteria is a heat index of 100 or greater with Extreme Heat Watch/Warning issued for 105 or greater (or 4 days or more in a row of 100 or greater). Heat headlines will be needed for much of this upcoming week.

Southwest surface flow will ensure our lakeshore region stays cooler given Lake Michigan is currently in the 60s. Even there, highs of 85 or warmer may still occur but that is tolerable compared to inland. The broader zone of some lake-modified air is likely to be near and north of Muskegon. NBM temperatures and dew points (low 90s/mid 70s respectively) in this region may be overdone so we will have to fine tune with time.

The NWS Heat Risk factors in forecast temperatures and humidity with respect to climatology, the duration of heat (eg. is it 1 day? 3 days? 5 days?), and also known thresholds that pose an elevated risk for heat illnesses. The four categories are Minor, Moderate, Major, and Extreme. The current NWS Heat Risk brings the region into the Major to Extreme categories, so impacts are likely especially given we haven't had much heat to deal with this year. Now is a good time to consider modifying the scope and duration of any planned outdoor work or activities particularly Tuesday through Thursday when temperatures will likely be the hottest and the risk for heat illness increases due to prolonged exposure.

- Dry Much of the Week, But Some Thunderstorms May Develop

As the warm front responsible for initiating the heat moves through the region Sunday night into Monday morning, a few showers and possibly thunderstorms may brush the area especially near and north of I-96. This threat should end by midday Monday and severe weather is not expected. Beyond Monday, it will be dry far more than it will be wet. While the Midwest will be under the influence of a solid LLJ of 40 to 50 kts at times, it is angled toward MN/WI and the Upper Peninsula. Slight buckling of the LLJ at times toward Lower MI is shown by some guidance and given the wealth of MUCAPE available (3000-5000 J/kg) can't rule out a ridge riding MCS trying to dive toward our better instability during the nighttime and early morning hours. However, there is currently not much support for this scenario. By the end of the week, some breakdown in the upper ridge may occur as well as a reorientation. This may allow for some shortwave impulses to traverse the region and increase the risk for some showers and thunderstorms to develop Friday into Sunday. That said, some ensemble guidance (ie. ECE members) do support a continuation of heat and humidity right into the holiday weekend along with largely dry conditions.

AVIATION /06Z TAFS THROUGH 06Z SUNDAY/

Issued at 105 PM EDT Sat Jun 27 2026

MKG and GRR have seen cigs lift to VFR over the last 1-2 hours. VFR is expected to continue through the rest of the TAF period. There is the caveat of a 10-20% chance of MVFR fog overnight driven by light easterly flow off of Lakes Huron/Erie, with confidence too low for TAF inclusion. Easterly flow aob 8 knots is expected with the exception of MKG which will see a northerly lake breeze this afternoon.

MARINE

Issued at 219 PM EDT Sat Jun 27 2026

Monday into Tuesday will feature an increasing risk for hazardous waves both to boaters and beachgoers especially near and north of Muskegon, where 3 to 5 feet will be possible. Small Craft Advisories and Beach Hazards Statements are a distinct possibility from Monday afternoon through Tuesday.

GRR WATCHES/WARNINGS/ADVISORIES

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