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KEY MESSAGES

- Dangerously hot and humid conditions through Thursday with heat indices in the upper 90s and low 100s

- Multiple rounds of thunderstorms tonight through Thursday with the potential for producing damaging winds, large hail, and heavy rain

- Heavy rain with storms and could lead to flooding impacts

DISCUSSION

Issued at 222 PM EDT Tue Jun 30 2026

Dangerous Heat and Humidity: A very warm and humid airmass remains anchored over northern MI tonight through Thursday. Surface observations are depicting heat indices reaching into the mid to high 90s and low 100s over much of the state today, even under a thick deck of stratus which was in place earlier this afternoon. Where sun can peak through, temperatures will quickly shoot up 3 to 5 degrees as surface dew points continue on their steady rise into the mid to high 70s. This airmass will persist through at least Thursday, possibly through Friday as well for spots south of M-72. Heat stress tends to be cumulative, and by Thursday even if temperatures are slightly lower than Tuesday it will still result in dangerous conditions especially for more vulnerable population's as surface Td are still expected to be in the 70s for northern lower. Multiple waves of severe storms are also possible during the next couple of days, which could provide temporary relief under the rain cooled air but could also result in power outages which would take out AC for some folks and build on these heat hazards. Severe impacts are not expected to be widespread (see discussion below), however this could exacerbate heat stress locally.

Thunderstorms and Flooding: Current satellite depicts a large scale upper -PNA pattern, which has continued to advect deep Gulf moisture northward around the upper ridge. A larger upper closed low spins over ND and is ingesting a dry slot near the base of the negatively tilted upper wave (over MN). The Great Lakes region remains in the deep moisture, where PWATs have doubled in the last 48 hours and are reaching around +1.5" per ALPW estimates this afternoon. Surface observations depict southwest winds with gust of 15 to 25 mph, which has been resulting in higher wave heights over Lk MI and thus moderate to high swim risks.

As the lower level stratus scatters out this afternoon, satellite depicts agitated CU beginning to bubble. Due to the high humidity present and earlier cloud cover, its unlikely widespread temperatures will reach their convective Ts in the mid 90s.. However some spots could this evening over NE lower. If convection can break the capping inversion, lots of energy awaits.. The 12z KAPX RAOB confirms higher mid level lapse raters of 7 to 8 C/km, with RAP model soundings continuing this through the afternoon. Marginal shear from 0-1km exists (15 to 20kts) with better shear in the 0- 6km (30kts) will be present this evening. Steering winds will be westsouthwest at around 25 to 35 mph. Over northern WI, there is a moisture gradient, which could also help initiate convection that would move near the tip of the mitt and over the U.P.. Due to the shear profiles and moisture present, discrete storms that tap into the energy aloft will have the ability to mature, possibly becoming super cellular. Storms will be moving, however the deep moisture will allow for efficient rainfall production (possibly HP super cell). This will likely result in flooding impacts, especially over areas that saw heavy rain in the previous 24 hours (soil saturations remain 50% or more). Stronger storms will be capable of producing damaging wind gusts, large hail, and with the lower cloud levels a tornado/waterspout or two cannot be ruled out as well. Storm coverage will likely remain isolated to scattered during this time due to the dependency on the convective initiation.

Northern MI will continue to intersect the deep moisture transport around the upper high tonight into early Wednesday. This will result in multiple waves of isolated to scattered convection. There will likely be breaks, however storm chances do persist tonight into early Wednesday morning. Warm and humid conditions will also linger into the night tonight, but could be temporarily received by rain cooled air under storms.

Starting Wednesday, the upper closed low over ND will start to open up and begin its slow progression eastward. At the same time, an upper shortwave (currently a lobe of energy spinning over the desert SW), will race northeast ward across the rockies and initiate convection over the north central plains (southern MN). This leads to chances for MCS/linear convection formation over southern MN/WI Wednesday, and the steering flow will likely direct that over northern WI, then northern Lk MI. These stronger storms will likely still have some dynamics and energy to work with while moving east, and could hold together when coming across the lake. Damaging winds, hail, and heavy rain are all possible hazards, as well as a tornado/waterspout or two. The coverage could be more than what we would have been seeing the last two days, as incoming convective boundaries could initiate new convection over northern MI. Training of heavy rain will also result in flooding concerns for Wednesday. There is growing confidence for multiple swaths of 1 to 2 inches over northern lower, with locally higher amounts up to +3".

Although the upper wave over southern CAN progresses east, the 596 dam upper ridging over the Ohio Valley will anchor in for the remainder of the week. Winds will turn west/southwest and a moisture boundary could set up near the U.P. for Thursday and beyond, however deep gulf moisture will continue to ride around the upper ridge and intersect MI. A few upper shortwaves will be possible Thursday and Friday, which could help spark more widespread convection. Storm chances will linger into the holiday weekend as well, although predictability remains low at this time with lots of uncertainty in the shorter term forecast.

AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 131 PM EDT Tue Jun 30 2026

Showers with embedded thunderstorms are seen on radar slowly tracking into the region. Uncertainty remains in regards to timing, organization, and intensity of these showers/storms and if another round of showers/ storms are possible this evening/ tonight. Storms will be capable of heavy rain, brief gusty winds, and hail. Southwest winds remain gusty through the evening with winds decreasing tonight.

HYDROLOGY

Issued at 255 AM EDT Tue Jun 30 2026

A few corridors of significant rainfall fell earlier Monday and resulted in areas of flash flooding, road washouts, and river rises. Areas hardest hit were from the Leelanau Peninsula east into portions of Antrim county, and then up across Emmett and Cheboygan county. The Jordan river has crested to ~6.6 ft, top 3 all time. Consequently, an area flood warning was issued along the river in Antrim and Charlevoix county. With additional thunderstorm chances within a very moist airmass, there was some pondering about a flood watch. Relative low predictability will preclude that consideration at this time, but if better agreement comes into play later this week then perhaps that could be discussed. Locally heavy rain will be possible with any thunderstorms through the end of the week, and the risk for flooding.

APX WATCHES/WARNINGS/ADVISORIES

MI...Heat Advisory until 8 PM EDT Thursday for MIZ016>018-020>036- 041-042-097>099. Beach Hazards Statement until 8 PM EDT this evening for MIZ025- 031. Heat Advisory until 8 PM EDT this evening for MIZ086>088-095- 096. MARINE...Small Craft Advisory until 8 PM EDT this evening for LHZ349. Small Craft Advisory until 5 AM EDT Wednesday for LMZ323-341- 342-344. Small Craft Advisory until 11 PM EDT this evening for LMZ345- 346.