textproduct: Marquette

KEY MESSAGES

- Hot and humid weather is expected through much of the week, with the hottest temperatures Tuesday through Wednesday. The heat index will reach the low 100s on Tuesday. Heat advisories have been expanded to cover most of the CWA.

-There is a slight risk (15% chance) for severe thunderstorms on Tuesday, mainly during the afternoon and evening. The main threat will be damaging winds though a few storms with large hail cannot be ruled out.

- There will be the chance for thunderstorms every day/night this week along with the potential for strong to severe thunderstorms.

DISCUSSION

Issued at 314 PM EDT Mon Jun 29 2026

Ridging was building over the area today ahead of an upper level low pressure over eastern MT. Thunderstorm formed early this morning as a warm from lifted north through the region. Thunderstorms lingered through the early afternoon hours over the eastern UP, northern Lake Michigan and parts of Lake Superior. Low level moisture advecting into the area and clouds shearing off of the anvils of storms to the west helped keep ample cloud cover over the region, putting a hamper on the expected heating today. At 130pm this afternoon temperatures were only in the 60s east and 70s south and west. Some patches of broken cloud cover were beginning to show up in the western UP and north central WI, lifting northwards. It is still likely that the sun will make an appearance in the western UP late this afternoon and evening and it will quickly feel quite muggy. That being said, knocked the expected highs down into the mid to upper 80s over the west. A line of severe thunderstorms was over north-cental MN moving eastwards.

Steepening mid-level lapse rates and deep moisture will bring increasing instability to the area through tonight, reaching upwards of 3000-4000 J/kg. While instability and shear will create a favorable environment for strong thunderstorms the best lift will remain closer to a jet streak and diffluent flow over northern MN. Models are in decent agreement that convection west of the region will push into western Lake Superior tonight, weakening as it moves into the central part of the lake, where the LLJ will be weaker. There is uncertainty as to how far south the storms will extend and can't rule out a stronger thunderstorm over the western UP this evening into the late night hours. The CAMs are not handling the ongoing bow echo over MN very well, if at all, but with loss of support as it moves east the southern part of the complex will likely dissipate. It will be warm and muggy overnight with lows in the mid 60s east to the low 70s west.

The upper level low will very slowly make its way to northern Manitoba through Wednesday night with ridging over the Upper Great Lakes. On Wednesday the ridge will begin to flatten out some with several shortwaves pushing through into early next week. 850mb temps of 22-24C are expected on Tuesday with highs well into the 80s and mid 90s. This, combined with dewpoints in the 60s to near 70 will bring the heat index to the low 100s over much of the UP. Expanded heat advisories to the rest of the CWA for Tuesday. Those who will be exposed to the heat for prolonged periods and those who are sensitive to the heat should have a plan to stay cool and hydrated. 850mb temps will remain warm but a bit cooler, 18-20C through the rest of the week into next weekend. This will likely keep the heat below advisory levels but very warm and muggy conditions will continue.

Extreme instability will develop through the day, with mucape at an impressive 5000+ J/kg along with 40+ kt bulk shear. A capping inversion should hold for much of the morning and early afternoon but will weaken in the late afternoon. At this time it appears a boundary (moisture gradient) will set up over western Lake Superior and UP, providing a focus for convection with the weakening cap. Confidence is fairly high that we will see thunderstorms in the region, some of which will be strong to severe. However, the location of the boundary could be highly influenced by the convection this afternoon and tonight to the north and west. Should severe storms develop they will likely grow upscale into an MSC with the main threat being strong, damaging winds up to 80mph. Secondary threats being large hail and an isolated tornado.

The chance for thunderstorms (possibly strong to severe) will continue through the week as multiple shortwaves round the flattening ridge. Predictability is low on timing and placement of thunderstorm activity, which will be highly dependent on the evolution of MCSs and resultant outflow boundaries.

AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/

Issued at 145 AM EDT Tue Jun 30 2026

VFR conditions prevail at all sites after the round of thunderstorms this afternoon swept through the UP and eroded away the prior low ceilings. Models are widely spread on the potential for thunderstorms early this morning and then again this afternoon/evening. Current guidance suggests only around 20% chances of thunderstorms early this morning, though the thread of LLWS has increased enough at IWD/CMX to include mention of for the overnight period. With 30-40% chances of afternoon thunderstorms at CMX/SAW, will include some PROB30 groups to cover those. Winds will have a primarily southerly component throughout the period, but may waver between southwest and southeast around 10 kt with gusts around 20 kt, except in the immediate vicinity of a thunderstorm.

MARINE

Issued at 314 PM EDT Mon Jun 29 2026

The weather will remain active across the Upper Great Lakes through the week with multiple rounds of thunderstorms, some of which could be strong to severe. Southerly winds will increase to 30kt tonight before falling back to 20kt or less on Tuesday. As a very humid airmass spreads across the lake areas of dense fog will continue to develop, especially anywhere rainfall is occuring. Issued dense fog advisories for the mid-lake through tonight but there is the potential for fog to increase in coverage tonight, lingering into the day tomorrow. Some of the thunderstorms this week could be strong to severe. For this evening and Tuesday strong winds will be the main threat with any of the stronger storms. There is a potential for winds up to 70kt should severe thunderstorms for Tuesday afternoon and evening.

MQT WATCHES/WARNINGS/ADVISORIES

Upper Michigan... Heat Advisory until 8 PM EDT /7 PM CDT/ this evening for MIZ002>007-009>014-084-085.

Lake Superior... Dense Fog Advisory until 8 AM EDT this morning for LSZ263-264.

Lake Michigan... None.