textproduct: Marquette

KEY MESSAGES

- Warmer than normal temperatures continue through early next week.

- A strengthening pressure gradient will bring a period of gales to far western Lake Superior this afternoon into the early evening.

- Multiple rounds of showers and thunderstorms are expected Sunday night through Tuesday. Some of the storms could be severe, especially over the southern half of the U.P. on Monday. Heavy rainfall is also possible.

DISCUSSION

Issued at 250 AM EDT Sun May 17 2026

Early this morning RAP analysis showed an upper level trough digging into the western U.S. with a ridge amplifying downstream over the north-central U.S. A nearly stationary front was over the central plains with sfc troughing deepening in the lee of the central Rockies and a strong 50kt+ LLJ over the southern plains. Along the frontal boundary and nose of the LLJ radar and ir satellite showed thunderstorms ongoing with a more organized MCS over southeast NE and northeast KS. 850mb waa was spreading northwards with isolated to scattered elevated convection initiating over southern MN and southwestern WI. Clouds at the top of the convection were streaming northeastwards with some high level cloudiness pressing into the southern U.P. The weather was still quite up here in the U.P. with light winds and temperatures in the upper 30s to 40s.

A stretch of active weather kicks off today as flow aloft pivots more southwesterly with the deepening of the trough over the western CONUS, paving the way for a series of waves to eject out of the Plains towards the Upper Great Lakes through the early part of next week. The first or these wave arrives today.

The forecast was a bit challenging today with respects to the timing and placement of showers and thunderstorms in the region as we progress through the day. The evolution of the convection and MCS ongoing in the central plains will likely have an impact on how the showers and thunderstorms progress northwards, most CAMs have a different idea on these details. However, models are in relative agreement on the big picture through tonight, the western upper level trough continues to dig with strengthening southwesterly flow over the plains. Meanwhile low pressure will push off the Colorado Rockies and track through the central Plains, reaching western Lake Superior by early Monday morning. This will lift the warm front northwards pulling deep Gulf moisture into the Upper Great Lakes. As the warm front lifts north today showers will spread into the U.P. as early as this afternoon, followed by increasing chances for thunderstorms by evening into the overnight as elevated instability increases. A few strong to severe elevated thunderstorms will be possible during the evening and overnight with SPC highlighting a marginal risk for severe thunderstorms over the area. HREF shows mean MUCAPE of 1000-1500 J/kg tonight with 0-6km shear of around 40- 50kt. However, effective bulk shear is lower, 25-30kt, given the elevated nature of the storms. The lower effective shear could make it harder to sustain any organized convection lifting into the area. The HREF is fairly limited on 0-2.5km helicity tracks but there is a clustering over NE WI this evening into Menominee County and over the far western U.P. overnight, closer to the surface low pressure where shear is a bit stronger. There are a few isolated tracks across the remainder of the U.P. The main threats would be large hail (1" diameter) and damaging winds.

Severe potential increases and expands further east into the U.P. on Monday with the arrival of the second wave and associated surface low, with MUCAPE increasing as high as 2000-2500 J/kg and and bulk shear values of 40-50 kt easily supporting more organized convection. SPC maintains a Day 3 slight risk (15% probability) for severe thunderstorms across much of the southern half of the U.P. on Monday, Will also need to monitor heavy rain potential Sunday into Monday as a rich feed of gulf moisture sends PWAT values into the 1.25-1.75" range which is above the 95th percentile of seasonal climatology for mid-late May. WPC maintains a marginal risk of enhanced rainfall through Monday, with HEFS guidance currently showing a 10-25% chance to get back to minor flood stage on a few western U.P. rivers. Will continue to monitor in the coming days.

Third wave and associated surface low arrives on Tuesday. Model soundings suggest more of a weak/elevated thunderstorm threat by this time as the layer near the surface begins to stabilize, but will continue to monitor trends in the coming days. Mid next week currently looking cooler and drier in the wake of the more active weather early in the week, but guidance begins to diverge as we get later into next week.

AVIATION /12Z TAFS THROUGH 12Z MONDAY/

Issued at 717 AM EDT Sun May 17 2026

VFR conditions are expected through the afternoon, deteriorating thereafter as showers and thunderstorms move into the U.P. A low pressure will track from the Central Plains to the western U.P. today into tonight. A warm front will lift north initially spreading high to mid-level cloudiness to the TAF sites this afternoon. Conditions will fall to MVFR this evening as showers and thunderstorms become more numerous, further falling to IFR and LIFR as the evening progresses. Periods of VLIFR are possible in the heavier showers and thunderstorms. A few strong to severe thunderstorms are possible with the potential for damaging winds and large hail. The thunderstorms threat will diminish late in the TAF period but IFR conditions are expected to linger along with spotty showers. LLWS is also expected late tonight into Monday morning, mainly at IWD and SAW.

MARINE

Issued at 250 AM EDT Sun May 17 2026

A low pressure approaching the region today increases winds to 20-30 kts for this afternoon and tonight. Gales are expected over the far western part of the lake this afternoon and evening. A secondary low Monday night and Tuesday brings winds back to 20-30 kts lakewide. Additional chances for gales are present with this system, but probabilities are capped at 30% as uncertainty regarding stability over the lake remains. Strong to severe storms are possible Sunday night into Monday as well as Monday night.

MQT WATCHES/WARNINGS/ADVISORIES

Upper Michigan... None. Lake Superior... Gale Warning from 2 PM EDT /1 PM CDT/ this afternoon to 11 PM EDT /10 PM CDT/ this evening for LSZ162.

Lake Michigan... None.