textproduct: Milwaukee/Sullivan

KEY MESSAGES

- A complex of thunderstorms is expected to roll across southern Wisconsin tonight. Hail and gusty winds will be the main threats, with gusty winds more likely toward the Wisconsin and Illinois border. A Slight Risk (Level 2 out of 5) for severe weather is currently outlooked southwest of Madison.

- Warm and increasingly humid conditions will bring rounds of storms Saturday night through Tuesday. Storms may strong to severe at times.

UPDATE

Issued 630 AM CDT Fri May 15 2026

Models have trended northward regarding tonight's severe weather threat, which places southern WI in a more favorable spot to see strong to severe storms.

06z runs of the HRRR, RRFS, and GFS have shifted a more favorable plume of 1200 to 2000 J/kg of MUCAPE further north along the MS River Valley and into south central WI. These models also suggest storm development further north along the Upper MS Valley, which would then lead to a line of storms moving ESE into our area after sunset. At this time, storms look to be fairly elevated when they move in, as model soundings show these storms rooted at 850mb. However, very steep mid level lapse rates around 8 C/km will be present aloft amid 50 knots of effective shear. The shear space may favor supercell structures capable of large hail in the line . These storms will also be raining into a somewhat dry sub storm layer with tdds around 15 degrees near the surface and DCAPE around 800 J/kg, leading to some potential for gusty winds despite a weak inversion at the surface.

The best overlap of CAPE and shear will exist between the MS River Valley and Madison, with a less favorable environment east of Madison toward the lakeshore. As such, storms will be strongest in our western CWA they move in between 9 and 11pm and track through Madison between 11pm and 1am. Storms are then projected to weaken as they travel east toward MKE and out over Lake Michigan through 4am.

CMiller

SHORT TERM

Issued 100 AM CDT Fri May 15 2026

Overnight through Saturday:

The thunderstorms diminished late in the evening but light showers are still crossing southern and central WI during the overnight hours. Temperatures will remain mild due to increasing southerly winds, with lows ranging from the mid 40s near the lake to the lower 50s west of Madison.

Breezy southerly winds with gusts up to 30 mph are expected Friday. The south component should allow a lake breeze to develop but remain close to the shoreline. With sunny skies, highs will range from the mid 70s east to the lower 80s near Wisconsin Dells.

A thunderstorm complex should develop in northwest IA Friday afternoon along the warm front, with support from the nose of a low level jet, a mid level shortwave, and the right entrance region of an upper jet. This is expected to track east through northern IA and into southwest WI during the evening hours. The southern WI environment should be primed with around 1000 j/kg of CAPE and very high shear (high-shear/low-cape situation). The complex should weaken as the LLJ veers, and also trend southeast into IL toward the better instability overnight. Severe storms will be possible if the storm complex has a well- organized line along the leading edge, but model soundings are hinting at a stable layer near the surface (below 900mb). Damaging winds are the main threat, although isolated hail and locally heavy rain are also possible. If a well-organized line tracks into southern WI, the 0-3km shear vectors will be around 30 kt out of the WSW, so we would need to watch for QLCS tors if the system became surface-based.

Lingering clouds over southern WI Sat from the decaying complex will keep temps modest during the morning, but expect a quick jump into the lower 80s by the afternoon with the sunshine. Dewpoints should be in the mid 50s which keeps us at fairly comfortable humidity levels.

Cronce

LONG TERM

Issued 100 AM CDT Fri May 15 2026

Saturday night through Wednesday:

Another thunderstorm complex (or two) is expected to develop in the Plains Saturday night where the low level jet, warm front, shortwave trough, and upper jet are all situated. This complex should track eastward and remain south of WI overnight.

With the upper trough amplifying over the west Sunday morning, the warm front over IA will push into MN and WI. Showers and thunderstorms will be possible along this front, and a few severe storms cannot be ruled out. However, the stronger forcing and better chance for severe will be over MN and western WI with the better dynamics, especially in the evening when the LLJ kicks in and the upper trough gets closer.

Southern WI may see remnants of any upstream storm complexes late Sunday night or Mon morning, but there is not a lot of confidence here. If there are remnants, clouds may inhibit some degree of instability for Monday afternoon. The stronger storms still look like they will develop in MN and western WI Monday afternoon/evening but we could be in their path and get severe storms out of it.

The main cold front will track through WI on Tuesday and bring one last chance for storms. Timing of this front is still uncertain and the severity of storms will depend on that timing. Cooler temps will briefly return Wed.

Cronce

AVIATION

Issued 713 AM CDT Fri May 15 2026

VFR conditions are expected through most of the day amid gusty southerly winds to 25 knots. Ceilings will lower tonight as a thunderstorm complex rolls through. Hail and gusty winds may occur with some of the storms.

CMiller

MARINE

Issued 100 AM CDT Fri May 15 2026

High pressure over Lower Michigan will continue to shift eastward overnight, with southerly winds developing on the back side of the high. These winds will increase on Friday behind the departing high and low pressure moving from central Manitoba into Ontario. Confidence is increasing in gale force gusts over the northern third of the lake Friday afternoon and evening, so we will be issuing a Gale Warning.

Southerly winds will gradually diminish on Saturday as the low pulls away to the northeast. Southerly winds will then pick back up Sunday into Monday ahead of approaching low pressure. Gales are possible Monday and Tuesday. Multiple rounds of showers and storms will be possible this weekend into early next week.

Cronce

MKX WATCHES/WARNINGS/ADVISORIES

WI...None. LM...Gale Warning...LMZ261-LMZ362-LMZ364-LMZ563...1 PM Friday to 1 AM Saturday.

Small Craft Advisory...LMZ643-LMZ644-LMZ645-LMZ646...10 AM Friday to 4 AM Saturday.