textproduct: Milwaukee/Sullivan

KEY MESSAGES

- A Severe Thunderstorm Watch is in effect through 5 AM CDT from Iowa to Milwaukee Counties southward. This is primarily for damaging winds and isolated hail up to 1 inch in diameter. North of the watch, rain and thunderstorms are still possible, but are not expected to become severe.

- Isolated thunderstorm development Saturday afternoon and evening, with more widespread rain and storms overnight Saturday. A storm or two could become severe, but confidence is not high at this time.

- Active start to next week, with multiple rounds of showers and storms expected. Some storms could be severe & produce locally heavy rainfall.

- Trending quieter & cooler Wednesday into Thursday.

SHORT TERM

Issued 1140 PM CDT Fri May 15 2026

Tonight through Sunday:

850 mb LLJ will strengthen existent storms as they progress southeastward tonight along and south of the I-94 corridor late tonight through early Saturday morning. A Severe Thunderstorm Watch is in effect for Iowa County through Milwaukee County southward. With storms already developing linear structure, expecting the hail threat to be lower than previously forecasted, although a few cores may become strong enough to produce 1 inch in diameter hail. Cloud bases do remain high, around 4000 ft, and combined with a stable to moist adiabatic airmass in the low levels, expecting very low tornado threat. An isolated tornado DOES remain possible in scenarios with an easterly bowing segment and strong rear inflow jet. Storms are expected to maintain intensity through the Lake Michigan Counties, but confidence decreases farther east. Regardless of intensity, expecting storms to exit offshore by 5 AM CDT.

Lingering light showers are possible after storms exit Saturday, with clearing conditions through the rest of the day. Low pressure developing in the central High Plains will bring warm frontogenesis back into the Upper Midwest late Saturday afternoon into Saturday night. If storms develop early enough on Saturday (HRRR favors this solution with MUCAPE around 1000 J/kg), probabilities for an isolated severe thunderstorm increase. However, confidence is low (1 out of 5, or Marginal on the SPC scale). More widespread rain and storms are expected to develop during the overnight hours along this wing of frontogenesis. Ingredients are not favorable for severe weather overnight, with minimal MUCAPE and mid-level lapse rates only near 6 degrees C/km as the surface front lingers just to the south.

Early Sunday morning, warm front finally lifts northward through southern Wisconsin, bringing increasing instability and shear. With overrunning precipitation, still not anticipating any organization, but storms may become briefly surface based. Southeasterly winds expected throughout the day Sunday, keeping a wide temperature margin from the Lake Michigan and central Wisconsin (low to mid 70s) to southwestern and inland counties (low to mid 80s). Sunday afternoon, low pressure ejects into the central Plains, bringing an additional frontogenesis region to southwestern Wisconsin. Severe storms may become possible Sunday afternoon into Sunday evening in this region, but confidence in timing and placement remains in question (slower low progression would result in less severe potential).

MH

LONG TERM

Issued 1140 PM CDT Fri May 15 2026

Sunday night through Thursday:

Synopsis: Longwave ridging will remain entrenched over the eastern CONUS Sunday night through the beginning of next week as a broad upper trough gradually migrates from the Great Basin to the Northern Plains. The upper pattern will support persistent high pressure off the Eastern Seaboard, with broad low pressure gradually building from the Central Plains into the upper Mississippi River Valley. The placement of surface features will support southwesterly winds and well-above normal temperatures through the day Monday. Muggy conditions will accompany the warm temperatures. Temperatures will slowly begin to trend downward Tuesday as low pressure continues to build from Michigan's Upper Peninsula into Ontario, sweeping a cold front across southern Wisconsin. The warm/muggy conditions & eastward progressing upper trough, surface low and cold front will support a continued active pattern featuring multiple chances for additional showers and thunderstorms Sunday night into Tuesday. Some storms could be severe, with locally heavy rainfall also possible. Conditions will trend quieter and cooler in the wake of the departed cold front Wednesday through Thursday.

Monday: Widespread thunderstorm development is forecast from the Central Plains into the mid-upper Mississippi Valley during the afternoon/evening hours as upper troughing & a surface cold front impinge upon a warm/moist & unstable air mass in place regionally. An overlap of ample instability with abundant speed & directional wind shear will be supportive for potentially substantial severe weather hazards in thunderstorm development. Precipitable water values in the 1.25-1.50"+ range will also be favorable for heavy downpours & locally heavy rainfall in any convection. Certainty of the severe weather forecast picture in the MKX CWA, however, remains cloudier relative to areas further west, as influences from early day showers/storms could impact the quality of the afternoon/evening warm sector over southern Wisconsin. Further, a number of different solutions regarding the forward progression of the ejecting upper trough & mirroring surface features remain apparent in available global/mesoscale forecast guidance. If slower solutions apparent in some guidance win out, the corridor of peak severe potential would trend further west/away from southern Wisconsin. Will thus be monitoring forecast trends very closely through the weekend, as small changes in expectations for early day storms and/or storm system progression will have large ramifications on the potential for severe weather in southern Wisconsin on Monday.

Tuesday: Could see shower/storm potential linger around the region, particularly in the scenario of a slower storm system evolution/delayed cold frontal arrival in southern Wisconsin. Wouldn't be able to rule out a few stronger storms with additional heavy rainfall in the event the cold front hangs up long enough & instability can build back into parts of the area. Will be watching trends over coming forecasts.

Quigley

AVIATION

Issued 1140 PM CDT Fri May 15 2026

VFR conditions expected through the remainder of tonight outside of organized thunderstorms. A line of strong thunderstorms is expected to progress west to east along and south of the I-94 corridor tonight. Confidence is high in widespread thunderstorm activity all the way to Lake Michigan, and will amend as storms move toward terminals for updated wind speed potential. LLWS will continue ahead of these storms as winds remain southerly at the surface, and will diminish after the strong storms move eastward and winds turn west-northwesterly. Expect light rain to continue with isolated lightning for a few hours after the line of storms exits. MVFR conditions are possible within and behind the storms.

Storms will exit into Lake Michigan by 5 AM CDT Saturday, with expectations for skies to remain VFR and become clear into Saturday afternoon. Northwesterly winds will continue through the day. Additional clouds around 3500 ft may develop Saturday late afternoon into Saturday evening as a warm front develops. Precipitation chances increase into Saturday night (50-70% chance), with lightning expected in the strongest cells. Winds will also shift to southeasterly overnight.

MH

MARINE

Issued 1140 PM CDT Fri May 15 2026

Low pressure around 29.1 inches exiting into the Hudson Bay will keep gusty southerly winds across the Lake through early Saturday morning. Expect winds to shift to become northwesterly and light into Saturday as a front crosses the lake. Thunderstorms are expected along this front, with a few becoming strong with gusty erratic winds possible. Storms will end shortly after sunrise. Lighter northwest winds continue through Saturday, with light and variable winds overnight. Sunday, low pressure around 29.2 inches will develop in the central High Plains, shifting winds to easterly. Winds remain easterly through Sunday, shifting to southwesterly and becoming gusty Sunday night through Monday night as low pressure ejects into the western Great Lakes region. A few strong thunderstorms are possible on Monday as low pressure crosses Lake Superior. Potential for a few strong thunderstorms remains into Tuesday as low pressure lingers in the region.

MH

MKX WATCHES/WARNINGS/ADVISORIES

WI...None. LM...Gale Warning...LMZ261-LMZ362-LMZ364-LMZ563 until 1 AM Saturday.

Small Craft Advisory...LMZ643-LMZ644-LMZ645-LMZ646 until 4 AM Saturday.