textproduct: Milwaukee/Sullivan

KEY MESSAGES

- Confidence continues to increase in strong to severe thundestorms pushing across southern WI overnight tonight (after 10 PM CDT). Hail and damaging winds are the main threat for southwestern Wisconsin as the line moves in, with damaging winds becoming the dominant threat across south-central to southeastern Wisconsin, especially along the WI/IL border.

- Very warm and increasingly humid conditions through the weekend.

- Additional thunderstorm chances Saturday night through Tuesday. Storms may be strong to severe at times with the best potential later on Monday.

UPDATE

Issued 720 PM CDT Fri May 15 2026

Scattered supercells have developed in northwest Iowa this evening along a developing frontal feature. Additional, more isolated, development has occurred in southern Iowa and eastern Nebraska. These storms are expected to remain discrete through the evening hours, potentially through the early overnight hours as they are driven eastward under 50 kt of bulk shear. As they impact southwestern Wisconsin (expected between 10 PM and 12 AM CDT), large hail and damaging winds remain possible under ~2000-2500 J/kg of MUCAPE. Around midnight tonight, an 850 mb LLJ around 35-40 kt is expected to develop, with the nose into south-central Wisconsin. This will serve to bring in MUCAPE around 1500-2000 J/kg, and should also allow storms to gain a more linear structure and transition to a wind threat. There will be an inversion/moist adiabatic layer in the low levels (~850 mb to the surface), but a strong enough rear inflow jet will be able to overcome this concern and result in damaging winds at the surface. Still expecting the worst conditions from southwestern Wisconsin through the WI/IL border. Areas near Lake Michigan may stay more stable and be largely gapped by severe potential.

MH

SHORT TERM

Issued 219 PM CDT Fri May 15 2026

Tonight through Saturday night:

Today is much warmer than previous days with temps in the 70s this afternoon. The milder temps associated with the low-level WAA will continue tonight along with milder dewpoints advecting into southern WI as a weak area of low pressure along the cold front draped across the regions lifts from NE into IA. This will be the focus for the increasing potential for storm development overnight as the upper- level trough moves across the Upper Great Lakes Region situating the upper-level jet's right entrance region over MN/WI. This upper-level forcing aligns with a mid-level vorticity max as well as the low- level WAA and band of frontogenesis as the front pushes eastward. Overall, synoptic forcing and dynamics align over the region to support increase thunderstorm chances.

The more favorable environment for severe storm development remains back to the west across IA into southern MN this afternoon/evening where higher instability (SBCAPE +2000 J/kg) and daytime heating pairs with +40 kt of effective shear along the cold front. However, hi-res models continue to project this environment to shift more into southwestern WI later this evening and tonight as a wedge of higher dewpoints push into the western portions of the CWA. While the upstream activity across IA gets going this evening, CAMs have it transitioning into a line of storms as it pushes toward the Mississippi River Valley. Although we lose a bit of instability (~1000 J/kg of MUCAPE) with daytime heating and questions remain on how storms would react to increasing capping inversion, there is ample deep layer shear (+40 kt) on the nose of an increasing LLJ tonight that continue to hint that stronger to severe thunderstorms remain possible as the line pushes in overnight. Given the steep mid- level lapse and linger MUCAPE values, large hail will be possible, but given the more linear convection mode and steep low-level lapse rates, gusty to damaging winds remain the main concern tonight. 12z HREF members all hint at a potential to see a bowing segment develop within the line as well and given the RAP and NAM showing 0- 3km shear of +30kt from west to east, could not rule out an embedded tornado or two with in the line associated with any bowing features. However, will be dependent on the LCLs which are progged to be around 1000m and if the lower-level saturate more than models suggest at this time.

Nevertheless, there continues to be a a growing concern for overnight strong to severe line of thundestorms bringing damaging winds and some hail to southern WI. Timing will be as early as 03- 05z (10pm-12am) for areas west of I-39, but more likely between 05- 07z (12-2am) and pushing east through 08z-10z (3-5am). Although the line seems like it may lose some momentum as it approaches Lake Michigan, it still may have enough energy to bring the severe threat that far east. Given the overnight timing, be sure to have a way to receive warnings that may wake you up about any potential severe threats.

Otherwise, may see some of the lingering showers behind the line linger through daybreak Saturday before things dry out with westerly winds behind the cold front. This front then looks to stall south of the WI/IL border during the day Saturday. While post-cold front, temps on Saturday will warm back up in the upper 70s and lower 80s. Will need to keep an eye on the placement of this front as it looks to be the focus for additional shower and thunderstorm development later Saturday evening/night as a weak shortwave trough sweeps through the area. While the chances for development along this boundary looks to stay mostly south of the Cheddar Curtain, there could be a few scattered showers and thunderstorms Saturday evening, but coverage remains limited at this time.

Wagner

LONG TERM

Issued 219 PM CDT Fri May 15 2026

Sunday through Friday:

A strong shortwave trough will dig sewd from the Pacific NW to the central Rockies for Sun-Sun nt, while upper ridging proceeds over the ern USA and the central to ern Great Lakes. Shortwave troughs within the swly flow aloft will be traversing through the central Great Plains into the Upper MS River Valley and wrn Great Lakes during this time. At the surface, low pressure over wrn KS will develop into ern NE by 00Z Mon with the warm front extending ewd into central WI. Thus low to mid level warm, moist advection and a shortwave trough passage will bring 60-80 percent chances for showers and isold to sct thunderstorms for Sun AM over south central WI with lesser chances to the east and toward the IL border. Temps will warm drastically with the warm frontal passage Sun afternoon and remain very mild Sun nt within the warm sector.

The aforementioned wave of low pressure over ern NE will then track to nw WI and Upper MI by 12Z Mon, while a decaying MCS from the Great Plains may be arriving after 09Z Mon over south central WI. Another wave of low pressure will develop from the central Great Plains to the NE and IA border by 00Z Tue, while the main shortwave trough tracks from the central Rockies to ern SD and NE. Both features will then continue to lift newd to the nrn Great Lakes and Ontario into Tue. The actual trailing cold front will likely not pass until Tue, but there is some confidence in rounds of showers and storms for Mon-Mon nt given srn WI is downstream of areas of new convective initiation and possible MCSs. The severe storm probabilities look very low for Sun-Sun nt but increasing into Mon- Mon nt. The SVR probs will then decline on Tue as the cold front will pass during the morning into the early afternoon. High pressure and a cooler air mass will then follow for Wed-Thu with lgt sly winds and warm advection returning late Thu into Fri as the high moves ewd and a shortwave trough approaches from the west. Thus high temps in the 60s for Wed-Thu will return to the 70s well inland from Lake MI for Fri.

Gehring

AVIATION

Issued 720 PM CDT Fri May 15 2026

VFR conditions through this evening into tonight before a line of strong thunderstorms progresses northwest to southeast between 10 PM CDT and 3 AM CDT. Have included PROB30 groups for stronger storm potential, with high confidence in at least thunderstorms late tonight and a lower chance of gusty winds. Even without stronger storm impacts, expecting LLWS as these storms move in with a strong low-level jet overnight. Will amend as needed as storms approach.

After thunderstorms exit, expecting lighter rain to linger for a few hours and conditions to improve back to VFR. VFR conditions and west-northwesterly winds are then expected throughout Saturday. Winds become light and variable Saturday night.

MH

MARINE

Issued 219 PM CDT Fri May 15 2026

Low pressure tracking across Ontario is bringing increasing southerly winds across Lake Michigan this afternoon into the evening. Still looking at a few gale force gusts to occur over the northern third of Lake Michigan into this evening and the Gale Warning remains in effect. Then a line of showers and thunderstorms will push across the region overnight into early Saturday morning along the low pressure's cold front. Looking at the the front to bring more southwesterly winds into Saturday as the front stalls south of Lake Michigan. Then another low pressure is progged to develop over the Plains Sunday and lift into the Upper Midwest into the start of next week. This will bring increasing southerly winds and additional shower and thunderstorm potential through Monday night/Tuesday. Then high pressure looks to build in for midweek behind the early week low pressure system.

In addition to today's gales, expect periods of small craft conditions to develop across southern WI nearshore waters with the stronger southerly winds this evening through the start of next week.

Wagner

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.