textproduct: Milwaukee/Sullivan

KEY MESSAGES

- Widespread showers and scattered thunderstorms expected Wednesday and Wednesday night. Still monitoring a conditional risk for severe storms (hail/wind), though the greatest chances will be to the south of the area.

- Locally heavy rain Wednesday into Wednesday night is targeting far southern Wisconsin near the WI/IL state line. - A Small Craft Advisory and High Swim Risk are both in effect Wednesday afternoon through late Wednesday night. Breezy southeast winds will lead to high waves and dangerous currents.

- The next substantial chance for thunderstorms arrives Sunday (45-65% chances).

SHORT TERM

Issued 1030 PM CDT Tue Jun 16 2026

Today (Wednesday) and Tonight:

A 500mb trough tracks southeastward across the Dakotas through the predawn hours this morning, with diffluent flow east of the trough overspreading IA. Meanwhile, the corresponding surface low pressure system draws a plume of gulf moisture northward into IA, igniting an MCS as the dynamics / thermodynamics align. 00z CAM model guidance tracks this MCS eastward into northern IL / southern WI with an arrival window of 5-10 AM this morning (from west to east), and an exit window from ~10 AM to the early- afternoon. Though our surface winds are forecast to be out of the southeast in advance of the MCS's arrival, note that we are well north of the effective warm front (most noticeable in the surface dewpoint model fields, draped from central IA to central IL at the time), and all of this convection will be elevated. With poor mid-level lapse rates and MUCAPE < 1,000 joules, this initial morning convection should pose practically no severe threat whatsoever, we'll mainly be watching to see how much rainfall it can make over southern WI (in advance of the potentially stronger afternoon / early evening convection); that and a few rumbles of thunder.

In a similar manner to last Thursday, the parent surface low will then attempt to lift the effective warm-front further north in IL, and the closer it gets to southern WI, the stronger our storm potential will be. The 00z guidance continues to stall the front well south of the WI/IL border, but depending on which model or initialization you pick, it could land as far south as I-80 or as far north as 1-county's distance south of WI (note the 16.21z RAP Theta-E and wind fields). If the boundary lifts close enough, we could observe 1,500+ joules of MLCAPE, enough to support hail or damaging winds with a few storms in the early/mid afternoon hours. On the other hand, if the boundary lands further south, the localized heavy rainfall and severe threat over far southern WI will reduce substantially. The latter scenario may open the door to late afternoon / early evening airmass recovery...

In the event that clouds can break apart and/or our region can remain relatively free of convection in the early afternoon, models like the 17.00z HRRR hint at the potential for the western half of our CWA to 'recover' to ~1,500 joules MLCAPE and > 7 C/km mid level lapse rates in advance of the approaching surface low, which could boost a late afternoon / early evening round of convection to marginally severe levels, with a weakening trend further east after the sun sets. Wind gusts and hail would again be the primary concern.

Sheppard

LONG TERM

Issued 1030 PM CDT Tue Jun 16 2026

Thursday through Tuesday:

Predominantly dry weather expected in the cool sector of the departing low pressure on Thursday. Some lingering 500mb vorticity on the north edge of the jet, some cool air aloft, and steepening low-level lapse rates through the course of diurnal heating (below the subsidence inversion) allows models to yield some slight chances of cellular rain showers Thursday (~10-20% chance). A similar story for Friday, not enough jet dynamics to support much else. Ensemble plots for High Temps suggest a daytime high around 70 / low 70s for Thursday, with roughly steady state temperature trends this weekend into early next week.

For Saturday, the longwave upper trough axis accelerates its eastward propagation, with a brief warm-up to the upper 70s possible. A developing shortwave ridge in the subtropical jet (axis from KS into IA) works with the polar jet to rapidly tighten the geostrophic gradient over WI, resulting in a strengthening WNW to ESE jet-streak. Precip chances remain relatively low on Saturday for now (below 20%), but the right- entrance of this potent jet streak will seed the development of a strong surface low over the central Plains, which could eject northeastward towards our region and bring showers + thunderstorms with it on Sunday (~45-65% chances). Both ensemble and deterministic systems indicate the preferred system track would be overhead or south of our region, which would help to prevent warm sector entry into our region and reduce the threat of stronger storms. The aforementioned ensemble temperature plots corroborate this conclusion by keeping ensemble IQRs for high temps in the low 70s or cooler, as do the NBM wind fields (E to NE winds off of Lake Michigan on Sunday). That said, we'll keep a close eye on the track of that system.

If all of this plays out as written, we'd see the surface high pressure dipole move in for Monday, leaving predominantly dry weather. Predictability drops for Tuesday, with the ensemble probability space split between amplifying or deamplifying the wave train of the jet early to mid next week.

Sheppard

AVIATION

Issued 1030 PM CDT Tue Jun 16 2026

Dry weather and VFR through much of the overnight hours, with light winds out of the west gradually backing south.

A thunderstorm complex is expected to develop over Iowa late tonight, crashing eastward into southern WI and northern IL with an arrival window of 5-10 AM Wednesday. Additional thunderstorms then continue to fill into southern WI and northern IL. Cloud bases will be initially VFR upon arrival, gradually lowering to Fuel Alt MVFR in a west to east manner late Wednesday morning as showers and storms continue, then potentially to IFR for the afternoon. Some gusty winds and hail cannot be ruled out with the afternoon and early evening storms, particularly along and south of the I-94 corridor and areas near / west of Madison. Locally heavy rainfall and frequent lightning are currently the main concerns with this activity.

Breezy southeast winds are expected throughout the day Wednesday, with the showers and storms causing some localized variability in the wind field.

Sheppard

MARINE

Issued 1030 PM CDT Tue Jun 16 2026

Open Waters / Synopsis:

A light westerly breeze continues into the overnight hours, decelerating as a weak surface ridge crosses the lake. Winds will turn south and accelerate Wednesday morning as low pressure around 29.2 inches tracks eastward across southern MN. Winds will turn southeasterly and become gusty into Wednesday afternoon, with a few gusts to gale force possible over southern portions of the lake. The low crosses the lake late Wednesday evening, leaving gusty west winds over the southern half of the lake Wednesday night, with a north breeze for the north half. Showers and thunderstorms are expected across the lake late Wednesday morning through Wednesday evening. A few strong to severe storms will be possible over southern portions of the lake Wednesday afternoon.

Breezy northwest winds are expected through Thursday as the low pressure system continues eastward. Weaker northwest winds are then expected to continue into Friday as weak high pressure around 29.9 inches builds into the midwest.

Nearshore Zones:

The gusty southeast winds ahead of the approaching low pressure today will result in high waves. A Small Craft Advisory has been issued this afternoon through late tonight. Expect winds to veer W / NW after the low pressure passes overhead this evening, with breezy winds continuing and waves diminishing late tonight.

Sheppard

MKX WATCHES/WARNINGS/ADVISORIES

WI...Beach Hazards Statement...WIZ052-WIZ060-WIZ066-WIZ071- WIZ072...1 PM Wednesday to 4 AM Thursday.

LM...Small Craft Advisory...LMZ643-LMZ644-LMZ645-LMZ646...1 PM Wednesday to 4 AM Thursday.