textproduct: Milwaukee/Sullivan

KEY MESSAGES

- Multiple windows for thunderstorms this week, with each having varying degrees of strong to severe potential, beginning Mon nt then each day possibly through Thu.

- Heavy rain and flooding potential will increase through the week as rainfall accumulates from rounds of showers and storms. Srn WI rivers are expected to rise with some reaching minor flood stage, and potentially higher levels of flooding.

UPDATE

Issued 811 PM CDT Sun Apr 12 2026

The widespread light to brief moderate rainfall is expected to end from west to east across srn WI through 06Z. The shortwave trough over ern IA responsible for the rain will reach Lake MI and IN shortly after 06Z. Meanwhile, farther to the north, cyclogenesis over nrn Ontario and srn Hudson Bay will support a dominant low developing in said region, while its cold front shifts ewd across WI. The front will weaken and stall over srn WI by 12Z Mon.

The front will then move back to central WI as a warm front during the afternoon as sly winds increase over srn WI. The sly winds are a result of low pressure tracking ewd along the SD and NE border. Very steep mid level lapse rates around 8C/KM will overspread srn WI, capping a very mild and moist airmass. The return of low level moisture beneath the cap will simply result in widespread stratus and possibly even patchy drizzle by later in the day. Will likely need to lower Max temps a bit to account for this.

A weak shortwave trough will then track from the central Great Plains across central or nrn WI Mon nt. This will shift the low pressure area and the nose of a strong low level jet across central WI. The warm sector will be strongly capped but right along and north of the front, convective initiation and severe convection is likely. There are differences among the models on the aforementioned features, but the CAMs and 12Z HREF trended north with the deep, moist convection. Some CAMS then suggest a MCS will slip esewd into central WI and possibly se WI from the late evening into the early morning hours. MUCAPE will range from 1500-2000 J/KG with mdt effective shear present. Damaging winds would be the main threat but cannot rule out the possibility of QLCS tornadoes if the MCS maintains maturity.

Gehring

SHORT TERM

Issued 130 PM CDT Sun Apr 12 2026

Tonight through Wednesday:

A very active spring pattern to start off the week across southern WI with multiple windows for severe weather with varying degrees of uncertainty and coverage. Regardless of the severe potential, will also be a wet period with numerous waves of rain and storms. NBM and global ensembles continue to forecast at 1-2 inches rainfall for much of the area now through Wednesday. However, localized heavier rain associated with any stronger storms and/or moving over the same are may result in higher amounts as well as flooding.

Late This Afternoon and Evening:

Low pressure will be tracking across the Dakotas and into MN this afternoon with a warm front extending into the Upper Great Lakes. Morning round of showers and storms have been focused along the surface warm front, but is progged to lift northward through the remainder of the afternoon. This places southern WI in the warm sector of this system with plenty of moisture as we are seeing dewpoints creeping into the lower 60s. Pair this with the mid-level shortwave trough trekking through MO and into IL this afternoon will be another window for showers and storms across southern WI. While additional precip chances increase, overall severe potential this afternoon looks low given meager deep layer shear (<35kt) and LLJ pushing east as well as the morning activity tamping down the surface based instability to under 1000 J/kg. Nevertheless, if things slow down and forcing lines up a bit more, cannot completely rule out a few stronger storms capable taping into the tall skinny CAPE and producing small hail along with pockets of gusty winds given the inverted-V on the 12z models soundings. Additionally with the northern low's cold front turning into a stall front across the area as the Plains' trough lifts in may resulting in shower and storms sitting and training over the same area this evening. Overall QPF amounts generally look to remain below three- quarters of an inch, still will need to keep an eye out for minor flooding concerns with any stronger cells given how saturated things have been the past week or two.

Monday- Monday night:

Overall upper-level synoptic forcing will be lacking as upper- level ridge sits to the eastern CONUS and looks to damping any upstream waves. Some mid-level ripples of vorticity looks to track across southern WI, but low-level WAA and 45-55 kt LLJ nosing up from the southwest through the day will be the main synoptic forcing. The focus for thunderstorm development on Monday will be along the aforementioned stalled boundary turned warm front. While the 12z run of the CAMs suggests there will be ample deep layer shear (40-55 kt), surface based instability exceeding 1500 J/kg, dewpoints well into the 60s all supportive of strong to severe convection, the main question is where will the warm front set up as forecasting warm front placement around the Great Lakes this time of year is rather tricky as well as how capped will it be south of the warm front.

Much of the 12z CAMs as well as the GFS and NAM are favoring the warm front to lift into northern WI as a low develops across the Plains and tracks into MN/western WI, but the global models such as the ECMWF and Canadian are not as aggressive with the northern push of the front and stalls it more across central WI, which will impact our neck of the woods. Regardless of where front sets up expect to see discrete convection initially develop with all severe hazards possible. Elevated hail will be possible along and north of the warm front with 1-2" hail. Along and south of the warm front in a narrow corridor between capped and uncapped environment could see storms capable of producing winds and even a few tornadoes in addition to hail where 0-1 SRH exceeding 250 m2/s2. After a couple of hours then see this activity looks to merge and cluster while training over the same area along the boundary resulting in heavy rain and potential flash flood concerns. So while strong convection is expected to develop along the warm front and trending north of our CWA, will need to monitor as we cannot discount the south trend seen on the ECMWF which could bring this activity impacts more into our northern half of the CWA.

Tuesday- Tuesday night:

There will be another window for severe on Tuesday, but potential will be conditional based on what happens Monday with the frontal boundary. Mid-range models do hint at a bit better upper-level synoptic forcing with a shortwave trough lifting across the region and a +100kt 250mb jet strewn across the Upper Great Lakes. Still looks to be some disagreement between the models on the surface feature and frontal placement with the 06z GFS as an outlier with a low tracking across the Upper Mississippi River Valley and dragging a cold front across the area while the ECMWF, Canadian, and NAM favor the warm front setting up across the area. Again will be ample surface based instability (+1000 J/kg) and deep layer shear (+40 kt) for convection initiation, but difficult to pinpoint exact timing and if the forcing aligns enough to produce stronger storms. Will continue to monitor trends, but a severe potential exists. Also cannot rule out additional flooding potential with any Tuesday activity as well.

Wednesday: As we get farther out, the more dependent the pattern will be on the previous day, but still worth noting of any strong to sever thunderstorm potential for Wednesday as well. Latest mid- range models continue to prog a deepening shortwave trough and associated surface low to dig across the Plains and lift into the Midwest for Wednesday. So there looks to be more upper level synoptic forcing and a surface warm front setting up across the area, but the low- level WAA as well as LLJ look meager as well a bit less moisture making up this way. Overall, not overly excited about this potential at this time, but will have to see how the next couple days shake out and go from there.

Wagner

LONG TERM

Issued 130 PM CDT Sun Apr 12 2026

Thursday through Sunday:

500mb cluster analysis are generally favoring an anomalous ridge building across the northeast and Mid-Atlantic regions, with a trough to the west of the Great Lakes gradually shifting through the area by later in the weekend. Ensemble means are generally showing low pressure shifting east northeast across and to the northeast of the Wisconsin Thursday into Thursday night. This may be accompanied by a fairly potent 500 mb shortwave trough.

This may bring continued chances for showers and some thunderstorms Thursday into Thursday evening. Not sure if any strong to severe storms would occur this far out in the forecast, though may need to monitor any additional rainfall on saturated ground for some minor flooding potential. Temperatures should continue to be warm, with small spreads in ensemble highs into the 70s in most areas.

May see a brief lull Thursday night, before another low pressure system develops and moves from the central Plains to northern Wisconsin Friday. This may shift east northeast across the region generally Friday night, with a strong cold front. This should bring yet another round of showers and some thunderstorms to the region, mainly later Friday into Friday night. Will have to watch for strong to severe storm potential, as well as heavy rainfall, as warm and humid conditions linger.

Strong cold air advection behind the front later Friday night into Saturday should cooler and more seasonable temperatures. There may be chances for lingering showers into Saturday, before the cooler and drier air clears them out.

Wood

AVIATION

Issued 811 PM CDT Sun Apr 12 2026

LLWS via a swly 40-45 kt low level jet this evening is expected. Otherwise, widespread MVFR Cigs and areas of Cigs below 1 kft developing through the evening into the overnight. The widespread rainfall will end from west to east across srn WI from 02Z-06Z. Areas of MVFR Vsbys expected within the rainfall, but areas of MVFR Vsbys via BR may also form toward sunrise over south central WI. Some clearing skies may occur toward central WI toward sunrise Monday and continue into the early afternoon.

Elsewhere across srn WI, Cigs below 1 kft will rise to MVFR Cigs with widespread MVFR Cigs spreading from srn WI into central WI during Mon afternoon. Patchy drizzle or very light rain may develop. LLWS will redevelop on a swly low level jet of 45-50 kt by late evening along with with chances for thunderstorms especially toward central WI. Finally, areas of Cigs below 1 kft may develop over south central WI late Mon nt.

Gehring

MARINE

Issued 130 PM CDT Sun Apr 12 2026

Two areas of low pressure will be tracking across Lake Michigan region through tonight bringing gusty southwesterly winds to southeastern WI nearshores as well as the open waters. Strongest winds look to occur across the northern third of the open waters with gales. Although gales tend to struggle to mix down in warm air advection patterns, the stronger winds just off the deck paired with passing showers may be enough to mix them down consistently. Thus will err on the side of caution and have upgraded to a Gale warning for this evening/tonight. Otherwise will see multiple rounds of showers and storms through the week with frontal boundaries lifting, stalling, and pushing through Lake Michigan. Winds will follow these boundaries and vary through the period, but primarily will favor southerly components through at least midweek.

Wagner

MKX WATCHES/WARNINGS/ADVISORIES

WI...None. LM...Gale Warning...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565 until 3 AM Monday.

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