textproduct: Milwaukee/Sullivan

KEY MESSAGES

- Multiple windows for thunderstorms this week, with each having varying degrees of strong to severe potential, beginning tonight then each day and night 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 530 AM CDT Mon Apr 13 2026

Forecast remains on track this morning with mild and muggy conditions in place regionally. Look for broken cloud cover to stick around through the day today with a stout EML based near/above 850 mb keeping low level moisture trapped in the lower portions of the column. Can't rule out some light spits of drizzle from some of the low cloud cover, though confidence in preferred locations of drizzle development is low. Will be monitoring trends through today and adjusting the weather grids where necessary.

Still watching for significant severe weather potential this evening/tonight toward central and north-central Wisconsin. Broad set up for storms is clarifying this morning, with low pressure currently centered over southwest Nebraska forecast to track northeast toward the Twin Cities vicinity by late afternoon. Low will drag an attendant dryline northeast with it into northern Iowa/southern Minnesota in the process. A surface warm front is expected to extend east-southeastward from the low into west-central and east-central Wisconsin by late afternoon. Convective initiation appears probable by late afternoon/early evening near the triple point intersection of the surface low, dryline, and warm front. Observational trends will refine specific initiation corridors through this afternoon, but an area generally extending from Wautoma-La Crosse north to the Twin Cities over to Wausau is currently favored for the zone most likely to see CI. Given this anticipated corridor, current expectations are for storms to stay north of the area through the initiation phase of the event (~3 - 7 PM). Any southward trends in the favored initiation corridor, however, would place the northwest corner of the CWA in line for discrete supercell development during late afternoon/early evening, with very large hail, damaging winds, and a few tornadoes all being possible. Will be watching trends regarding initiation very closely through this afternoon.

Barring a last-minute southward shift in initiation corridor, the most likely scenario for storms & severe impacts in southern Wisconsin remains centered on the mid-evening to early overnight timeframe (~ 7 PM - 1 AM), with locations north of I-94 being at highest risk. Following discrete supercellular development during late afternoon/early evening, general expectations are for convection to steadily grow upscale into one or several clusters as the low level jet increases regionally. Propagation of these clusters will prove critical for severe potential in southern Wisconsin, with forward- propagating Corfidi vectors & Bunkers vectors implying a general east-northeast movement of storms away from the initiation zone. This places far northern counties---specifically Marquette, Green Lake, Fond du Lac, and Sheboygan---at greatest risk for strong/severe storms this evening. Given the expected linear mode of storms, damaging straight line winds & QLCS-type tornadoes would be the greatest severe hazards. Torrential downpours would also support some potential for flooding impacts, particularly in any areas of training. Continue to expect a sharp cutoff in the southern extent of the severe threat given the aforementioned propagation vectors & significant capping moving south/away from the warm front. Will nevertheless need to monitor for some activity trying to make it as far south as I-94. In the event storms do approach I-94, straight line winds would be the primary concern.

Quigley

SHORT TERM

Issued 1208 AM CDT Sun Apr 12 2026

Tonight through Monday night:

The widespread light to brief moderate rainfall is expected to end from west to east across ern WI by 07Z, as the shortwave trough responsible for the rain moves away across Lake MI. 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 nrn WI. The front will weaken and stall over central or 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. Have lowered Max temps a bit to account for this.

A weak shortwave trough will then track from the central Great Plains to srn MN by 00Z Tue, then across 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 00Z 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

LONG TERM

Issued 1208 AM CDT Sun Apr 12 2026

Tuesday through Sunday:

A stationary front will reside in the vicinity of srn WI on Tue into Tue evening, while a vorticity maximum approaches in swly flow aloft. In the lower levels, low pressure will track across the central Great Plains into ern NE with the nose of a strong low level jet developing into srn WI. MLCAPE around 2000 J/KG and mdt deep layer shear is expected, and with the aforementioned lift, the cap is expected to erode. Scattered to numerous strong to severe convection will ensue either late afternoon or early evening and continue to expand. Large hail and damaging winds will be the primary threat with lesser chances for a few tornadoes.

A more organized shortwave trough will then approach from the southwest Wed nt, but not before a vorticity maximums lifts nwd ahead of it into se WI by late Wed afternoon. Thus additional rounds of showers and storms are expected from Wed afternoon through the night. Some showers may linger on Thu as the shortwave trough passes in the morning.

Yet another larger and stronger upper trough and n-s sfc trough will move out of the Great Plains and into the wrn Great Lakes for Fri nt-Sat. In summary, multiple rounds of showers and storms are forecast from Tue-Sat, with a few inches or more of rain possible. River rises are expected with some flooding possible. Some severe storms will likely occur at times.

Gehring

AVIATION

Issued 530 AM CDT Mon Apr 13 2026

MVFR and IFR flight categories continue into this morning at all terminals, with gradual CIG improvements occurring from late morning into the afternoon hours. Stout layer of warm air aloft will keep at least SCT-BKN cloud coverage around through the entirety of the day. Can't rule out some -DZ from some of these clouds, though confidence in development & location is far too low to support any mentions at this time. Will be watching trends through the day. Significant convective development remains forecast over central and north-central Wisconsin this evening, with impacts currently most likely at SBM. Have inserted prevailing -TSRA mentions in the coming 12Z SBM update. How far south storms make it remain uncertain. Will need to monitor for some activity trying to make it as far south as I-94, though confidence in this occurring is much lower compared to areas further north. Have thus maintained PROB30 groups at MSN, UES and MKE in the 12Z update, with quiet conditions expected further south. Will be watching for LLWS potential once more tonight, with insertions possibly becoming necessary in coming updates.

Quigley

MARINE

Issued 1208 AM CDT Sun Apr 12 2026

A broad area of low pressure around 29.4 inches over Ontario becoming deeper over southern Hudson Bay early this morning. This will bring breezy south winds to all of Lake Michigan, but with gales expected over the north third into early this morning. A Gale Warning is in effect.

Winds will weaken later this morning as the low quickly moves away and the trailing cold front stalls over central Lake Michigan. Then, a couple passing low pressure system along the stalled front will lead to intermittent showers and thunderstorms through the middle of the week. Areas of dense fog will likely occur at times due to a relatively mild and humid airmass over the cold waters.

Gehring

MKX WATCHES/WARNINGS/ADVISORIES

WI...None. LM...None.