textproduct: Milwaukee/Sullivan

KEY MESSAGES

- Thunderstorms with higher-end severe potential and heavy rainfall is expected this afternoon and early evening. An Enhanced Risk (3 out of 5) for severe weather exists over the western half of the CWA. Heavy rain over saturated ground may lead to flash flooding.

- A Flood Watch in effect for all of southern Wisconsin until 7am Saturday.

- Marine Dense Fog continues through tonight.

UPDATE

Issued 244 PM CDT Fri Apr 17 2026

The storms fired along that leading cumulus field that was in eastern Iowa right around noon. They grew and evolved into a broken line of supercells and are now tracking a little slower to the east. The MLCAPE is on the 2000-2500j/kg gradient and the bulk shear is 40 to 50kt. The significant tornado parameter is pushing 4 in this region, and the sig hail parameter is 3. These parameters diminish as the cells congeal into a line over the next few hours, but wind and line-embedded tornadoes will become a higher risk.

Radar and satellite are also showing the cold frontal boundary (just north of Twin Cities to Rochester to just east of Des Moines) light up with numerous mini supercells. If this initial round of storms take control for the afternoon, which they are expected to, this will diminish the intensity of the storms along the cold front as it tracks across southern WI during the early evening between 6 and 9 PM. However, if this first round underperforms, meaning does not produce widespread thunderstorms (and a cold pool), then the storms along the frontal boundary will be able to maintain its strength as it moves through.

The other scenario noted in the most recent small-scale model runs is that the storms along the frontal boundary may accelerate and catch up to this initial round. The way things are moving, this seems like a likely scenario. The 18Z HRRR is hinting at this. This would mean the convection would rush out ahead of the main front and there will still be a chance for showers and isolated thunder to linger until that front clears our area during the overnight hours.

Heavy rain and flash flooding are still a threat due to high precipitable water and repeated rounds of storms over already- saturated soil.

Cronce

SHORT TERM

Issued 1207 PM CDT Fri Apr 17 2026

Tonight and Saturday:

Nearly all of southern WI has sunshine as of 1130 this morning. The fog and low stratus are hanging on over east central WI, but the increasing southerly winds will help that to erode quickly through the early afternoon. Temperatures are rising quickly, with areas west of Madison already in the lower 70s. Dewpoints are rising as well, with lower 60s in the southwest. Gusty winds are developing as we start to mix out our low layer inversion and diurnal cu are also popping up. This is all conducive for priming our environment for convection this afternoon.

The main change for today is that we are expecting storms to develop and move into southwest WI earlier than forecast, now between 1 and 5 PM. Storms triggered along a gravity wave just south of the Twin Cities around 8 am. If we trace that gravity wave down into Iowa on satellite, the cumulus clouds are focusing along it. The leading edge of this is now entering northeast IA and far southeast MN. The timing of it to arrive in Madison is 230PM/1930Z, give or take a half hour. There is no lighting in eastern IA yet, but we are watching for that to happen within the next 30 to 60 minutes.

Whatever develops in IA and tracks eastward is our main show in southern WI for today. All hazards are on the table. High CAPE, shear and helicity all point to a large hail (greater than 2 inches), damaging wind and tornado potential. If tornadoes develop, they would be on the strong side. As any discrete cells congeal into a line as they head into eastern WI, the severe threat will be mainly for damaging wind and QLCS tornadoes. With 0 to 3 km shear vectors up to 50 kt and aligned perpendicular to the line of storms (and streamwise vorticity of 45-50 kt), brief spin- up tornadoes are favorable along the leading edge within any notches/bowing line segments.

There is still a chance that the storms that develop in Iowa could skirt just to the south of WI and be the strongest across northern IL this afternoon. If I had to guess, the strongest storms should be over the south half of southern WI. However, there is still a severe threat for all of WI, including north central all the way to Green Bay.

The actual cold front is back in south central MN and central IA. This is a very strong front, with over a 20 degree temp drop behind it. Storms will be well ahead of the front, but once our afternoon storms clear, we will still have a chance for scattered showers and thunderstorms (not severe) until the actual front passes during the late evening and overnight hours.

Cronce

LONG TERM

Issued 244 PM CDT Fri Apr 17 2026

Saturday through Friday:

Gusty west-northwest winds up to 30 mph are expected behind the cold front Saturday. Sunshine should be prevalent, although cyclonic flow aloft will allow for some high-level diurnal cumulus clouds. Saturday highs will only be in the 40s, a stark contrast to the highs today. Lows Saturday night should dip below freezing nearly everywhere except the immediate Lake Michigan lakeshore.

The upper low will remain over Ontario Sunday and a shortwave trough digging into central and southern WI may steepen lapse rates enough to produce a few flurries. It looks like moisture will be very limited with this feature, but the forcing is certainly there. Highs Sunday will be similar to Saturday but with slightly lighter northwest winds. Lows Sunday night have a better chance of dropping below freezing, with lows forecast in the 25 to 30 degree range.

As high pressure departs the Upper Great Lakes on Monday, southerly winds will usher milder air back into southern WI. The sunshine will also help temps get into the mid 50s inland and lower 50s near the lakeshore. A clipper system will cross Ontario and Lake Superior Monday night, but southern WI will remain dry. However, this could push a cold front down Lake Michigan Tuesday. Not all models have picked up on this potential yet, so the forecast highs Tue are in the lower 70s inland and mid 60s lakeshore. Lakeshore temps could end up being colder, but the highs will depend on when that back door cold front actually makes it onshore.

There will be a stark temp gradient across southern WI from west to east on Wednesday as low pressure develops in the Plains. It looks like the storms associated with the warm front of this low will be over northeast MN and northern WI Thu. All We should see temps get back into the upper 70s inland over southern WI. The next potential for showers and thunderstorms, potentially severe, looks like next week Friday.

Cronce

AVIATION

Issued 1207 PM CDT Fri Apr 17 2026

See the main part of the discussion for thunderstorm chances. Timing of the storms for southern WI looks earlier than previously forecast. All severe hazards are possible.

A strong cold front will cross southern WI late tonight. Showers and a few storms will remain possible until the front clears. MVFR ceilings are expected behind the front on gusty WNW winds, and IFR ceilings are possible for a few hours early Saturday morning. Saturday ceilings should recover to VFR by mid/late morning and probably scatter out.

Cronce

MARINE

Issued 1207 PM CDT Fri Apr 17 2026

Winds will become southerly this morning as a surface low of 29.5 inches approaches from the central plains. Dense fog will continue over the lake through this evening and likely during the overnight hours until the cold front clears the lake. This surface low will quickly move from Omaha to Duluth from this morning into the afternoon. Winds will become gusty and southeasterly tonight, and strong to severe storms will cross the lake as a cold front sweeps through the area. Winds then become northwesterly behind the cold front Saturday morning. Breezy northwesterly winds then continue Saturday through Sunday as the low then races northeast toward Hudson Bay. Winds ease Monday morning as high pressure tracks southeast over the upper Great Lakes.

Cronce

HYDROLOGY

Issued 1207 PM CDT Fri Apr 17 2026

This afternoon / evening's convection is expected to produce an additional 0.50 to 1.25 inches of rainfall across the region, with locally higher totals up to 2.00 inches possible. Probs of exceeding 2.00 are only 10% or less based off of most guidance. Highest rainfall potential exists in the western half of the CWA, given the potential for initial supercells with heavy rain followed by additional showers / storms along the advancing cold front (eg. multiple rounds of heavier precip). Further east, still expecting a good burst of heavy rain from short line segments or QLCS to be more progressive / fast- moving by that time of day, but increased the storm total amounts toward an inch.

Cronce/Sheppard

MKX WATCHES/WARNINGS/ADVISORIES

WI...Flood Watch...WIZ046-WIZ047-WIZ051-WIZ052-WIZ056-WIZ057-WIZ058- WIZ059-WIZ060-WIZ062-WIZ063-WIZ064-WIZ065-WIZ066-WIZ067- WIZ068-WIZ069-WIZ070-WIZ071-WIZ072 until 7 AM Saturday.

LM...Dense Fog Advisory...LMZ080-LMZ261-LMZ362-LMZ364-LMZ366-LMZ563- LMZ565-LMZ567-LMZ669-LMZ671-LMZ673-LMZ675-LMZ777-LMZ779- LMZ868-LMZ870-LMZ872-LMZ874-LMZ876-LMZ878 until 4 AM Saturday.

Small Craft Advisory...LMZ643-LMZ644-LMZ645-LMZ646 until 10 PM Saturday.