From the National Weather Service Milwaukee/Sullivan Forecast Office, Sunday at 2:30 p.m.

“Hot and humid conditions are expected Monday, and especially Tuesday. Temperatures will reach 90 to 95 on Monday with heat indices close to the air temperature. Humidity will increase on Tuesday, and highs will reach 95 more uniformly over much of southern WI, with heat indices between 100 and 105.
What has changed? Dew points will not reach as high as previously anticipated on Monday. While it will still be hot, heat indices will be similar to the air temperature on Wednesday, owing to drier dew points and a southwesterly breeze.
Confidence remains high that Tuesday will be the hottest day, potentially worthy of a Heat Advisory.”