The weather across Wisconsin and the Upper Midwest has remained cool the last several weeks, with daily temperatures averaging below normal on most days. The main reason for the cooler conditions has been a persistent westerly upper level wind flow. Weak weather systems moving along these westerly steering currents kept temperatures below normal most days, with several bouts of rain.
However a change is in the offing, promising warmer temperatures for southern Wisconsin and the Upper Midwest for the rest of this week. Beginning tonight and continuing on Tuesday, low pressure will amplify and strengthen over the northwest U.S. into southwest Canada. Since a ridge of high pressure is expected to remain nearly stationary over the southeast U.S., increasing south winds are expected to develop in the mid levels of the atmosphere between these two systems. These increasing winds will pull warmer air into the region the rest of this week. High temperatures on Tuesday are expected to climb into the lower to mid 70s inland areas, with cooler mid 50s to mid 60s near Lake Michigan. Highs in the mid to upper 70s are then expected for Wednesday and Thursday, with a noticeable increase in humidity.
However, the increasing south winds will carry an active frontal boundary, that has been located well south of Wisconsin the past few days, northward into Wisconsin and the Upper Midwest. This warm front will move northward toward southern Wisconsin Tuesday night, possibly triggering thunderstorms with hail.
Then several low pressure systems will move northeast across the Northern Plains and western Great Lakes Wednesday through Friday along this boundary. These weather systems, along with the increased humidity, will result in scattered thunderstorms across the region. A few of the thunderstorms could become strong to severe, especially Wednesday afternoon and Wendesday night. Keep up with the latest forecasts regarding this period.
NWS - Milwaukee/Sullivan