A low-pressure system moved from the central High Plains toward the Dakotas overnight Saturday night, May 26, into the morning of Sunday, May 27. A surface warm front extending eastward from this system pushed north toward Wisconsin early Sunday. The arrival of the warm front had been anticipated for several days; locations south of the warm front were expected to be under the influence of southerly winds, which would bring with them exceptionally warm temperatures. High temperatures for Sunday were expected to be in the lower to mid 90s across most of southern Wisconsin. A lake breeze was expected to develop off Lake Michigan and have a cooling influence on temperatures, but only in the immediate vicinity of the lake (along the shore but not at inland locations).
A warm front is often associated with showers and thunderstorms: as warm, moist air flows up and over a region of cooler air, the lifting of the moist air may produce precipitation. This process occurred during Saturday afternoon and night over Wisconsin, producing rounds of thunderstorms. Rain-cooled air that flowed out of these storms remained over eastern Wisconsin and Lake Michigan in the early hours of Sunday morning. The presence of this cool "outflow" air helped to strengthen the lake breeze as it developed. See the depiction below (click on it for a larger version).
Skies cleared across southern Wisconsin by late morning as the warm air deepened and dissipated the scattered cloud cover. The surface warm front pushed well north of the Iowa border into Minnesota, and the La Crosse, Dells, and Madison areas all already felt temperatures in the 80s by 11 a.m. However, the pool of colder air around the lake greatly restricted the northward movement of the front, so at this time, areas like Fond du Lac, Milwaukee, and Kenosha saw easterly winds with temperatures still in the upper 60s and 70s.
The lake breeze was able to hold off the advancement of the warm front into the afternoon and through the peak daytime heating hours. The result? High temperatures were as high as 97 degrees in south central Wisconsin, with lower 90s observed at most places west of a line from Montello to Burlington. On the other hand, temperatures remained in the 70s or lower 80s throughout the afternoon where the warm front was unable to penetrate.
Afternoon imagery from the MKX radar showed a clear line of convergence where the easterly winds of the lake breeze met the strong southerly flow in the "warm sector," that is, areas behind the warm front. Convergence refers to the fact that winds are flowing together in an area. Dust, insects, smoke, and other airborne particles can "pile up" in areas of convergence and make an otherwise invisible boundary appear on radar. It is not unusual to be able to see the lake breeze boundary on radar, but the sharp contrast in temperature between the two air masses may have enabled particles to be suspended/accumulated even more effectively, highlighting the boundary even more.
Sunday's temperature forecast was a "bust" for many locations near Lake Michigan. It is very difficult to account for the effects of thunderstorm outflow on temperature trends, and in this case was not anticipated to be a factor in Sunday's temperature. Even if outflow events like the one documented here were anticipated, it would be hard to know exactly where the outflow would form and how far it would reach. Predicting the precise timing and location of thunderstorms, even just a few hours out, is still a challenging task.
National Weather Service, Milwaukee/Sullivan, WI