An area of weakening showers and thunderstorms moved across southern Wisconsin early Friday morning. Despite the innocuous look of the precipitation, strong gusty winds developed on the north edge of the precipitation area over parts of south central Wisconsin.
At 545 am CDT, wind equipment at the Jack Young Middle School in Baraboo in Sauk county reported a wind gust to 45 mph. At the same time, the Kalahari Water Park in the Wisconsin Dells reported a wind gust to 41 mph. 12 to 15 acres of corn was also flattened approximately one mile south of Plain in Sauk County.
At 645 am CDT, the automated observing equipment at the Lone Rock airport in Sauk county reported a wind gust to 44 mph. An hour later at 745 am CDT, wind equipment at the Barneveld School in Iowa county reported a wind gust to 42 mph while a wind gust to 41 mph was recorded at Madison West High School in Dane county.
Here is a radar image from 556 am CDT Friday morning, shortly after the 45 mph wind gust was reported in Baraboo.
The strong winds were occurring on the northern edge of the large precipitation area.
A large cold pool of air developed beneath the widespread precipitation over southwest Wisconsin and northwest Illinois Friday morning, which contributed to the development of a small, compact area of high pressure called a meso-high over northwest Illinois.
At the same time, descending winds near the trailing edge of the large precipitation area over southwest Wisconsin resulted in a compact area of low pressure, called a wake low, as illustrated by the below 7 am CDT surface analysis.
The resulting strong pressure gradient between the meso-high and wake low produced the strong gusty winds over southern Wisconsin early Friday morning. The pressure gradient did not last long however, as illustrated by the below 9 am CDT surface analysis.
Below is a graph of barometric pressure on the northeast side of Madison. Notice the large drop in pressure between 6 am and 7 am CDT Friday morning as the strong pressure gradient moved across the Madison area.
Typically, the compact wake low occurs near the back edge of a large precipitation area where the winds are descending most rapidly, near where the precipitation has already evaporated or ended. Wake lows appear to be most common with decaying meso-scale convective systems with an asymmetric, or non-linear structure.
Below is a radar animation between 5 am and 845 am CDT Friday as the rain diminished across southern Wisconsin.
Mark Gehring - Senior Meteorologist
Marc Kavinsky - Senior Meteorologist
Penny Zabel - Meteorologist