August 20, 2010 Severe Weather, Including 100 mph Winds Near Village of Green Lake

 Strong thunderstorms affected much of southern Wisconsin in the afternoon and early evening on August 20th.  Many of these storms were accompanied by strong winds producing damage across the region.  Rock, Jefferson and Dodge counties received widespread tree damage from a bow echo that moved through between 3:30 and 5:30 pm.

The radar image on the left below shows the bow echo moving through eastern Dane county and across Jefferson into northern Walworth county at 4:20pm.  A separate storm producing some damage was in Sauk county at this same time.

The image on the right is the base velocity measured by the radar a little later in the life of this bow echo. The bright red into pink colors are winds moving to the northeast at 50 to 70 mph. This velocity image is from 5:10pm, about the time a 100-year-old barn was blown down near the town of Eden in eastern Fond du Lac county.


Green Lake County Event

A small, intense microburst hammered the northeast corner of the Green Lake shoreline (Green Lake County) about 540 pm on Friday, August 20, 2010, and then moved northeast for about 1.8 miles.  

Here are the facts:

* Maximum wind speeds were an estimated 100 mph.  One shoreline home with a partially sheltered wind anemometer had a measured wind gust of 90 mph.

* Event time started about 540 pm and ended about 542 pm.

* There were no fatalities or injuries.

* Heavy rains and poor visibilities accompanied the hurricane-force winds.

* Damage was inflicted on homes, boats, boat piers, and many trees.

* Start location was 1.5 miles east-southeast of the Village of Green Lake on the shoreline of Green Lake.

* End location was about 2.3 miles east-northeast of the Village of Green Lake, about 1/4 mile south of State Highway 23.

* The path length of the microburst was about 1.8 miles and the maximum width at the shoreline was about 150 yards.

Damage survey was conducted Saturday morning, August 20, 2010, in conjunction with the Green Lake County Emergency Management team. A special thanks is extended to them for their quick and thorough documentation done prior to the NWS being able to travel to the damage areas.

Below are two radar images from the Green Bay WSR-88D Doppler radar, which had a better angle to this storm for measuring the winds than the Milwaukee/Sullivan WSR-88D.  The image on the left shows base reflectivity.  The microburst was increasing in strength as it approached the far northeast corner of Green Lake.  The image on the right shows the base velocity.  Note the bright green colors in the circle. This represents the powerful microburst winds in the Green Lake area.  Blue and green colors indicate that the targets are approaching the Green Bay radar site.

base reflectivity 540 pm     base velocity 540 pm

Below is a graphic showing the location and path of the microburst:

 Location of microburst

There were two areas of concentrated damage: 1) along the extreme northeast shoreline of Green Lake, and 2) about 1 to 1.5 miles northeast in a residential development.  Below is a description of the damage in the two areas:

1. Green Lake shoreline area - four homes sustained damage to roofs, or siding, or rain gutters.  One pontoon boat was overturned and damaged. One boat pier was destroyed and another pier had a ripped canopy and tilted floor boards. A couple dozen large trees were either uprooted or had broken large branches.  Flying debris left impact craters on the ground.

Here are some of the damage pictures from area #1 marked on the map:

picture 1  picture 2  picture 3

picture 4  picture 5  picture 8 

2. Residential area about 1 mile northeast of Green Lake shoreline - four homes sustained damage to roofs, siding or rain gutters.  Several roof panels were ripped off of one home after the garage doors were buckled in.  From this residential area going northeast for another half mile, a couple dozen large trees were either uprooted or had broken large branches.

Here are some of the damage pictures from area #2 marked on the map:

picture 6  picture 7  picture 9

Walworth County Microburst Event

A microburst affected the area just southeast of the Village of Walworth in Walworth County about 530 pm with gusts of about 60 mph out of the south.  These winds blew right into a south-facing 3-sided pole shed and destroyed about half the building.  Unfortunately the open end of the building faced right into the winds.  Below is a picture of the damage taken by Lt. Kevin Williams, Walworth County Emergency Management Director.

walworth shed damage

Elsewhere in Walworth County - in and around Whitewater, several large trees were either uprooted or had large limbs broken as three separate storms moved northeast through the city between 345 pm and 430 pm.  Additionally, about 2 inches of rain fell in the city within that 45 minute period resulting in some urban flooding.  This information was supplied by Sgt. Mike Cardo of the Whitewater Police Department, who is also the Emergency Management Director for the city.

Meteorological Information for August 20, 2010 

The environment this day was favorable for strong wind events more so than tornado or hail events.   The following graphic shows the sounding observation from Davenport, Iowa on the morning of the 20th.  The red line on this graphic is temperature, and the green line dewpoint.  The bottom of the plot represents the surface of the earth, while the top of the plot is thousands of feet vertically in the atmosphere. When the red and green lines have a large gap between them, as they do in much of this sounding, this is indicative of dry air in the atmosphere. With strong heating in the afternoon, the red and green lines had an even larger gap between them over southern Wisconsin around the time storms were forming. 

When thunderstorms develop in this atmosphere, rain falls through this air that is initially very dry.  The water drops evaporate, cooling the surrounding air.  With enough evaporative cooling, the air above the surface within the storm environment becomes heavier than the air below, and will rush downward.  This results in very strong winds reaching down to the surface and leaving behind the damage seen in southern Wisconsin on August 20th.


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