Severe Thunderstorms in the Central U.S.; Heavy to Excessive Rainfall in the Ohio Valley and Central Appalachians

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Autumn bird migration is starting and weather radar often detects large flocks of birds as they start their day. A large group of birds was detected leaving Lake Poygan in Winnebago County to feed just before sunrise on August 2, 2017. This feature, known as a "roost ring," occurs when the radar beam detects thousands of birds simultaneously taking off from their roosting site around dawn to forage for insects.

August 2, 2017 radar reflectivity animation from 5:00 AM to 5:45 AM.

Leading up to fall migration, a number of bird species are known to gather at large communal roosting sites, which are often detected by NWS Doppler radar. The unique doughnut pattern of these roost rings is the result of birds departing their roosting site in all directions, roughly in equal densities. As they travel further from their roosting site and reach higher altitudes, they are detected by radar until they either rise above or drop below the radar beam. Some species typically return to the same roosting sites in the evening, which are usually situated near bodies of water. As a result, the roost rings are often observed in the same locations on radar over the course of several mornings.

Here are examples of other roost rings in northeast Wisconsin:
- August 2010
- August 2006

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Severe Thunderstorms in the Central U.S.; Heavy to Excessive Rainfall in the Ohio Valley and Central Appalachians

Radar Detects Birds Leaving Lake Poygan