The rollover occurred between exits 60 and 61, DOT said. The accident scene is about 20 miles east of New Haven.
Ambulances, police and fire trucks have responded, and several people were standing outside the bus at the crash scene.
The accident is near Exit 61 for Route 79/Durham Road.
The accident was reported at 12:28 p.m. It was not known how long it would take to clear the accident scene and reopen the highway.
Scanner reports indicated about 50 people were onboard, and many were outside the bus.
There was no information on any injuries or on the cause of the crash.
The entire Connecticut shoreline is under a Winter Weather Advisory until 6 p.m. Monday. The highway is snow-covered with falling and blowing snow continuing at the scene at 12:55 p.m.
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