A windstorm that struck the region Sunday evening quickly knocked out power to more that 20,000 customers, downed trees, and caused transformer explosions and road closures.

It was the most outages in the region since Superstorm Sandy, which occurred five years ago Sunday.

The National Weather Service issued a high wind warning and a flash flood watch for New London County, effective until 6 a.m. Monday. A coastal flood advisory for southern New London County is also in effect until 9 a.m. Monday.

"We have a storm system organizing along the mid-Atlantic coast, lifting northward up the coast and across New York, as we go through the overnight hours," said Gary Lessor, the assistant director of The Weather Center at Western Connecticut State University. During tomorrow it will lift into southeast Canada."

The region can expect heavy rains at times overnight and very windy conditions, with winds of 25 to 30 miles per hour with gusts as high as 65 to 70 miles per hour, he said.

According to Eversource, as of 11 p.m. Sunday, North Stonington was the hardest-hit town with 81 percent or 2,348 customers without power. Stonington wasn't much better with 73 percent or 5,878 customers without power. In Ledyard, 58 percent or 3,848 customers were without power and another 4,909 customers or 50 percent were out in East Lyme.

Also out were 1,561 customers in Waterford, 277 in Montville, 365 in Old Lyme, 4, 909, 142 in Lyme, 169 in Preston and 37 in Salem. Groton Utilities reported multiple power outages. A tree also fell on a home in Ledyard but there were no injuries. 

Peak gusts and the heaviest rain are expected from 11 p.m. to 3 a.m. Rainfall is anticipated to total 2 to 4 inches, with the possibility of 5 inches or more in some.

Rain will wind down Monday morning  but it will still remain windy with gusts up to  45 mph into the mid-afternoon, he said. Dry and cooler weather are forecast for Tuesday and the rest of the week.

In Waterford, known power outages were in the areas of Boston Post Road near Brill Avenue, Route 85 at Interstate 395 and Spithead Road/Fog Plain Road to Willets Avenue, according to the Waterford Police Department's Facebook page.

East Lyme Public Safety's Facebook page reported that Chesterfield Road is closed in the vicinity of Aces High RV Park.

In New London, a tree was down on Jefferson Avenue and wires down on Howard Street. In Stonington, trees and wires were down on Pequot Trail and Al Harvey Road. Power was also out in the Hazelnut Hill Road area of Groton.