The heat wave that enveloped much of the region in ­recent days broke with a bang Wednesday afternoon, as a barrage of lightning and heavy rain swept across Massachusetts, flooding roadways, downing trees, and leaving more than 20,000 people without power.

The entire state was under a severe thunderstorm watch until 9 p.m., the National Weather Service in Taunton said.

The storms moved across the state quickly and brought much more comfortable weather for the remainder of the week, according to Weather Service meteorologist Charlie Foley. He said sun and low ­humidity is expected starting Thursday and continuing through Sunday. Daytime temperatures will be in the low 80s, dipping into the 70s on Friday.

“The big thing is that this frontal passage will mark the end of the hot and humid air mass that we’ve been experiencing,” Foley said.

State Police said the North Shore experienced roadway flooding Wednesday in several locations, including portions of Route 1 in Peabody, which was under 2 feet of water. The Nahant-­Lynn rotary and the Lynnway at Commercial Street also flooded, and the Nahant Causeway had standing water, as did Route 1 southbound at Route 60, State Police said.

In Malden, Route 99 was backed up for about a mile at around 3:15 p.m. and a deep puddle nearly 50 yards in width had amassed in the parking lot of a mobile home park located just off the road.

Ed Bryan, 56, said the flooding in the lot, which spread to the front entrances of some homes, was nothing new.

“This has been happening since the 1950s,” Bryan said. “When it comes, it looks like a river. It’s a couple hours of hell.”

National Grid at one point reported about 20,000 customers without power, mostly on the North Shore, but shortly ­after 7 p.m. that number had dropped almost in half.

National Grid spokeswoman Charlotte McCormack said ­extra crews had been brought in before the storm.