NEWBURYPORT — The second nor'easter in less than a week dumped heavy, wet snow overnight Thursday, knocking down trees and power lines and leaving tens of thousands of people without power.

National Grid officials said the lights might not come back on in many areas until late Friday night or Saturday.

Mayor Donna Holaday said Thursday morning that National Grid reported major power lines severed in Groveland, and company workers were struggling to find the location of the breaks. Those lines supply power to Newburyport, Amesbury, Merrimac, Rowley, Ipswich and Newbury, among other local municipalities.

National Grid planned to send out a helicopter to scan the route "to trace the lines to see if they can find the problem" but had to call off that plan, the mayor said late Thursday.

As of 5 p.m. Thursday, the Massachusetts Emergency Management Agency said 255,000 National Grid customers had no electricity, as well as 65,386 Eversource East customers.

By late afternoon, Holaday said it was "looking like a longer-term event" and she hoped the power line break in Groveland could be found before dark, so repairs could begin more quickly.

In the meantime, warming centers were opened in Newbury at PITA Hall on Plum Island, the Salvation Army on Water Street in Newburyport and at the Hilton Senior Center on Lafayette Road in Salisbury. The City of Newburyport contacted the Red Cross to find out what help might be available, and National Grid was sending a van with blankets, flashlights, coffee and electrical charging ports to the Senior Community Center so people could charge phones.

Holaday said city officials were trying to get another generator so an additional shelter could be opened.

The mayor said a meeting was scheduled for 10 a.m. Friday at PITA Hall to assess the state of Plum Island, which experienced major problems several years ago when the sewage pumping system failed. On Thursday, Holaday said the situation on Plum Island was stable in terms of the sewage pumping station.

"In terms of Plum Island, I think this is the first time I ever really prayed for snow instead of rain," Holaday said. "We were in real trouble out there. The flooding was something like I've never seen." 

She praised firefighters and Department of Public Services workers who filled hundreds of sandbags to help hold back flooding.

"We did OK last night with high tide. There was some washover, as expected, but not as high as the last storm," the mayor said.

Newburyport police reported on Facebook that as many as 157,000 residents of the Merrimack Valley were without power, including 99 percent of Newburyport.

There were a few bright spots, with Nick's Pizza on Merrimac Street, and the Black Cow restaurant on the waterfront, both open for business with power from generators. Kelly's True Value Hardware on State Street was open Thursday as well.

Seabrook Town Manager Bill Manzi said Thursday morning about 1,100 customers in Seabrook didn't have power but most of the town and business areas were up and running.

Amesbury police Lt. Craig Bailey was coordinating police efforts for the city and said downed trees were the biggest concern in the morning.

"We try to solve the problems as we get to them but there are a lot of problems," Bailey said.

One of the few places offering hot coffee in the city, Dunkin' Donuts on Route 110, became Bailey's biggest late-morning concern since the drive-through lines of cars stretched up Rocky Hill Road and onto Route 110, all the way back to the neighboring McDonald's.

"People have been rubbernecking over there and we've had some minor traffic accidents," Bailey said.

A more serious traffic incident took place at roughly 10:30 a.m. when an unidentified man was arrested and charged with driving under the influence of narcotics just past the Salisbury town line before Elm Street.

"I told the guys, in 24 hours, 90 percent of our problems will be solved," Bailey said. "In 48 hours, most of our problems will be solved. You need to keep that in mind because guys get so busy running from call to call to call."

Firefighter/paramedic Rob Serino had been on duty since 3 a.m. on Thursday and said his department received roughly 40 calls since then.

"It's been mostly electrical problems," Serino said. "We have had things like a tree down on Rocky Hill Road and that really becomes a coordinated effort. We go out and assess, the Police Department has to block the roads, the tree company and National Grid take care of everything else."

Shawn Roberts, the deputy director of the city's Emergency Operations Center, began his volunteer shift taking nonemergency phone calls at 9 a.m. Thursday but the call center had been staffed since 7 p.m. Wednesday.

"It was calming down when I got in but it seemed to be cranking up again (by 10 a.m.) when it seems like a lot more trees were coming down all of a sudden," Roberts said. "Mostly, people are calling to see how long the power will be out for. They would also like to know about any shelters being open."

Mayor Ken Gray gave his City Hall employees a delayed start time of 1 p.m. on Thursday but headed to his office by 9 a.m.

"The police station, the fire station and the Emergency Operations Center are all right here," Gray said. "I was able to spend a fair amount of time with each one this morning. That gives me a better feel of what is going on and I can then call the DPW and see what is going on on their side. The roads are actually in pretty good shape because it is warm. That is a good thing."

Gray said he keeps in 24-hour contact with the Police, Fire and Public Works departments via text and emails.

"It evolves over the course of the day," Gray said. "I heard this morning that National Grid has got over 240,000 people without power in the state. So this is pretty bad. It is not as many as the storm this past October but it is a lot. We have a lot of main transmission lines down."

Major thoroughfares such as Route 110 take priority during restoration efforts, according to Gray.

"There were two main transmission lines down, one was on Route 110 and one was at the intersection of Elm Street via Monroe Street, before the Laundromat," Gray said. "It looks like those were the two biggest ones."