The National Weather Service has confirmed that a tornado struck the Merrillville/Hobart area during heavy storms Friday night and is investigating whether a tornado touched down in Portage and/or Valparaiso as well.

The Merrillville area had tree damage, downed power lines and utility poles, and fences knocked down by the storm, as well as damage to homes, including a tree that fell on a house, according to weather service and town officials.

Meteorologists can determine whether damage is from a tornado or heavy winds by “the direction the debris was blown,” Doom said, adding if debris are erratic, the cause is likely tornadic and not straight line winds.

The weather service made its determination of the Merrillville/Hobart area Sunday night, though did not have details about the path of the tornado and other metrics yet on Monday afternoon. The weather service, Doom said, also was in contact with trained spotters and first responders in Portage and Valparaiso.

“We haven’t made a determination yet whether it was tornadic or straight line winds” in those communities, Doom said, adding he expected that to come by the end of the day Monday.

With another line of severe storms expected Tuesday, he said, meteorologists had “a narrow window” to make that determination.

As of 2 p.m. Monday, NIPSCO said on its power outage webpage that crews and contractors continued to work to restore electricity to the 320 customers who did not yet have power, down from more than 27,000 customers at the height of the storm.

The majority of the customers were in Portage.

“Due to the substantial amount of damage requiring individual repairs, we expect power to be restored for the majority of those outstanding customers, who reside primarily in Merrillville, Hobart and Portage,” by midnight Monday, the utility said online. “Some customer outages may extend into late evening on Tuesday, April 4, due to the extensive damage. We continue to recommend that you make plans to keep yourself and your family safe during this extended outage.”

The weather service reported a wind gust of 74 mph at Porter County Regional Airport on Friday night.

The Merrillville tornado, an EF-1 with winds between 86 and 110 mph and considered weak according to the National Weather Service’s website, was the 16th identified in the Chicago and Northwest Indiana area Friday night.

Ross Township Trustee Eric Blackman Sr. said he was busy Monday as his office continues to offer assistance to residents impacted by the tornado. Ross Township has 49,000 residents in Merrillville, on Crown Point’s north side and on the south side of Hobart.

His office provided assistance for two families to stay in hotels over the weekend “but the phone is ringing off the hook now.”

Blackman’s office has been communicating with the Merrillville Fire Department and the American Red Cross, among other entities, since the storm hit, including assisting with search crews to make sure people with disabilities and older folks could safely get out of their homes.

Town officials said in a release that the Madison Meadows and Forrest Hills neighborhoods were most damaged by the tornado.

“It’s very hard to give you an actual number (of residents impacted by the tornado),” Blakcman said. “We’re still getting calls from people in need.”

The Dean and Barbara White Community Center remains open as a warming center, town officials said. The center is located at 6600 Broadway, and it is open until 11 p.m. Anyone who needs to use the facility after 11 p.m. can go to the Merrillville Police Department at 7820 Broadway and get in contact with an officer who can assist them.

Additionally, those in need of support are encouraged to visit the Trustee’s Office at 26 W. 73rd Ave. or call there at (219) 769-2111; the office also is taking donations to assist residents. The Red Cross can be reached at 1-800-RedCros.

With more storms forecast for Tuesday, Blackman said, “we put a plan in place in case, God forbid, anything like this happens. We’ll be on standby.”

Lance Bella, director of the Porter County Emergency Management Agency, said he had staff taking notes and pictures of storm damage in Portage and Valparaiso to assist the weather service in its determination of whether a tornado or tornadoes struck here as well.

The staff also are trained weather spotters, he added, and were sending their information to the weather service, which would combine that with other data they have “and get a picture of what maybe happened,” Bella said.

Emergency sirens in Porter County came on at 8:59 p.m. Friday after they were triggered by the National Weather Service because of the storm path, Bella said, adding that “99% of the time,” that’s what triggers the sirens.

In all, 24 of the county’s 52 sirens sounded, he said, adding that while Portage received the other storm warnings throughout Friday night, the sirens there did not go off because the weather service’s computer program didn’t see Portage as the path of the storm.

“Straight line winds don’t trigger it,” Bella said. “Rotation doesn’t mean tornado, it means there’s rotation in the clouds.”

A meteorologist with the NWS said Saturday that rotation had triggered the sirens for Porter County.

alavalley@chicagotribune.com