It will take "months" for the citizens of Goderich, Ont. to clean up the town after it was hammered by a devastating tornado over the weekend, says a meteorologist with Environment Canada.

The agency has ranked the tornado an F3, with sustained winds of 280 kilometres per hour and a path of destruction a half-kilometre wide. The tornado hit the town Sunday afternoon as a violent storm swept through southwestern Ontario. One man died, and 37 people were injured.

On Monday the town council declared Goderich a disaster area, which will allow the community to access millions of dollars in assistance pledged by the provincial government.

Goderich council is also asking Ottawa for aid money.

On Monday, Ontario Premier Dalton McGuinty said the province had activated its emergency plan to help towns affected by the storm. McGuinty said $5 million has been set aside specifically for clean-up and rebuilding efforts in Goderich.

McGuinty called the devastation the worst he had seen in his time in public office. The tornado downed power lines, flattened trees, broke windows, toppled cars and left all manner of debris strewn on the streets.

Arnold Ashton told CTV's Canada AM on Tuesday that the most intense part of the tornado cut straight through the centre of Goderich, damaging homes, commercial buildings and the town's church.

He said the town is in for a sustained clean-up and rebuilding period.

"It almost takes months to get everything together," Ashton said. "The degree of damage to these buildings, especially the commercial buildings and a lot of the residential areas along the core just east of town, were devastated."

According to Ashton, the agency's initial grading of the tornado as an F2 was based on "a very cursory look" through Goderich. The upgrade to an F3 was based on further investigation by three separate teams that analyzed the devastation in Goderich.

The rating system is based on engineers' calculations of how strong winds have to be to, for example, knock down a wall. Investigators then survey the damage caused by the tornado and make their conclusions, Ashton said.

"With this we saw crumbled walls, the church was just devastated," he said. "There were century-old, double-layered brick buildings that have withstood the test of time with those howling winds off Lake Huron. And they were just crumbled."

The storm was the post powerful tornado recorded in Ontario in more than a decade. According to the agency, fewer than five per cent of tornados in Canada are ranked at F3 or above.

An F3, or Fujita Scale 3, rating means that the tornado is "severe," according to Environment Canada. The type of damage expected with an F3 includes exterior walls and roofs blown off homes, metal buildings collapsed or severely damaged, and forests and farmlands flattened.

The storm hit around 4 p.m. local time, and not long after Goderich Mayor Delbert Shewfelt declared a state of emergency.

Late Sunday, OPP confirmed one fatality. Norman Laberge, a 61-year-old resident of nearby Lucknow, Ont., died as he worked his shift at the Sifto salt mine. The cause of death has not been released.