Post-tropical cyclone Irene, downgraded from a monster Category 1 hurricane, is whipping up ferocious winds and dumping rain upon Quebec and the Maritime provinces as it hurtles north toward the Canadian border from the U.S.

Even as a weakened storm carrying sustained winds of 95 km/h, Irene knocked out power to 248,500 Quebec homes early Sunday evening after having walloped the U.S. northeast, leaving massive flooding and potentially billions of dollars worth of destruction in its wake. At least 21 deaths have been blamed on the storm.

Coastal areas are on guard for the threat of dangerous storm surges and more power outages, and wind warnings are still in effect for much of the Maritimes. The Eastern Townships, the Gaspé Peninsula and western New Brunswick are also under rainfall warnings.

The storm, which began as a hurricane and was later reduced to a tropical storm and then downgraded again to a post-tropical cyclone, delivered enough rain to cause flooding in Lower Manhattan Sunday morning. According to the U.S. National Hurricane Center, Irene can still carry heavy rains and high winds as a post-tropical storm.

Irene's impact was felt midday Sunday in south and eastern Quebec, where winds of 65 km/h and blowing rain started in the early afternoon, knocking out power across the province and leaving thousands of clients in the dark. Most of the outages were in the Montreal area, according to Hydro-Québec, as Irene's outer bands lashed at the area and hovered over New England.

Trees whipped by heavy rain and wind fell to a thud in Montreal and traffic signals were malfunctioning. Police were forced to close a section of a downtown street after gusts blew away two windows from the seventh and 19th floors of a building. Nobody was injured.

Environment Canada's René Héroux said the worst will come as the storm reaches Quebec's Lower North Shore Monday.

"The corridor, Montreal, Townships … then lower St. Lawrence and the North Shore, those regions will have very high winds, up to 90 km/h and even 110 in the Quebec City area," Héroux said Sunday, adding some regions will get close to 100 millimetres of rain over the next 24 hours.

Rain in New Brunswick

Nearly 50 millimetres of rain had already fallen in parts of southern New Brunswick by late afternoon. Hundreds of homes and businesses were without power.

In Fredericton, the city warned residents to expect up to 66 mm of rain and wind gusts of up to 95 km/h. More than 30 homes in the area had their basements flooded during heavy rain.

By late evening Sunday, tree branches had reportedly fallen onto roadways in Western Nova Scotia and Southern New Brunswick. About 5,000 customers were without electricity in New Brunswick.

Environment Canada said tropical storm warnings are still in effect for the Fundy coastlines of New Brunswick and Nova Scotia as well as the Atlantic coast of Nova Scotia southwest of Porters Lake.

mi-irene-montreal-2-620

A woman in Montreal battles Irene's winds with her umbrella Sunday. (CBC)

CBC meteorologist Peter Coade said Irene is a huge storm, estimated to be about 700 kilometres wide, with a cloud shield stretching from the Carolinas into southern Quebec and as far west as Lake Ontario.

"As the storm moves closer the wind is going to continue to increase. We've already had some extremely high gusts, and that really is going to be the biggest problem, I believe, for Nova Scotia and Prince Edward Island," Coade said.

Generally, the heaviest rain falls to the left of the track of the storm and the highest winds to the right.

Storm surge warnings are in place for Yarmouth and all of western Nova Scotia, southwestern New Brunswick midnight Sunday and into Monday, while the Canadian Hurricane Centre predicts strong winds in the Bay of Fundy. There is also a threat of higher winds for southern New Brunswick and western Nova Scotia and in the St. Lawrence regions.

Coade said the Yarmouth area of Nova Scotia saw wind gusts of up to 90 km/h by midday Sunday.

Chris Fogarty, with the hurricane centre, said Irene brings a flood threat for Chignecto and Minas basins.

Millions of Americans along the eastern seaboard were without power Sunday as the storm pounded New York. At least 18 deaths in the United States are being blamed on Irene as it powered its way northward.

Hydro-Québec has sent 54 emergency teams to New Hampshire to help Irene relief efforts south of the border.

Storm could last 12 hours

High winds could cause storm surges and waves from the southwest into the Bay of Fundy.

Many parts of Eastern Canada have received above normal rainfall this summer, which could raise the risk of flooding and streams overflowing.

The Emergency Measures Organization warned Sunday that people should have three days of supplies ready and have a family emergency plan in place.

Fogarty said to expect the storm to last at least 12 hours — high winds are expected even through Monday.

People are warned to stay away from coastal regions.