The city of Calgary said Thursday up to 100,000 residents could be forced to leave their homes as communities across southern Alberta deal with heavy rains and widespread flooding.

The evacuation is likely to take place in stages over the next few days.

Mandatory evacuation orders have already been issued for several Calgary communities along the Elbow River, as parts of southern Alberta face rising flood waters and prepare for the possibility of more rain to come.

“Residents are encouraged to find shelter with family or friends for the next 72 hours minimum,” the city said in a statement.

By late Thursday, emergency evacuations were underway in the communities of Beltline, Bonnybrook, Bowness, Bridgeland Industrial Area, Chinatown/Eau Claire, Cliff Bungalow, Deer Run, Discovery Ridge, Downtown/East Village, Elbow Park, Erlton, Inglewood, Hillhurst, Mission, Montgomery, Quarry Park, Rideau, Riverbend, Riverdale, Roxboro, Stanley Park/Elboya, Sunnyside, Victoria Park, Westmount and Windsor Park.

All schools in the Calgary area were closed on Friday.

Environment Canada has issued a rainfall warning for parts of southern Alberta, forecasting up to 100 millimetres of rain in the next two days. A flood watch warning has also been issued for the Bow River upstream of the city, and flood warnings have been issued for parts of the Elbow River.

Reception centres have been set up for residents who can’t find alternate accommodations at the Southland Leisure Centre and the Acadia Recreation Complex.

Members of the Calgary Police Service and the Calgary Fire Department will be going door to door in the affected communities to notify residents of the evacuation order. As well, Calgary Transit is on standby to help residents who cannot leave on their own.

Residents in the evacuated communities would not be allowed back as of 3 p.m. Thursday, the statement said.

Calgary Police will maintain a presence in the evacuated areas for security purposes, the city said via Twitter.

Calgary is one of 12 Alberta communities to issue a state of emergency as surging waters grip the region.

Prime Minister Stephen Harper issued a statement on the flooding Thursday afternoon, offering thoughts and prayers to the families affected in Calgary and Southern Alberta and hope for a speedy end to the flooding.

“I want to assure Albertans that the federal government has offered any and all possible assistance to the Province of Alberta in response to the situation. Canadian Armed Forces assets, including a Cormorant and Griffon helicopter, have been deployed to the area to assist the Alberta Provincial Emergency team with rescue and evacuation efforts.

Premier Alison Redford, who was returning to Alberta from a conference in New York Thursday, said she would be touring the affected areas.

"I plan to visit the affected areas as soon as possible on Friday to see the situation firsthand and to thank those who have been working so hard at keeping everyone safe," Redford said in a news release.

During a news conference Thursday morning, Director of the Calgary Emergency Management Agency Bruce Burrell said current data indicates the flow rate will likely exceed the rate reached during the floods of June 2005.

Calgarians are being warned to stay away from river banks and creeks and move their valuables to higher levels in their homes.

Officials said during the news conference that the city’s drinking water remains unaffected by the flooding.

Residents are encouraged to continue checking the city's Twitter account for updates on the evacuations.

Helicopters survey flooded Highwood River

The RCMP confirmed to CTV Calgary that support from the Canadian Military has been requested to help the flood-stricken town of High River.

Three helicopters and additional ground support have been dispatched to High River, south of Calgary, from Canadian Forces Base Cold Lake, reports CTV Calgary.

A number of High River residents were reported to be stranded on rooftops as the Highwood River water levels rose.

According to Wildrose Opposition leader and High River resident Danielle Smith, area residents are saying this is the worst flood they’ve seen in years.

Earlier on Thursday one of two people believed to have been swept into the river was rescued.

Cam Heke, of the STARS air ambulance agency, said the agency received reports that a man and a woman went missing, after their mobile home was swept into the Highwood River near the town of Black Diamond.

Both were later located and the man was rescued, he said. The woman is still missing.

"One female adult had been stranded on a trailer and also a second adult male had been stranded on a nearby flatbed," Heke said.

"We did respond to the area. The female adult was no longer on the trailer and was missing. We did conduct a search along the river and we were unable to locate that missing person.”

Communities face critical alerts

The following Alberta communities were also facing critical alerts:

Turner Valley, Hazardous Materials Alert: Residents are being warned of the release of sour gas from a pipeline containing around one per cent hydrogen sulphide. Later Thursday afternoon officials said the well feeding the line had been shut off, but crews are still working to seal off the leak. Residents are being asked to remain in their homes.

Rock View County, High Water Alert: A mandatory evacuation for the Bragg Creek area has been issued. Residents are asked to go to the emergency evacuation centre at Springbank Park for All Seasons or the Banded Peak School.

Lethbridge County, Flash Flood Alert: An evacuation order has been issued for residents living in the Oldman River valley. An evacuee reception centre has been set up at the County Administration building.

High River, High Water Alert and Flash Flood Alert: Residents are directed to go to the emergency evacuation centres at Highwood High School and the Tom Horneker Recreation Centre in Nanton.

Black Diamond, Overland Flooding Alert: Due to localized flooding, residents are being evacuated to the reception centre at Oilfields Arena. Residents are asked to conserve water.

Municipal district of Foothills No. 31, Flash Flood Alert: A mandatory evacuation has been issued for Foothills residents who live close to rivers, creeks and streams in the region. Residents are advised to seek shelter, as there is a risk of being swept away in the current.

Residents can check the Alberta Emergency Alert website for updates.

State of emergency in Canmore

In Canmore, about an hour’s drive west of Calgary, emergency evacuations centres have been established for residents at the local civic centre and Canmore Collegiate High School.

The power is out in various locations across the town and authorities say the rain has caused Cougar Creek to overflow, closing traffic on Highway 1A and the Trans-Canada Highway.

People living along the creek have been evacuated and other residents are being notified to prepare for possible evacuation, CTV Calgary reported Thursday afternoon.

Sewage has been reported to be leaking into a number of basements in the southern part of the town. Authorities warn that the sewage is toxic and should not be touched.

Town spokesperson Sally Caudill said a number of motorists had to be rescued by helicopter overnight due to the floodwaters.

"We had about 20 or so people on the highway ... who got stuck ... (when) water covered the highway in two places, so we used a helicopter to get those folks out,” she said.

Authorities are also concerned that a pedestrian bridge over the creek is at risk of falling.

The town’s website advises all residents to stay away from the creek, as its banks are unstable.

A mudslide has reportedly shut down all traffic on Highway 1 about 40 kilometres east of the town. Throughout the day, the RCMP has been turning all traffic in the area around.

Two other mudslides have also cut off traffic in other locations: one on Highway 40 and another near Banff.

All public schools in Canmore and neighbouring Banff have been closed, authorities said.

With files from CTV Calgary and The Canadian Press