There are concerns Medicine Hat could see flooding similar to what happened a year ago as heavy rains continue to soak areas to the west of the southern Alberta city.
Alberta Environment's Evan Friesenhan stresses that prediction is a worst-case scenario and will only happen if the Oldman and Bow rivers to the west peak at the same time where they meet the South Saskatchewan River.
And he added that scenario is still three days away, so a lot can change.
The city says it has opened its Emergency Operations Centre and has begun sandbagging using intelligence it gained last year.
About 1,000 Medicine Hat homes were hit by high water in the flood of 2013 that ravaged southern Alberta and 10,000 people were forced to head for higher ground.
Forecasters are calling for as much as 200 millimetres of rain in the southwest corner of the province by Thursday morning.
To the west of Medicine Hat, officials in Lethbridge County declared a local state of emergency Tuesday citing extremely high water levels on the Oldman River.
One of several communities under flood threat in southern Alberta has already started evacuating residents.
The town of Claresholm says overland flooding has reached some homes and affected people are being moved to the local arena.
Spokeswoman Karine Wilhauk says a downpour of early morning rain has flooded streets on the west side of the town and some sewers are backing up.
All schools have been closed but students writing diploma exams can take buses from the evacuation centre to their schools.
The Blood Reserve, Cardston and Lethbridge County are also preparing residents for possible evacuations due to rising river levels.
There's potential to reach levels seen in 1995, when heavy rains pushed rivers over their banks from Pincher Creek to Medicine Hat and more than 3,000 people had to flee their homes, the county says.
Farmers are being advised to move their livestock and people are being told to stay away from riverbanks.
In total, 100,000 people had to flee their flooded homes in southern Alberta in June 2013. Communities hit the hardest — including Canmore, Calgary and High River — are further to the north of area currently experiencing high water and are expected to be fine.
In 2013, more than 300 millimetres of rain fell in some places over a 2 1/2-day period.
"We're looking at a peak flow of 5,300 cubic metres per second through Medicine Hat on the South Saskatchewan River, which is very similar to last year," Friesenhan said.
"Obviously that's the worst-case scenario and assumes that the flow coming out of the Oldman and the flow coming out of the Bow will merge together at the same time."
States of emergency are also in effect in the Municipal District of Willow Creek, Crowsnest Pass and Medicine Hat.