Alberta Environment's Evan Friesenhan stressed Tuesday 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 opened its Emergency Operations Centre and began sandbagging using intelligence it gained last year.
Late Tuesday, Mayor Ted Clugston tweeted that "with a heavy heart, we have declared a state of emergency tonight. I want everyone to know that the city is prepared."
The city also posted a map highlighting potential evacuation areas, including residences along the river and a 16 by five block region in the city's southeast, and started setting up an evacuation centre.
"If you live in one of these areas, please be aware that you may be required to evacuate your home in the next 48 hours, possibly on short notice," warned the notification.
"If evacuation becomes required, someone will come to your door to advise you."
People were also advised to consider moving their belongings to floors above ground level and make arrangements for the care of their pets.
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. Similar states of emergency were issued in the Municipal District of Willow Creek and Crowsnest Pass.
The county says Oldman River valley residents should prepared for an evacuation order sometime on Wednesday, depending on 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.
The city of Lethbridge, which is also on the Oldman river, said on its website that it has not declared a state of emergency, but was monitoring the situation.
Friesenhan said most of Lethbridge is on high ground and, while parkland may flood, homes should be OK.
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."
— With files from CJOC
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