A severe thunderstorm is heading north from Montana into the southeast corner of the province, warned Environment Canada early Tuesday morning.
In the southwest corner of the province, from Okotoks to Waterton Lakes National Park, the agency is predicting 50 to 70 millimetres of rain by the evening.
A weather watch for the communities of Lethbridge and Taber has also been updated to a "warning." A storm is expected to produce large hail, strong wind gusts and heavy rain.
In Calgary, residents near the Bow and Elbow rivers are being told to prepare for possible basement floods, as water levels are expected to rise this week.
The city opened its Water Emergency Operations Centre on Monday because of flood concerns.