After days of unusually warm temperatures a weather pattern change will bring a significant amount of moisture into central and southern Alberta. As of 9 a.m. Friday, both yellow and orange-level heat warnings remained over western Canada – extending from Alberta’s eastern border to southwestern Ontario. This is part of a weather pattern known as an Omega Block, named for the shape that is formed when two low pressure systems (with their counterclockwise rotation) sit southeast and southwest of a strong ridge of high pressure (which contains a clockwise rotation). The opposing rotations inhibit the movement of the ridge, so warm air persistently funnels in under the high, creating a “heat dome” and often results in multiple days of extreme temperatures. It is common to see unsettled weather where the lows sit – especially in this particular case on the western low in southern Alberta. Pacific moisture gets pulled into the counterclockwise rotation around the low-pressure system and often that low will stall out against the physical barrier of the Rocky Mountains. This can result in a substantial amount of moisture falling over a small area for a long period of time due to the overall blocking pattern of the systems to the east. These synoptic scenarios produce an upsloping component and make it very difficult for forecasters to pinpoint the highest precipitation totals ahead of the actual event. It is reasonable to expect 80 to over 100 millimetres of rain in the foothills – which will add strain to the waterways there (including the Bow and Elbow Rivers upstream of Calgary) due to accelerated snow melt. Calgary will likely see at least 50 millimetres of rain over a three to four-day period, but those totals could be higher if the low sets up further east. It is important to note - this rain event is different than the 2013 flooding event. The snowpack in the Rockies in 2013 was closer to normal compared to a much higher snowpack in 2025/26 (100 to 200 per centy higher than normal in some areas this season). However, the snow this spring has been gradually melting during intermittent periods of above-average temperatures. In 2013, there was a delayed melt prior to a major rain event from June 19 to 21 that brought between 200 to 300 millimetres of rain in some areas of the foothills. The rain event this weekend is not expected to produce as much moisture, and it will occur over a slightly longer period of time; from Saturday until Tuesday. That said, there will be some localized flooding as the ground becomes saturated and is unable to absorb that much moisture over such a short period of time. Waterways will continue to be dangerous due to increasing volumes and high streamflow rates. Stream banks should be avoided as they will become compromised, and some streams will likely see their banks breached. While we have better forecasting models in 2026 compared to 2013 and more flood mitigation options in place, there is still a legitimate cause for concern and there are still actions people can take to minimize some of the impacts. Homeowners might want to ensure gutters are cleared and that any areas that typically accumulate water around properties are assessed and tended to. For folks planning to travel, highway conditions should be evaluated as there will be water pooling in low-lying areas or at the base of hills, and slides are possible in terrain that has been previously compromised by fire and/or prior flooding. Temperatures in Alberta and B.C. will be much cooler this weekend. In Calgary the highs will range from 16 C to 9 C from Saturday to Tuesday.
