Thursday will be mostly calm and warm across the Calgary area, with a minimal chance of light rain in the afternoon as clouds build and winds pick up throughout the day. The forecast high in Calgary is 25 C for Thursday, and by the afternoon, wind will flow in from the south and southeast at 20 to 40 km/h. There is a chance of some scattered and weak thunderstorms along the foothills and in central and southern Alberta Thursday. Stronger thunderstorms are possible in the north and north-central regions later in the day, where a less stable air mass is expected to collide with a hot and stable air mass that has prompted yellow-level heat warnings in northern Alberta and the Northwest Territories. Starting Friday, an incoming and strong low-pressure system from the north Pacific basin will become the main weather-maker, bringing a significant amount of moisture across Alberta and Saskatchewan. A number of anchoring high-pressure systems (to the north, east and west) will hold that low in place once it crosses the Rockies. Persistent rain is expected over a period of at least three days, with some locations in Alberta likely to record between 30 and more than 50 millimetres in total. While many areas will see heavy rain this weekend, a special weather statement has been issued by Environment and Climate Change Canada (ECCC) for the capital region. That area is already experiencing issues with a ground that remains saturated from last weekend’s rain as well as the higher-than-average amount of rain from the month of June. Anytime a location receives rainfall totals of 30 to 50 millimetres over a short period of time, the ground is going to become too water-laden to absorb moisture at the same rate it falls. In this case, “additional rain will increase the risk of overland flooding, washouts and potential impacts to infrastructure.” The national weather agency explains the challenge with this type of synoptic set-up where a low-pressure system (with its counterclockwise rotation) ends up on the leeward side of a mountain range and becomes “stuck” within a blocking pattern. “There remains low certainty regarding the exact location of the rainfall across the region. The area of heaviest rain is forecast to fall in a narrow band, which could lead to more localized impacts than we observed last weekend. The area will become clearer as the system approaches later this week.” The region most likely to see heavy rain initially will be along the Yellowhead highway and in the north-central portion of Alberta. The western edge of that low is expected to cut down along the foothills and eventually track through the Calgary area. Again, it is difficult to pinpoint the most vulnerable areas until the low crosses the Rockies, but it seems likely Calgary will see the heaviest rainfall on Sunday (around 20 to 25 millimetres) with another 10 to 15 millimetres on both Saturday and Monday. The overall forecast will be unsettled after Friday, with strong wind, cooler temperatures and rain that could last until Wednesday. It’s possible the rain will start in Calgary on Friday, but it is also possible Calgary might not see any rain until later Saturday. It is too far out to predict with certainty, but at this point, it looks like the Canada Day forecast will be OK, with a high of 20 C, a mix of sun and cloud and a 60 per cent chance of light rain. Daytime highs should return to seasonal by Thursday and be nice for the Calgary Stampede parade, with a high of 24 C and a mix of sun and cloud.
