Calgary weather: Thunderstorms possible Friday with intense rain on Sunday

Cloudy and bit cooler, heading into rain for the weekend. Jodi Hughes has a look at the five-day forecast from the Sky Watch Weather Centre.

Another major rain event will impact Alberta and Saskatchewan this weekend.

An intense low-pressure system from the Pacific basin will cut through southern B.C. before swinging back into Saskatchewan and into Alberta.

The first point of entry for Alberta will be around the capital region and Environment and Climate Change Canada (ECCC) issued a special weather statement to explain the potential concerns.

That area is already dealing with a very saturated surface from last weekend’s rain event and a wetter-than-average June, so another prolonged rain event will lead to issues like water pooling, washouts, overland flooding and runoff since the ground will not be able to absorb moisture at the same rate it is falling.

The national weather agency is expecting between 40 to 70 millimetres of rain from Friday through Sunday along the Yellowhead Highway, with another system likely to impact central Alberta on Sunday.

“Rainfall warnings will be issued for some areas of central Alberta later today.”

Any time a moisture-laden low-pressure system sets up along the eastern side of the Rockies, it can be prone to producing significant precipitation accumulations due to the counterclockwise rotation as the low encounters the topographical barrier of the mountains.

In this case, the risk is enhanced because this low will be positioned south of a high-pressure system and in between two larger highs.

The opposing clockwise rotations will serve as a blocking pattern and essentially hold that low in place, so the collective assembly will funnel moisture in from the western high, where it will get sucked into the low over Alberta and then resist tracking east from the eastern and northern highs.

Eventually, the high to the north will squeeze that low, and central and southern Alberta will sit under a period of heavy and persistent rain.

Some models suggest areas west of Calgary will record over 60 millimetres of rain and Calgary between 35 to 45 millimetres on Sunday alone, while other models have those totals closer to 40 millimetres west of Calgary and around 25 millimetres for Calgary by Monday morning.

Until forecasters can see where the low ends up in Alberta, there will continue to be differing forecasts.

The one certainty is that portions of the province will see significant accumulations and that the bulk of rain for southern Alberta will likely be on Sunday.

The temperatures will continue to fall over the weekend, and wind will increase in strength as instability heightens.

There is a chance of some thunderstorms forming along the foothills Friday afternoon and evening as well as in southeastern Alberta.

These could intensify rapidly if they do form.

The rain is expected to continue until at least Wednesday, with temperatures becoming more consistent until the following weekend.