More than a month’s worth of rain expected in portions of southern Alberta this weekend

Rain expected to begin Friday morning. Jodi Hughes has a look at the five-day forecast from the Sky Watch Weather Centre.

Rainfall warnings and special weather statements have been issued for southern Alberta ahead of a major weather event starting Friday.

Rainfall warnings are issued in Alberta when rainfall totals are expected to reach at least 50 millimetres over a period of 24 hours or at least 75 millimetres within 48 hours.

Calgary is likely to see around 50 to 75 millimetres of rain this weekend, which would be close to, or exceed the average monthly precipitation for the city if this were any other month. June is typically Calgary’s wettest however, with an average of nearly 113 millimeres of rain by month’s end.

The next closest month for total precipitation is July, which averages 65.7 millimetres over the 31 days.

There is a high level of certainty about the prolific amount of rain that will fall over central and southern Alberta this weekend, however precipitation totals will be highly variable throughout the region and the most vulnerable locations are difficult to pinpoint due to the synoptic factors creating this scenario.

The main weather maker for the next few days is a low pressure system which is on track to reach the southernmost corner of Alberta and B.C. on Friday.

That Pacific low will be heavily influenced by a more intense low pressure system in Northern Canada that will block normal west to east progression in the upper patterns.

As these two systems edge closer together late on Friday, moisture will be picked up from the Pacific, wrap around the counter-clockwise rotation of the western low, and persistently drive precipitation from east to west into the topographical barrier of the Rocky Mountains (in what is often referred to as upsloping).

Complicating things, cold air will start to flow around the western edge of the northern low before getting caught up in the system impacting southern Alberta.

At the surface, this will translate to a rapid drop in temperatures, strong north winds, and wind gusts up to 60 to 80 km/h on Saturday.

Daily highs in central and southern Alberta will be colder than average until early next week, with this factor amplified in the foothills and Rocky Mountains.

Early Saturday rain is expected to transition to heavy, wet snow on some mountain highways in Alberta and B.C, and potentially lead to winter driving conditions. According to ECCC, “10 to 20 centimetres of snow is possible over the highest elevations, such as Highwood Pass, as snow levels drop to near 1400 metres.”

There are many concerns associated with a setup like this because most surfaces – especially non-vegetative ones - are incapable of absorbing such a large amount of moisture in such a short period of time.

Within Calgary risks from this weekend’s rain include overland flooding, water pooling, clogged storm drains and eavestroughs, and downed trees linked to saturated soil and strong winds.

Additionally, banks near creeks, rivers and streams should be approached with caution as heavy rain can impact stability.

Finally, the timing for this weather event is interesting.

This weekend marks the 12-year anniversary of the 2013 floods, and June 20 is also the first day of astronomical summer.