Wet, icy surfaces to start the week with high close to 20 C by Thursday

Upward swing in temperatures for Calgary. Jodi Hughes has a look at the five-day forecast from the Sky Watch Weather Centre.

Rapid melting is expected at the beginning of this week as another weather pattern change occurs.

A dominant ridge of high pressure will set up over the western portion of the country, funnelling warm, moist, Pacific air into British Columbia in a phenomenon known as an “atmospheric river.”

A number of weather warnings have been issued by Environment and Climate Change Canada (ECCC) due to the potential hazards.

Rainfall warnings and winter storm warnings have been issued along the west coast, with ECCC explaining, “An atmospheric river will remain over the region until Tuesday bringing heavy snow and rain. Snowfall accumulations of 10 to 15 centimetres have fallen since Sunday afternoon. The heavy snow has changed to rain as temperatures have gradually risen above the freezing mark. Heavy rain is expected for the rest of day with up to 70 millimetres of rain expected by Tuesday morning. This may result in localised flooding as the rain collects on snow-covered surfaces before the snow completely melts.”

Snowfall warnings have been issued for portions of central and eastern B.C. with the national weather agency noting, “A frontal system combined with cool air is bringing significant snow. The region received 10 to 15 centimetres of snowfall last night, with another five to 10 centimetres expected this morning. The snow will turn to rain by late this morning or early this afternoon as temperatures warm. Roads and walkways will likely be difficult to navigate due to accumulating snow. Visibility will likely be reduced at times.”

Special weather statements have been issued north of Golden and Revelstoke to the south and south of Valemont in southeastern B.C. due to heavy rain (20 to 40 milimetres) forecast to fall after Sunday’s snow.

The greatest risk is rapid mountain snowmelt. “Additional rainfall is also possible on Tuesday night into Wednesday. A mix of mountain snowmelt and rainfall could lead to localized flooding in valleys and low lying areas.”

Some of that moisture will fall over the Rockies and into Alberta – including on Monday and Tuesday. Rain is possible throughout central and southern Alberta Monday – including in Calgary, and freezing rain is possible in east-central Alberta overnight.

Surfaces could become icy overnight with rain falling onto frozen areas and strong wind contributing to a faster freeze rate.

Yellow-level wind warnings have been issued for southern Alberta with west wind gusts of up to 100 km/h expected to start Monday night.

ECCC advised, “Strong winds that may cause damage are expected. Winds will gradually weaken Tuesday afternoon. Driving conditions will likely be difficult. High-sided vehicles will be pushed around by the wind.”

Calgary is expecting west winds of 20 to 40 km/h on Monday.

Both the daytime highs and overnight lows will sit well above seasonal this week, amplifying melting. Daytime highs will range from 10 C on Monday to 18 C on Thursday, with lows between 4 C and 6 C. The normal range this time of year is 4 C for a high and -8 C for a low.

Water is expected to pool on frozen surfaces as ground is unable to absorb at the same rate it melts.

Water will also pool at the base of hills, in parking lots, shady locations, under downspouts and in storm drains if they are clogged with debris.