Intense rain prompts rainfall warnings, flood watches, and high streamflow advisories

Rex Block dumping more rain on Calgary. Jodi Hughes has a look at the five-day forecast from the Sky Watch Weather Centre.

According to Environment and Climate Change Canada (ECCC) between 30 to 40 millimetres of rain fell over the Calgary area as of 6 a.m. Monday with at least another 10 millimetres expected throughout the day.

In the Banff/Canmore area, ECCC reported between 35 to 40 millimetres of rain as of 6 a.m. Monday, with another 10 to 40 millimetres possible throughout the day.

There will be localized areas with significantly more rain due to the nature of the incoming system coupled with the synoptic setup.

The main weather-maker for the region is a low-pressure system situated over the southern portions of Alberta and Saskatchewan.

This low is struggling to move due to the blocking pattern in the upper levels – including strong high-pressure systems east and west, with rainfall totals being enhanced by another high-pressure system situated north of the low.

When this type of synoptic setup – known as a Rex Block – develops in areas on the leeward side of a mountain range, you will often see prolonged periods of precipitation with significant accumulation totals.

This is due to the combination of an ample source of moisture being fed into the opposing flows - the counterclockwise rotation around the low and the clockwise rotation around the high to the north – in conjunction with the topographical barrier of the mountains, and the overall blocking pattern.

A yellow-level rainfall warning has been issued for southern Alberta with the national weather agency explaining rainfall totals in the warned region will range from 50 to 100 millimetres.

One of the primary concerns around this much rain, is the inability for the ground to absorb moisture at the same rate it is falling – a problem common during these types of rain events.

ECCC cautions, “Water will likely pool on roads and in low-lying areas. and slides may occur in vulnerable areas such as steep slopes, deforested areas or recent burn scars. Fast-moving and rapidly rising water can sweep vehicles away and damage infrastructure.”

There have also been advisories issued for several rivers and streams in Alberta due to rapidly increasing volumes and flow rate.

By 6 a.m. Monday, there were still widespread high streamflow advisories across the province – including for the City of Calgary.

There were also some flood watches north and west of Calgary.

Just after 8 a.m. Monday, the flood watch for the Elbow River west of Calgary was upgraded to a flood warning, with river forecasters explaining, “Water levels have risen by around 2.0 m and an additional rise of up to 0.5 m is possible. Homes upstream of Bragg Creek may have their access roads/driveways cut off. Based on provincial flood mapping, there may be impacts at Glencoe Golf Course.”

Rain is expected to continue off and on across southern Alberta for the next five days, including possibly on Canada Day, and parade day for the Calgary Stampede.

Temperatures will remain close to seasonal values, but there will be frequent periods of instability. Late Tuesday and on Wednesday there is a chance of moderate to severe thunderstorm development along the QEII north of Calgary.

Wind speeds will be intense for most of the day Monday.

By 9 a.m., wind gusts in Calgary were measuring around 50 km/h from the north.