Friday, June 21, 2013

Devastating Floods in Southern Alberta, Calgary

Downtown Calgary Friday
Credit: Justin Kripps - Twitter
Heavy rain falling just east of the Canadian Rockies in southwestern Alberta resulted in catastrophic flooding on Thursday and Friday across much of southern Alberta, including the city of Calgary.

More than a dozen towns in southern Alberta declared states of emergency after swollen rivers crested their banks Thursday, forcing mandatory evacuations for entire communities.  An estimated 100,000 people were evacuated from downtown Calgary.  There have been two fatalities attributed to the flooding.

The Province of Alberta. Calgary is red dot.
The flooding in southern Alberta resulted from a combination of an unusual upper level weather pattern and geography. The Bow River flows from near Lake Louise, through Banff and Calgary, eventually flowing into the South Saskatchewan River, which flows northeast to Hudson Bay. There are a number of rivers in the Bow River drainage area, which lies just east of the Rockies. The eastern part of the basin is 6,000 to 9,000 feet above sea level. Any precipitation that falls in the basin ends up in the rivers and eventually the Bow River. That's the geography part.

The Bow River basin in southern Alberta

Remember the unusually strong, nearly stationary upper level high pressure system that brought record hot weather to Alaska? Well, that high plays a part in the Calgary flooding. This high acted as a block to weather systems moving in from the Pacific. A trough of low pressure moving from west to east from the Pacific was forced south of the high.

The 700 mb map (about 10,000 feet) at 6:00 p.m. MDT June 20.

The combination of clockwise winds around the high and counter clockwise winds around the low streamed moisture-laden air from the central U.S. into southern Alberta. The formation of the rain was aided by the air being forced to rise as it encountered the Canadian Rockies.
This map shows the winds at 850 millibars (~5,000 feet)
and precipitable water at 6:00 p.m. MDT June 20.
The light green shading indicates PW values from 1.5 to 2.0 inches.
The dark green arrows show the general airflow.

Radar image for 10:30 p.m. MDT June 19. Note the more intense cells south and west of Calgary

Over half the Bow River basin received more than 50 mm (~ 2.0 inches) of rain, with many areas in the foothills seeing over 100 mm (~4.0 inches).

Radar estimated precipitation for period ending at 9:00 a.m. MDT June 20.

Here’s a graph of the rainfall from the rain gauge at Three Sisters Dam, 10 km south of Canmore, showing about 100 mm (~4.0 inches) of rain in less than 9 hours, and 245 mm (9.65 inches) for the event.This rain gauge is located within the area of maximum precipitation on the radar map.

This rain event was actually well forecast by the Canadian forecast model up to a week before.  Translating the precipitation forecast into specific river stage forecasts in advance is complicated, even more so over complex terrain.

Precipitation forecast by Canadian model valid for 6:00 p.m/ MDT June 20.
Credit; Environment Canada and The Weather Network
For photos and videos of the flooding, follow the links below.

Images shared on social media - The Globe and Mail

Photo gallery of Calgary streets before and during the flood - Global News

Flooding coverage by The Weather Network

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