The June 2013 Alberta Catastrophic Flooding Event: Part 1—Climatological aspects and hydrometeorological features

Abstract

In June 2013, excessive rainfall associated with an intense weather system triggered severe flooding in southern Alberta, which became the costliest natural disaster in Canadian history. This article provides an overview of the climatological aspects and large-scale hydrometeorological features associated with the flooding event based upon information from a variety of sources, including satellite data, upper air soundings, surface observations and operational model analyses. The results show that multiple factors combined to create this unusually severe event. The event was characterized by a slow-moving upper level low pressure system west of Alberta, blocked by an upper level ridge, while an associated well-organized surface low pressure system kept southern Alberta, especially the eastern slopes of the Rocky Mountains, in continuous precipitation for up to two days. Results from air parcel trajectory analysis show that a significant amount of the moisture originated from the central Great Plains, transported into Alberta by a southeasterly low level jet. The event was first dominated by significant thunderstorm activity, and then evolved into continuous precipitation supported by the synoptic-scale low pressure system. Both the thunderstorm activity and upslope winds associated with the low pressure system produced large rainfall amounts. A comparison with previous similar events occurring in the same region suggests that the synoptic-scale features associated with the 2013 rainfall event were not particularly intense; however its storm environment was the most convectively unstable. The system also exhibited a relatively high freezing level, which resulted in rain, rather than snow, mainly falling over the still snow-covered mountainous areas. Melting associated with this rain-on-snow scenario likely contributed to downstream flooding. Furthermore, above-normal snowfall in the preceding spring helped to maintain snow in the high-elevation areas, which facilitated the rain-on-snow event.

A Numerical Study of the June 2013 Flood-Producing Extreme Rainstorm over Southern Alberta

“…An upper ridge is observed to the east of the trough extending from central Alaska toward the U.S. Great Plains. The locations of the upper trough and ridge are consistent with the evolution of the system discussed in Liu et al (2016). The simulation also captures observed strong jets upstream and downstream of the trough.…”

“…It was found that a train of Rossby waves across the North Pacific Ocean helped to form a blocking pattern in the northeastern North Pacific. A detailed analysis of the evolution of weather patterns during the heavy precipitation period can be found in Liu et al (2016). It was shown that a slow-moving upper low with an associated surface low pressure system moved into southern Alberta, which, combined with terrain effects, brought continuous precipitation during the period 19-21 June 2013, especially over the foothills and mountains.…”

“…2a,e) show that the main feature in the J13 storm was the enhanced quasi-linear precipitation structures that were roughly aligned parallel, but with a slight angle, to the foothills. In addition, the strong lowlevel easterly flow that developed during 19 June (Liu et al 2016) facilitated enhanced moisture transport toward the foothills. These results suggest that the orographic lifting of the strong easterly flow could have played a critical role in producing the precipitation.…”

“…Regional water budget assessments and air parcel back-trajectory analyses are carried out to examine the sources and transport of the remote moisture for the J13 storm. This article is a follow-up to two companion papers that discuss the synoptic and observational aspects of the J13 storm (Liu et al 2016;Kochtubajda et al 2016).…”

“…significant flooding events that occurred in 1965, 1973, and 1982 in Alberta (Liu et al 2016), the storm environment was much warmer and very convectively unstable, with CAPE values substantially higher.…”