ORI GIN AL PA PER

Characterization and assessment of the devastatingnatural hazards across the Canadian Prairie Provincesfrom 2009 to 2011

Julian Brimelow • Ronald Stewart • John Hanesiak • Bohdan Kochtubajda •

Kit Szeto • Barrie Bonsal

Received: 5 August 2013 / Accepted: 20 February 2014� Springer Science+Business Media Dordrecht 2014

Abstract From 2009 to 2011, the Canadian Prairies were subjected to exceptionally

variable precipitation regimes, ranging between record drought and unprecedented flood-

ing. Adjacent regions concurrently experienced droughts and floods, and individual areas

transitioned rapidly from pluvial to drought conditions and vice versa. Such events had

major impacts; for example, damages from floods in the Assiniboine River Basin (ARB)

have exceeded $1 billion, and forest fires ravaged the town of Slave Lake, Alberta. This

study first characterizes, and then assesses, these devastating natural hazards in terms of

their physical processes (across multiple spatial and temporal scales) related to both the

spatially contrasting precipitation states and rapid temporal transitions between these

states. Subtle differences in large-scale atmospheric flow had marked impacts on precip-

itation. Primary factors controlling the distribution and amount of precipitation included

the location and persistence of key surface and upper-air features, as well as their inter-

action. Additionally, multiple events—rather than individual extremes—were responsible

for the flooding over the Saskatchewan River Basin and the ARB. Very heavy rainfall

events (C25 mm d-1) accounted for up to 55 % of warm season rain at some locations,

and the frequency of heavy rainfall events was critical for determining whether a region

experienced drought or pluvial conditions. This study has increased our knowledge of the

characteristics, impacts and mechanisms of rapidly transitioning disparate precipitation

J. Brimelow (&) � R. Stewart � J. HanesiakDepartment of Environment and Geography, Centre for Earth Observation Science (CEOS),468 Wallace Building, University of Manitoba, Winnipeg, MB R3T 2N2, Canadae-mail: brimeljc@cc.umanitoba.ca

B. KochtubajdaEnvironment Canada, Edmonton, AB, Canada

K. SzetoEnvironment Canada, Downsview, ON, Canada

B. BonsalEnvironment Canada, Saskatoon, SK, Canada

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Nat HazardsDOI 10.1007/s11069-014-1107-6

states on the Canadian Prairies and will aid in better understanding both past and projected

future hydro-climatic extremes in the region.

Keywords Flooding � Heavy precipitation � Drought � Wildfires �Canadian Prairies � Precipitation variability � Natural hazard

1 Introduction and background

The Canadian Prairie region of Canada (Fig. 1) has one of the most variable climates in

North America. Drought, flooding and forest fires sometimes occur simultaneously across

the region, while extreme dry and wet conditions can be observed within a single growing

season at a given location (Hanesiak et al. 2011; Szeto et al. 2011).

This spatio-temporal variability makes it challenging for governments and communities

to effectively respond and adapt to hazardous events. For example, when multiple hazards

occur close in space and time, the cumulative impacts can negatively affect the capacity for

communities to respond and this in turn increases their vulnerability (Bonsal et al. 2011;

IPCC 2012). Benestad and Haugen (2007) noted that concurrent events (which need not be

extreme) can have negative impacts beyond those expected from the individual events.

Further, Turner et al. (2003) identified the importance of including multiple stressors when

conducting vulnerability analyses. Consequently, it is important to quantify the impacts

asscociated with the dramatic temporal shifts and spatial contrasts in precipitation on the

Canadian Prairies from 2009 to 2011, as well as improve our understanding of the pro-

cesses associated with both wet and dry precipitation extremes.

In the last decade, the Prairies have been subjected to large variations in precipitation

with devastating consequences. For example, damage to the Canadian economy from the

1999–2005 Prairie drought, part of possibly the worst drought over western North America

in 800 years (Schwalm et al. 2012), was estimated at $4.5 billion for 2001 and 2002 alone

(Wheaton et al. 2008). The western Prairies again experienced a short-lived, yet severe

drought in 2009. According to Agriculture and Agri-Food Canada, the spring of 2009 was

the driest in 70 years, and Saskatoon, Saskatchewan experienced its driest spring since

records began in 1892 (Phillips 2010). The Prairie drought was followed by widespread

and historic flooding in 2010 and 2011 (see Fig. 1). Saskatoon experienced its wettest

growing season (April through September) on record in 2010, and the Canadian Wheat

Board estimated that about 5 million ha of farmland went unseeded—the largest area since

1971 (Phillips 2011). The unprecedented and devastating flooding observed over the

Assiniboine River Basin (ARB) was rated as Canada’s number one weather event for 2011

(Phillips 2012). The Canadian Wheat Board estimated that almost 5.5 million hectares of

farmland did not produce crops in 2011 due to flooding (Phillips 2012). Simultaneously,

northern portions of the Prairies experienced extensive forest fires (Fig. 1), including one

that destroyed parts of the town of Slave Lake, Alberta and caused at least $750 million of

insured damage—the number two weather event for 2011 (Phillips 2012).

In 2009, southern Manitoba transitioned from spring flooding to the driest July since

records began in 1887, and some farmers in southern Manitoba claimed flood and drought

insurance in the same growing season. Impacts of the 2011 floods are ongoing, with many

people still displaced. At the time of writing, the cost of the floods is estimated to be

greater than $1 billion (Government of Manitoba 2012). Further, 5 of the 10 largest insured

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weather-related events on the Prairies (1991–2011) occurred between 2009 and 2011 and

accounted for 75 % of the insured losses (Insurance Bureau of Canada 2012).

Research has been undertaken to examine some of these types of events, with the main

focus being on droughts. One of the most comprehensive studies was the Drought Research

Initiative that focused on the main physical factors associated with the severe 1999–2005

Prairie drought (Stewart et al. 2011). A comprehensive review of Canadian Prairie drought

research is provided in Bonsal et al. (2011).

The objective of this paper is to characterize and assess the devastating natural hazards

experienced across the Canadian Prairies from 2009 through 2011. This is realized through

an examination of two issues. Firstly, the physical impacts related to the extreme wet and

dry events from 2009 to 2011 are characterized. Secondly, the atmospheric processes (i.e.

upper-air and surface circulation) related to the temporal shifts and spatial contrasts in

Fig. 1 Canadian PrairieProvinces study area, includingthe Assiniboine River Basin(ARB). Solid-coloured areasrepresent burn scars. Shadedbrown and blue regions wereaffected by drought and floodingfor at least three consecutivemonths between 2009 and 2011,respectively. ‘‘PC’’ is the PeaceCountry region. Insert shows thelocation of the study area. Urbancentres referred to in the text areshown by yellow dots

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precipitation are investigated. The paper is organized as follows. Section 2 outlines the

study area and datasets. In Sect. 3, the spatio-temporal variability of the surface temper-

ature and precipitation is documented, including heavy precipitation events. Lightning and

wildfire characteristics during the study period are reviewed in Sect. 4, whereas Sect. 5

examines atmospheric forcing mechanisms associated with key break points and concur-

rent wet and dry regions. The paper concludes with a discussion and conclusions in Sect. 6.

2 Study area and datasets

The Canadian Prairie Provinces (Fig. 1) encompass an area of approximately 1.9 mil-

lion km2. The region typically receives 350–500 mm of precipitation annually, of which

approximately 65–75 % falls in the warm season (May through September). Higher annual

precipitation is observed near the Rocky Mountains. Mean accumulated snowfall ranges

between 110 and 160 cm, contributing 25–35 % of the annual precipitation (National

Climate Data and Information Archive at http://www.climate.weatheroffice.ec.gc.ca). Land

cover over the Prairie Provinces is diverse. The southern agricultural region comprises

mainly seasonal cropland and is bordered to the north by the boreal plains, which consists

of deciduous and coniferous forest. Over extreme northern Saskatchewan and Manitoba,

the boreal shield is covered by coniferous forest and taiga. To gain better insight into the

processes associated with the spatial variability of precipitation, the study area is divided

into two sub-regions, namely the agricultural zone and the boreal zone (see Fig. 1).

Because this analysis investigates the variability of precipitation on a variety of tem-

poral and spatial scales, gridded climate data sets are used. These include the 50-km

monthly precipitation and temperature data from the Canadian gridded (CANGRD) data

product and the 0.2-gridded 6-hour precipitation data from the Canadian Precipitation

Analysis project (CaPA) (Mahfouf et al. 2007). CANGRD temperature values are derived

from climate station data that have been corrected for homogeneity issues caused by station

relocation, changes to instrumentation and observing times (Vincent 1998; Vincent and

Gullet 1999), and precipitation has been adjusted for gauge under-catch, wetting loss and

trace events (Mekis and Hogg 1999). To better understand the physical processes asso-

ciated with the identified precipitation extremes, key atmospheric variables including

vertically integrated moisture, mean sea-level pressure, vertical and horizontal motion, and

geopotential heights from the NCEP-NCAR reanalysis product (Kalnay et al. 1996) are

utilized.

Lightning data are monthly quality-controlled records of cloud-to-ground strikes (CG)

from the Canadian Lightning Detection Network (CLDN) (Burrows and Kochtubajda

2010). Streamflow values are from archived hydrometric data maintained by the Water

Survey of Canada (http://www.wsc.ec.gc.ca/applications/H2O/index-eng.cfm). Point and

polygon data from the Canadian National Fire Database (Canadian Forest Service 2012)

are used to characterize lightning-caused fire and associated area burned statistics.

3 Precipitation and temperature

3.1 Spatial and temporal characteristics

The high spatio-temporal variability in precipitation during 2009 through 2011 is high-

lighted by the area-averaged monthly CANGRD precipitation departures (relative to the

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1971–2000 averages) in Fig. 2. These data show that over the agricultural zone, 50 % of

the 36 months from 2009 to 2011 received above-average precipitation ([110 % of nor-

mal), and 25 % of the months received below-average (\90 % of normal) precipitation.

Similarly, over the boreal zone, 53 % of the months experienced above-average precipi-

tation, compared to 28 % with below-average values.

Despite the preponderance of months with above-average precipitation, some regions in

both zones experienced very dry monthly conditions (\40 % below average) in each of the

3 years (Fig. 1, also see Figs. 11b, 14a). In 2009, south-western Saskatchewan and central

and southern Alberta experienced very dry conditions from late May through September.

In 2010, the agricultural zone was very dry in February and March, as was the Peace

Country in the late summer. In May and June 2011, central Alberta and west-central

Saskatchewan experienced very dry conditions, as did portions of the agricultural zone

(especially southern Manitoba) between July and September 2011. Over parts of the boreal

zone (Fig. 1), very dry conditions were observed in May and November 2009, February

and June 2010, and May and September 2011.

Closer inspection of the precipitation departures and temperature anomalies reveals four

key time periods (Fig. 2). The first period, from April through September 2009, was

characterized by below-average precipitation (\90 % of average) and temperature (Fig. 2).

In May, June and July 2009, most of the agricultural zone experienced dry conditions (with

central Alberta receiving\40 % of its average precipitation in June). Two exceptions were

the central boreal zone, where precipitation was 110–130 % above average in June (up to

200 % above average over northern Saskatchewan), and far southern Alberta, where heavy

rainfall (up to 125 mm) in early July alleviated drought conditions. Monthly temperature

departures (Fig. 2b) indicate that below-average temperatures (-1 to -2 �C) accompanied

the dry conditions in the spring. The cooler regime continued into August, with large areas

experiencing below-average temperatures (-0.5 to -2 �C) interspersed with near-average

temperatures.

The second period, between October 2009 and March 2010 (Fig. 2), was dominated by

below-average precipitation and above-average temperatures (?2 to ?6 �C). With the

exception of October 2009 and January 2010, precipitation was typically below average

over the agricultural zone, with precipitation for some months being 50–60 % below

average and some locations receiving\40 % of their average. Early in 2010, a shift from

an extremely dry (\40 % of average) March to an extremely wet ([200 % of average)

April marked the onset of a prolonged wet phase.

During the third period, between April 2010 and July 2011, precipitation was above

average ([160 % in some months) and temperatures were typically near to below average

(Fig. 2). The 2010 warm season over the agricultural zone was especially wet, with pre-

cipitation over much of Saskatchewan being 200 % above average. In contrast, the Peace

Country and the boreal zone were dry and warm in June and July, with precipitation at

\40 % of average in places; the dry and warm ([?2 �C) conditions over northern Sas-

katchewan and northern Manitoba were accompanied by forest fires. Above-average pre-

cipitation was observed over most of the Prairie Provinces in August and September 2010.

Although precipitation during the winter of 2010–2011 was characterized by high spatial

variability across the Prairie Provinces, monthly values were average to above average

([150 % in January 2011). During spring, the boreal zone and north-central agricultural

zone were very dry (\40 % of average precipitation in places); elsewhere precipitation was

average to above average, but much-above average over the ARB ([160 %). In the early

summer, most of the agricultural and boreal zones were wet (120–140 % above-average

precipitation), with near-to average temperatures. June and July were especially wet over

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parts of southern Saskatchewan, the southern boreal zone and central Alberta, with pre-

cipitation being 160–200 % above average.

The fourth period marked a shift to progressively drier (55–90 % of average precipi-

tation) and much warmer conditions (up to ?5 �C) from August 2011 onwards (Fig. 2).

Southern Manitoba was especially warm and dry (\40 % of average precipitation) from

July onwards, as were portions of the boreal and western agricultural zones from August

onwards.

3.2 Spatial distribution of heavy precipitation events

The CaPA data were used to identify the spatial distribution and frequency of heavy daily

precipitation events in the Prairie Provinces. Here, heavy and very heavy precipitation days

were defined as days with at least 10 and 25 mm, respectively. Specifically, CaPA grid

points with C10 mm of accumulated precipitation (GE10) in a 24-h period (between 06

and 06 UTC) were identified. Similarly, precipitation events producing C25 mm (GE25) in

B24 h and C50 mm (GE50) in B48 h were identified. Zhang et al. (2011) suggested a

threshold of 10 mm d-1 to identify heavy precipitation days. The threshold of 25 mm d-1

is consistent with that used in other studies (e.g. Higgins et al. 2007, Akinremi et al. 1999).

We considered very heavy precipitation events only during the warm season (1 May

a

b

Fig. 2 Monthly anomalies for a precipitation and b temperature over the agricultural (solid trace) andboreal (dashed trace) zones of the Canadian Prairie Provinces between 2009 and 2011

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through 30 September). The rationale for focusing on heavy precipitation events is because

the preponderance, or scarcity, of heavy precipitation events is important for determining

whether a region is in drought or a pluvial (e.g. Evans et al. 2011). Specifically, Evans et al.

(2011) found that droughts were characterized by a scarcity of heavy rainfall events,

whereas pluvials were characterized by a preponderance of heavy rainfall events.

We identified 63 GE50 events in the 2009–2011 warm seasons, of which 39 were over the

agricultural zone. In 2009, most GE50 events occurred over the boreal zone (Fig. 3), with grid

points over the western boreal receiving up to two events. In 2010, there was a paucity of GE50

events over Saskatchewan, whereas up to three events were identified at grid points over

northern Manitoba. In 2011, up to three events were observed at grid points over the boreal zone

and Peace Country of Alberta. Several areas over southern Saskatchewan and Manitoba were

affected by GE50 events. In contrast, parts of the southern Prairies experienced only one GE50

event.

An analysis of daily rainfall data for five stations (Edmonton, Saskatoon, Medicine Hat,

Winnipeg and Prince Albert) across the agricultural zone from 1981 to 2010 found that the

stations typically experienced a total of ten GE10 events between May and September, two

of which were GE25 events. In 2009, the frequency of GE25 events was highest over

south-eastern Manitoba, northern Saskatchewan and south-central Alberta (Fig. 4a). The

mean frequency over the agricultural zone was 0.95 per grid point (or 2,229 events for all

grids), with similar values (1.0; 4,366) over the boreal zone (Fig. 4a). The mean contri-

bution of GE25 events to the warm season rainfall totals was 24 and 19 % over the

agricultural zone and the boreal zone, respectively (Fig. 5a).

The frequency and spatial extent of GE25 events increased markedly in 2010 (Fig. 4b).

The frequency was greatest over Manitoba, with up to 10 events; central Saskatchewan and

southern Alberta were also active. The mean frequency of GE25 events was 1.6 (4,017 events)

over the agricultural zone and 1.5 (8,238 events) over the boreal zone. The contribution to

warm season rainfall (Fig. 5b) was near 20 % over both the agricultural and boreal zones.

The 2011 warm season was similar to that of 2010 (Fig. 4c). The most active regions

were west-central Alberta and the Peace Country (consistent with the drought being broken

there), along with most of Manitoba and portions of central Saskatchewan. In contrast to

2009 and 2010, very few GE25 events were identified over southern Alberta and south-

eastern Manitoba. The mean frequency was 1.8 for the agricultural zone (4,366 events) and

\0.5 (1,982 events) for the boreal zone. The mean contribution to warm season rain totals

(Fig. 5c) over the agricultural zone was 25 %, compared to 22 % over the boreal zone.

The spatial distribution of the GE10 events during the warm season (not shown) was

very similar to that of the GE25 events, although the frequency of GE10 events was higher.

Specifically, the mean frequency for GE10 (GE25) events was 7.0 (1.5) over the agri-

cultural zone and 7.5 (1.0) over the boreal zone. The mean contribution of GE10 events to

warm season rain totals was also higher, at 51.5 and 46 % over the agricultural and boreal

zones, respectively.

The frequency of GE10 events for the cold seasons (1 October through 30 April) of

2009–2010 and 2010–2011 was much lower than that for the warm seasons (Fig. 6). The

highest occurrence of GE10 events was over southern Manitoba and south-eastern Sas-

katchewan, where up to 11 events were identified for the 2010–2011 cold season, com-

pared to GE10 maxima of up 20 events in the 2010 and 2011 warm seasons. Overall, GE10

events occurred mostly over the eastern Prairies and along the foothills of the Rockies;

very few GE10 events occurred over the western Prairie Provinces for the years studied.

In summary, for the three warm seasons considered here, GE25 events were infrequent

(B5 events per grid point) and limited to the far south-eastern part of the Prairies. Drier

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regions typically received fewer GE25 events than did areas experiencing pluvial condi-

tions. Also, very heavy events accounted for up to 55 % of warm season rain in places.

Typically, the mean contribution of GE25 events over the agricultural zone during the

warm season was between 20 and 25 %.

4 Lightning and fire activity

4.1 Lightning

Cloud-to-ground (CG) lightning flashes are common over the Canadian Prairie Provinces

(Burrows and Kochtubajda 2010). The vast majority of flashes are observed between May

and September, with the greatest frequency occurring in July. The number of lightning

days (d) and the total length of time (h) during which lightning occurred (on a 0.2� 9 0.2�grid) between May and September from 1999 to 2012 are illustrated in Figs. 7a and 8a,

respectively.

The temporal variability in lightning days and in total duration (from May to Sep-

tember) during the 2009–2011 period was atypical when compared to the 1999–2012

Fig. 3 Occasions at each CaPAgrid point when C50 mm (GE50)was calculated in B48 h between1 May and 30 September fora 2009, b 2010 and c 2011.Green indicates one event, bluetwo events and red three events

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climatology. The number of lightning days over the agricultural zone varied between 13

and 20 d between 2009 and 2011, and mean durations were between 3.5 and 6.0 h.

Lightning activity over the boreal zone displayed similar variability, although the number

Fig. 4 Number of occasions at each CaPA grid point when C25 mm (GE25) was calculated in B24 hbetween 1 May and 30 September for a 2009, b 2010 and c 2011

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of lightning days (9.5–12.0 d) and the lightning duration (2–3 h) were fewer and shorter,

reflecting the more northerly location of the boreal zone and shorter warm season (Burrows

and Kochtubajda 2010).

Fig. 5 Contribution of GE25 events to the warm season total precipitation for a 2009, b 2010 and c 2011

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Annual maps depicting regions of above-normal or below-normal lightning days and

duration reveal considerable spatial variability (Figs. 7, 8). The anomaly map for 2009—a

dry period—shows a less active thunderstorm season than average. Large areas experienced

below-normal activity both in the number of lightning days and in the lightning duration over

the agricultural zone (Figs. 7b, 8b). A reduction in lightning occurrence by C4 d and dura-

tions by C4 h below normal was noted over many areas of the agricultural zone. A few areas

in southern Alberta experienced C2 more lightning days and 2 h longer lightning duration

than normal. A large portion of the boreal zone also experienced reduced lightning activity,

except over north-central Saskatchewan where activity was above normal.

The agricultural zone experienced an active lightning season in 2010 (Figs. 7c, 8c);

nearly 60 % of the zone experienced an increase in C4 lightning days above normal.

Lightning duration was more than 6 h above normal over the southernmost region of the

Saskatchewan-Manitoba border and over east-central Alberta. These data suggest longer-

lived and/or more electrically active thunderstorms. Lightning activity over the boreal zone

was mixed. Some areas experienced C2 fewer days than normal and C2 h less lightning

duration (e.g. northern Manitoba), while some regions experienced a more active season

(e.g. north-western Alberta).

Fig. 6 Number of occasions at each CaPA grid point when C10 mm (GE10) was calculated in B24 hbetween 1 October and 30 April a 2009–2010, and b 2010–2011

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The anomaly patterns in 2011—a wet period—show near-normal lightning activity over

the agricultural zone and above-normal activity over the boreal zone (Figs. 7d, 8d).

Anomalies of at least 4 lightning days more than normal are evident over several regions

(e.g. the foothills of Alberta and south-western Saskatchewan). Some areas experienced

Fig. 7 May through September thunderstorm day counts for 1999–2012 a, and thunderstorm day anomaliesfor 2009, 2010 and 2011, respectively b–d. Crosses indicate where large fires ([200 ha) initiated

Fig. 8 May through September thunderstorm duration (h) for 1999–2012 a, and thunderstorm durationanomalies for 2009, 2010 and 2011, respectively b–d

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fewer lightning days than normal, including far southern Manitoba and east-central

Alberta. The lightning duration anomaly map for 2011, however, indicates that the sea-

sonal duration was C2 h lower over several areas in the agricultural zone, and throughout

many areas in the boreal zone. These patterns suggest that thunderstorms in 2011 were

likely shorter lived than average and did not produce much lightning.

Thunderstorms can produce varying amounts of convective precipitation and lightning,

triggering flash floods when intense precipitation falls over short times (Soula et al. 1998),

or igniting wildfires when little or no precipitation falls to the surface (Rorig and Ferguson

1999). A pattern of increasing average lightning activity associated with increasing

amounts of convective precipitation was found across Canada’s ecozones; this is consistent

with the findings of (Kochtubajda et al. 2013).

4.2 Forest fires

Forest fires are a common disturbance on the Canadian Prairies. Based on data from 1981

to 2011, over 2,300 fires occur annually and burn an average of over 1 million ha

(Canadian Forest Service 2012). Various factors can influence areas burned, including:

weather, topography, fuels and composition, and human factors such as land-use man-

agement and fire suppression policies and priorities (Parisien et al. 2006).

The annual variability of lightning-caused fires and associated area burned and the

distribution of large fires (fires larger than 200 ha in area) over this period is described in

Fig. 9. CG lightning flashes trigger over 1,200 forest fires (about 52 % of the total number

of fires) annually on the Prairie Provinces and burn about 0.9 million ha. The total area of

the boreal forest in the Prairies Provinces is about 144 million ha (Canadian Boreal

Initiative 2013). The least active lightning-caused fire season occurred during 2011, with

only 438 lightning-caused fires; 1998 was the most active season, with 2,273 fires. Large

fires in Canada represent only 3 % of the fires, yet account for about 97 % of the area

burned (Stocks et al. 2003). The Prairies typically experience about 90 large lightning-

caused fires annually, which is just over 7 % of the lightning-caused fires, yet accounts for

about 94 % of the area burned. A large fraction of these large fires (47 %) occurs in

northern Saskatchewan (Fig. 9). According to Saskatchewan’s Wildfire Management

Strategy (2013), minimal fire suppression is carried out in this region and fires are gen-

erally allowed to burn naturally, with the exclusion of communities and major infra-

structures such as mines. This, combined with the quick-drying shallow soils

characterizing the region, explains the higher fraction of large fires over northern Sas-

katchewan (Flannigan pers. comm. 2013).

The number of lightning-caused fires and area burned between 2009 and 2011 was highly

variable (Fig. 9). During this time, 2,700 lightning-caused fires burned nearly 2.3 million ha.

The most active season was 2010, when over 1,400 fires burned nearly 1.8 million ha. Large

fires in 2010 accounted for *11.5 % of the fires and over 96 % of the area burned. The

majority of these fires (55 %) occurred in northern Saskatchewan (Fig. 1). The fewest

number of lightning-caused fires occurred in 2011, whereas the smallest area burned was in

2009. The highly variable spatial distribution of burn areas is shown in Fig. 1. In 2009, large

fires were found along a swath extending from southern Manitoba into northern Alberta.

Large burn areas in northern Saskatchewan are evident in 2010, while in 2011 large burn areas

are observed in north-eastern Alberta and north-western Saskatchewan.

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5 Large-scale and event-scale forcing mechanisms

5.1 Temporal shifts and break points

The transition between precipitation regimes in early 2010 can be explained by the large-

scale flow (Fig. 10). In March 2010, the circulation over the Prairies was characterized by

persistent ridging (Fig. 10c), with troughs off the west coast and over eastern Canada.

Height anomalies at 500 mb approached ?100 gpm over southern Manitoba (Fig. 10e),

with the greatest anomaly over western Ontario. Consequently, ascent over the agricultural

zone was muted (not shown). In contrast, enhanced ascent and above-normal precipitation

(Fig. 10a) was present over northern Saskatchewan. At the surface (not shown), a rela-

tively dry downslope flow over the agricultural zone was unfavourable for precipitation.

According to the NCEP-NCAR reanalysis, the height anomaly observed over western

Ontario (50�N; 90�W) in March 2010 was the largest recorded since 1948, with one

exception: only March 2012 had a greater height anomaly at the aforementioned grid point

over western Ontario.

In April 2010, the upper-air long-wave pattern moved eastwards with the upper-air

trough over the west coast of Canada (Fig. 10d). This eastward shift proved critical in the

Prairies transitioning from dry conditions in March to wet conditions in April. Specifically,

the region of positive height anomalies over southern British Columbia was replaced by

negative anomalies, while positive anomalies persisted over north-eastern Saskatchewan

and Manitoba (Fig. 10f). This configuration of the upper-air flow resulted in enhanced

ascent over most of the Prairie Provinces, with the exception of a small region over central

Manitoba. At low levels (not shown), the flow was predominantly easterly over the agri-

cultural zone, resulting in the upslope transport of moisture, which in turn favoured heavy

precipitation. This regime led to much-above-normal precipitation for April over most of

the Prairies (Fig. 10b).

The second marked transition occurred late in the summer of 2011 and was charac-

terized by progressively unfavourable large-scale conditions for precipitation between July

and September. In July, a long-wave ridge (Fig. 11c) dominated over the eastern Prairies

(consistent with the drought over southern Manitoba). A dipole anomaly pattern was

evident at 500 mb, with a -60 gpm anomaly over northern British Columbia and ?50 gpm

over Wisconsin (Fig. 11e). As a result, descent dominated over the south-eastern Prairies

(not shown), while the mean upper trough over British Columbia favoured ascent and

above-average precipitation over the western Prairies. By September, the upper ridge

(Fig. 11d) and associated descent dominated the circulation over most of the Prairie

Provinces (anomalies[60 gpm), and a strong westerly flow developed at the surface. The

positive height anomalies over the Prairies in September 2011 were the fifth highest in the

NCEP-NCAR reanalysis since 1948. This configuration was not favourable for precipi-

tation, and most of the Prairies experienced much-below-normal precipitation in Sep-

tember 2011 (Fig. 11b).

5.2 Concurrent wet and dry regions

The monthly data for the contrasting events discussed below suggest that the atmospheric

flow patterns that resulted in the precipitation gradients tended to be persistent and/or

stagnant. For example, although June 2009 was near the peak of the drought over the

agricultural zone, precipitation anomalies of up to 200 % above normal were observed

over the western boreal zone only 250–300 km away (Fig. 12a). Reasons for the

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Fig. 9 Number of lightning-caused fires (bars) and the area burned (solid line) over the Canadian PrairieProvinces between 1981 and 2011 (left panel). The distribution of the large fires ([200 ha) by province isshown in the right-hand side panel

Fig. 10 Monthly fields for March and April 2010 for a, b precipitation departures, c, d 500-mb heights, ande, f 500-mb height anomalies

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contrasting precipitation over a relatively short distance included a split flow in the 500-mb

heights (Fig. 12b), with a trough over the coast of California and zonal flow over the

Canadian Prairies. Heights were above average over British Columbia and below average

over most of the Prairies (Fig. 12c), especially in the southeast. A weak upper-air trough

over the northern Prairie Provinces is evident in Fig. 12b. Although subtle, this feature

resulted in ascent over northern Saskatchewan and Manitoba (Fig. 12d). In contrast, mean

descent was observed over most of the agricultural zone, especially in the west. At 850 mb,

vector wind anomalies show enhanced westerly to south-westerly flow over most of the

agricultural zone (Fig. 12e), with an axis of convergence extending from north-eastern

Alberta to Manitoba. These factors worked in concert to produce the sharp precipitation

gradient over the western Prairies that persisted through the summer.

In June 2010, while severe flooding was occurring over the central and southern Prairies

(Phillips 2011), forest fires were raging over northern Saskatchewan and a drought was

occurring in the Peace Country of Alberta. Over a distance of only *250 km, monthly

precipitation amounts transitioned from[150 % of average over southern Saskatchewan to

\40 % of average over the boreal zone (Fig. 13a). A trough was present over British

Columbia with ridge downstream (Fig. 13b). Heights were below average over British

Columbia, but were near normal over the Prairies (Fig. 13c). Enhanced ascent was present

downstream of the trough over most of the Prairies, with descent over northern Sas-

katchewan and Manitoba (Fig. 13d). At 850 mb, vector wind anomalies indicated a per-

sistent cyclonic flow over the southern Prairies (Fig. 13e), with northward transport of

moist air from the USA. The combination of enhanced ascent and high low-level moisture

led to above-normal precipitation over most of the agricultural zone (110–180 % of

average). In contrast, mid-tropospheric descent caused very dry conditions (\40 % of

average) over most of the boreal zone.

In May 2011, very dry conditions were observed over most of the boreal zone (as well

as central Alberta and Saskatchewan), with wet conditions over far south-eastern Sas-

katchewan and southern Manitoba (Fig. 14a). A trough was present along the west coast at

500 mb (Fig. 14b), with an omega block over the boreal zone (anomalies[?80 gpm near

Great Slave Lake were the second highest on record for this region in May in the NCEP-

NCAR reanalysis). Consequently, heights were above average over most of the Prairie

Provinces, with below-average heights evident south of the Canada–US border (Fig. 14c).

Strong descent was observed over drought areas (Fig. 14d), with enhanced ascent over

southern Alberta, southern Saskatchewan and Manitoba. A moist easterly flow was evident

over southern Manitoba and south-eastern Saskatchewan. Heavy precipitation (monthly

precipitation *150 mm or [200 % of average) occurred along the southern base of the

omega block over south-eastern Saskatchewan and far south-western Manitoba (Fig. 14a).

5.3 Composite maps for heavy precipitation events

High-impact heavy rainfall events between May and September for 2009, 2010 and 2011

were identified. To qualify, a precipitation event had to be associated with C50 mm of rain

in 48 h in the CaPA data or station data and had to be associated with a damage report or

societal disruption. Events over the agricultural zone were divided into nine events west of

105� W and six events east of 105�W.

Composite maps for the nine high-impact events over the western agricultural zone are

shown in Fig. 15. All but one of the events was associated with a 500-mb closed low over

southern British Columbia or southern Alberta. This is reflected by the closed 5,600 gpm

contour over British Columbia (Fig. 15a). This observation is consistent with previous

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research that has identified upper-air closed lows as being associated with heavy rainfall

events in this region (e.g. Brimelow and Reuter 2005; Reuter and Nguyen 1993).

A broad zone of low pressure at the surface is evident in Fig. 15c, suggesting a spread in

the position of surface lows. The combination of low-level upslope flow around the surface

low (Fig. 15e) and dynamical forcing resulted in strong ascent ([15 Pa s-1) extending

from the Peace Country south-eastwards to south-western Saskatchewan (Fig. 15b).

Another key factor was the advection of warm, moist air (precipitable water contents

approaching 25 mm) within the region experiencing strong ascent (Fig. 15b, d).

Maps for high-impact precipitation events over the eastern region (Fig. 16) show a

sharp trough over Saskatchewan, with a surface low over southern Manitoba (Fig. 16a, c).

Three of the six events were associated with an upper-air closed low. The events were

associated with strong vertical ascent over eastern Saskatchewan and southern Manitoba

(Fig. 16b), with moist air (precipitable water contents[30 mm) drawn northwards into and

around the surface low, resulting in heavy rainfall over southern Saskatchewan and

Manitoba (Fig. 16d, e).

Fig. 11 Monthly fields for July and September 2011 for a, b precipitation departures, c, d 500-mb heights,and e, f 500-mb height anomalies

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6 Discussion and conclusions

This study has characterized the devastating natural hazards that occurred on the Canadian

Prairies from 2009 through 2011 in terms of the physical processes related to spatially

contrasting precipitation states and rapid temporal transitions between precipitation

regimes. Several key observations concerning the spatio-temporal distribution of precipi-

tation, their causes and impacts have been made. Subtle differences in large-scale flow had

marked impact on precipitation, and primary factors controlling the distribution and

amount of precipitation included the location and persistence of key surface and upper-air

features and their interaction.

Fig. 12 Monthly maps for June 2009 for a precipitation departure (per cent of normal), b mean 500-mbheights (gpm), c 500-mb height anomalies (relative to 1981–2010 climatology), d 500-mb vertical motion(Pa s-1) anomalies and e 850-mb vector wind anomalies (m s-1)

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Two break points in precipitation regimes were observed. The first in April 2010

marked the transition to pluvial conditions following a prolonged period of drought. The

second occurred between July and September 2011 when pluvial conditions transitioned to

drought. The former shift was the result of an eastward progression of the Rossby wave

train, whereas the latter was in response to the retrogression of the wave train. Shifts in

Rossby waves may have affected the storm track and the precipitation distribution. For

example, much-below-average cyclonic activity over the southern prairies in March 2010

transitioned to much-above-average activity over the south-central regions in April as the

upper trough shifted eastwards.

Three months during which adjacent regions had contrasting precipitation regimes (June

2009, June 2010 and May 2011) were found to share similar, but sometimes subtly

Fig. 13 Same as in Fig. 12 except for June 2010

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different, large-scale forcing. The sharp gradient in precipitation was controlled primarily

by the position of the Rossby waves and associated surface features. For example, in May

2011, a trough was present along the west coast, with an omega block over the Prairies.

Consequently, dry conditions dominated over most of the Prairies, with wet conditions

over the far southeast. The precipitation distribution was consistent with the storm tracks—

above-average cyclone activity was observed over the south-eastern Prairies downstream

of the upper trough, while below-normal activity dominated elsewhere because the

blocking high forced storm systems to the north.

The Pearson correlation coefficient between GE25 event counts over the agricultural zone

from May to September and the concomitant monthly precipitation departures was 0.70.

Thus, pluvial regions typically received more GE25 events than did areas experiencing

drought conditions. This supports the finding by Evans et al. (2011) that the frequency of very

Fig. 14 Same as in Fig. 12 except for May 2011

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heavy rainfall events is critical for determining whether a region experiences drought or

pluvial conditions. Further, GE25 events are an important component of the hydrological

cycle in this region and can account for up to 55 % of warm season rain at some locations,

although the mean contribution averaged over the agricultural zone is *20 %. Station data

for three Prairie sites in Evans et al. (2011) showed that from circa 1900 to 2004 the con-

tribution of 25 mm events to warm season rainfall was 20–25 %.

All but one of the nine high-impact heavy rainfall events over the western Prairies were

associated with a closed 500-mb low over southern British Columbia or southern Alberta,

with a surface low in the lee of the Rockies. Over the eastern Prairies, high-impact heavy

events were associated with a sharp upper trough over Saskatchewan, with a surface low

over southern Manitoba. Only three of the eastern events were associated with a cold low;

thus, cold lows may not be as critical for producing heavy rainfall events in this region. In

both regions, advection of moist low-level air from the US beneath strong dynamically

Fig. 15 Composite maps for nine high-impact heavy rainfall events (locations indicated by ‘‘X’’) over theagricultural zone west of 105�W. Shown are a 500-mb heights (gpm), b 500-mb vertical motion (Pa s-1;negative values indicate ascent), c mean sea-level pressure (mb), d columnar integrated water vapour(kg m-2) and e 850-mb vector winds (speeds in m s-1)

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forced ascent led to heavy rain. In the west, upslope flow processes forced by the Rockies

likely enhanced the lift and precipitation (Flesch and Reuter 2012). The synoptic-scale

setting for western events is consistent with previous research on heavy events over the

western Prairies (e.g. Chung et al. 1976; Brimelow and Reuter 2005; Szeto et al. 2011).

Over the Canadian Prairies, temperature is modulated primarily by cloud cover and

precipitation in the spring and summer months (Tang and Leng 2013); specifically, tem-

perature is typically negatively correlated with cloud cover and precipitation in this region.

However, between April and September 2009, drought conditions were associated with

below-average temperatures. This particular combination is rare but not unprecedented on

the Prairies—for example, Stewart et al. (2012) identified 15 months from 1900 to 2009

when the temperature and precipitation were both below normal over central Alberta.

The Slave Lake fire in May 2011 and large fires in northern Saskatchewan in June and

July 2010 were preceded by anomalously warm and dry conditions. Although weather is

but one of several factors that influence seasonal fire activity and burn area (e.g. Flannigan

et al. 2005), it does play an important role. To elucidate this, concurrent anomalies of

Fig. 16 Same as for Fig. 15, except for six high-impact heavy rainfall events (locations indicated by opencircles) over the agricultural zone east of 105�W

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monthly temperature, precipitation and thunderstorm day counts at grid points closest to

initiation point for 226 fires observed over the Canadian Prairie Provinces between 2009

and 2011 were identified.

For 58 % of the fire initiation points, the temperature was above- to much-above normal

for that time of the year. Similarly, the majority of fires (55 %) started at locations with

below-average precipitation. In contrast, 32 % of the fires started at grid points with above-

normal precipitation. About 45 % of the fires started at locations that experienced above-

normal temperatures and below-normal precipitation, whereas 20 % of the fires started at

locations that experienced near- to above-normal temperatures and above-normal precip-

itation. A possible explanation for this last finding is that dry conditions likely preceded the

time the fire started, with rain falling after the fire start date. An analysis of thunderstorm

day counts revealed that there were almost equal chances of fire start locations being

associated with either below-, near- or above-normal thunderstorm day anomalies. This is

perhaps expected given the stochastic nature of lightning and the fact that only one CG

strike under ideal conditions is needed to start a fire.

The high spatio-temporal variability of precipitation identified in this study corroborates

similar findings in the literature (e.g. Soule 1993; Potop et al. 2013) and underscores the

challenges of issuing reliable monthly and seasonal forecasts. Importantly, multiple heavy

rainfall events—rather than individual extremes—were responsible for flooding over the

Prairies and the ARB. This is somewhat analogous to the conditions that caused the 1993

floods over the midwestern USA (Kunkel et al. 1993; Guttman et al. 1994; Junker et al.

1999) when extreme flooding was aggravated by multiple mesoscale convective complexes

that produced heavy rain (Kempf and Krider 2003).

This study showed how varying types of natural hazards can occur close together in

space and time as the result of persistent circulation patterns. We, however, examined only

a three-year period and cannot speak to the significance of the observed spatio-temporal

variability in the historical record. Work is underway to quantify spatio-temporal changes

in precipitation variability close together in space and time in the climate record, to

examine the atmospheric and surface processes at play in the development of the historic

flooding over the ARB, and to quantify the contribution of thunderstorm rain to warm

season rainfall. We anticipate that these efforts will present a cohesive understanding of

precipitation-related extremes on the Prairies, in addition to helping develop appropriate

adaptation strategies.

Acknowledgments This research was supported by the Canadian Foundation for Climate and Atmo-spheric Sciences and the Changing Cold Regions Network funded by the Natural Sciences and EngineeringResearch Council of Canada. The authors appreciate the assistance of Lucie Vincent, Eva Mekis and EwaMilewska for providing the CANGRD temperature and precipitation data, respectively; Vincent Fortin andBruce Davison for providing the CaPA data; and John Little of the Canadian Forest Service for providingthe wildfire data and area-burned shapefiles. NCEP reanalysis data were provided by the NOAA/OAR/ESRLPSD, Boulder, Colorado, US, from their website at http://www.esrl.noaa.gov/psd/. The authors would alsolike to thank the two anonymous reviewers for their very helpful reviews.

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