the june 2013 alberta catastrophic flooding event: … · r. goodson,1 y. li6 and j. pomeroy6 1...

HYDROLOGICAL PROCESSESHydrol. Process. 30, 4899–4916 (2016)Published online 13 October 2016 in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/hyp.10906

The June 2013 Alberta Catastrophic Flooding Event: Part1—Climatological aspects and hydrometeorological features

A. Q. Liu,1* C. Mooney,1 K. Szeto,2 J. M. Thériault,3 B. Kochtubajda,1 R. E. Stewart,4 S. Boodoo,5

R. Goodson,1 Y. Li6 and J. Pomeroy61 Environment and Climate Change Canada, Edmonton, Alberta, Canada2 Environment and Climate Change Canada, Toronto, Ontario, Canada

3 Université du Québec à Montréal, Montreal, Québec, Canada4 University of Manitoba, Winnipeg, Manitoba, Canada

5 Environment and Climate Change Canada, King City, Ontario, Canada6 University of Saskatchewan, Saskatoon, Saskatchewan, Canada

*CEdE-m

Co

Abstract:

In June 2013, excessive rainfall associated with an intense weather system triggered severe flooding in southern Alberta, whichbecame the costliest natural disaster in Canadian history. This article provides an overview of the climatological aspects andlarge-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 multiplefactors combined to create this unusually severe event. The event was characterized by a slow-moving upper level low pressuresystem west of Alberta, blocked by an upper level ridge, while an associated well-organized surface low pressure system keptsouthern Alberta, especially the eastern slopes of the Rocky Mountains, in continuous precipitation for up to two days. Resultsfrom 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, andthen evolved into continuous precipitation supported by the synoptic-scale low pressure system. Both the thunderstorm activityand upslope winds associated with the low pressure system produced large rainfall amounts. A comparison with previous similarevents occurring in the same region suggests that the synoptic-scale features associated with the 2013 rainfall event were notparticularly intense; however, its storm environment was the most convectively unstable. The system also exhibited a relativelyhigh 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-normalsnowfall in the preceding spring helped to maintain snow in the high-elevation areas, which facilitated the rain-on-snow event.Copyright © 2016 John Wiley & Sons, Ltd.

KEY WORDS Alberta flooding; climate; hydrometeorological features; synoptic conditions; moisture transport; rain-on-snow

Received 25 September 2015; Accepted 29 April 2016

INTRODUCTION

Heavy precipitation in Alberta associated with vigorousweather systems can trigger floods and result in severesocio-economic damage. For example, a catastrophic rainevent in June 2002 brought record-breaking rainfall andmajor flooding to many locations in southern Alberta(Szeto et al., 2011). In June 2005, a series of four stormswith similar tracks caused extensive flooding overnumerous municipalities in southern Alberta (Ou, 2008;Shook, 2015). The floods claimed four lives, and

orrespondence to: A. Q. Liu, Environment and Climate Change Canada,monton, Alberta, Canada.ail: [email protected]

pyright © 2016 John Wiley & Sons, Ltd.

insurance losses because of property and infrastructuredamage were estimated at $400 million.A historic, widespread flooding event occurred in the

Rocky Mountains, foothills and downstream areas ofsouthern Alberta in June 2013. Dozens of communitieswere flooded, including the downtown region of Calgary,the largest city in Alberta. Over 100000 people wereevacuated. Infrastructure damage was vast and costly.Remarkably, given the significance of this event, onlyfive fatalities were attributed to the flood. This floodingevent was Canada’s costliest natural disaster withfinancial losses and recovery costs estimated at $6 billion(Phillips, 2014; Pomeroy et al., 2015).Past studies of Alberta rainstorms resulting in major

large-scale flooding have illustrated complex interactionsbetween atmosphere, hydrological processes and the

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4900 A. Q. LIU ET AL.

Rocky Mountains (Szeto et al., 2011; Flesch and Reuter,2012; Pennelly et al., 2014). These storms are typicallyassociated with a deep short-wave trough or an upper cut-off low (COL) pressure system and surface low pressuresystem development east of the Rockies (Hage, 1961;Reuter and Nguyen, 1993). These floods were mainlybecause of rainfall in the foothills and front ranges, withprecipitation primarily generated east of the centralranges. Some events may include snowmelt and rain-on-snow influences (Shook, 2015; Pomeroy et al., 2016).Brimelow and Reuter (2005) also found that the transport ofwater vapour to the province occurs inmoist warm conveyorbelts from distant sources. Recent model simulationssuggest that the Rocky Mountains affect precipitationamounts and duration in the mountains and foothills as aresult of orographic lifting (Flesch and Reuter, 2012).The unique setup of mountains and prairies makes

flood processes in Alberta slightly different from thoseaffecting other provinces that have less significant terrainchanges (Buttle et al., 2016). It does share somesimilarities with flood processes in other parts of theworld, such as the west central U.S., which is also on thelee side of the Rockies, and central Europe, downstreamof the Alps. It is noted that a few months after the 2013southern Alberta flood, another severe flood, the greatColorado flood, occurred farther to the south, withdamages exceeding $2 billion (Gochis and Coauthors,2015; Friedrich et al., 2016). Another flood, with at least25 fatalities and more than EUR 12 billion damages, hitcentral Europe in June 2013 (Grams et al., 2014), thesame month as the southern Alberta flood. There aresome comparisons made later in this paper betweenthese events.Milrad et al. (2015) studied the 2013 Alberta flooding

with a focus on antecedent large-scale atmospheric flowpatterns and synoptic-scale dynamic characteristics. Liet al. (2016) employed the Weather Research andForecast (WRF) model to numerically investigate dy-namic features that led to the heavy rainfall triggering theflood in southern Alberta. However, given the enormityof the 2013 event and the need to better understand it toimprove the prediction of such events, the objective ofthis work is to study the hydrometeorological factors thatled to the flooding, as well as to provide a climatologicalview of the storm system.The following specific questions will be addressed: (1)

What were the unique features of the extreme precipitationand the associated storm system that triggered flooding insouthern Alberta? (2) What were the sources of themoisture that supported the extreme precipitation, andhow was this moisture transported into southern Alberta?(3) What were the snow preconditions, and did the rain-on-snow enhance the overall snowmelt process andrunoff?

Copyright © 2016 John Wiley & Sons, Ltd.

STUDY AREA, DATA AND METHODOLOGY

Overview of the study area

Southern Alberta is located at the intersection of theRocky Mountains and the northern Great Plains region ofNorth America (Figure 1). The landscape is diverse,bounded by the humid and sub-humid Rocky Mountainsand their foothills to the west, and a semi-arid prairie eco-zone extending eastward. This area includes the Bow andOldman river sub-basins, which are part of the Saskatch-ewan River Basin. The climate of the region is continentaland characterized by short cool summers and coldwinters. Monthly average temperatures range from�5 °C to �15 °C in January to around 20 °C in July.Precipitation generally varies from about 300mm annu-ally over the southeastern area to over 600mm in thefoothills, and can exceed 1000mm in the mountains. Juneis typically the wettest month. Snowfall accounts for 30–35% of the annual precipitation, whereas in the mountainsabout 60% of the annual precipitation falls as snow(Phillips, 1990). Snow cover in the high mountains lastsuntil July (DeBeer and Pomeroy, 2010), and there areextensive glaciers at the highest elevations in the centralranges (Demuth and Pietroniro, 2003).

Data and methodology

A variety of observational and operational modelproducts are utilized in this study. These include satelliteobservations, upper air soundings, surface weatherstations, synoptic analysis and lightning network infor-mation. It also includes gridded model precipitationproducts and reanalysis data.

Observational and analysis datasets. The operationalweather analyses and forecasts produced by RegionalDeterministic Prediction System (RDPS) (Mailhot et al.,2006) from Environment and Climate Change Canada(ECCC) were used to examine the synoptic conditions ofthe event. As well, a comparison with previous events insouthern Alberta was performed using the JRA-55reanalysis (Kobayashi et al., 2015) and ANUSPLIN 10-km gridded daily precipitation dataset (Hutchinson et al.,2009). Snowpack in the mountains was examined withthe ECCC and National Oceanic and AtmosphericAdministration (NOAA) snow analyses. ECCC snowanalyses are produced by a snow model that combinesprecipitation accumulation from the operational weatherforecast model and snow measurements at varioussynoptic stations (Brasnett, 1999; Brown and Brasnett,2010). NOAA’s Snow Data Assimilation System(SNODAS) utilizes a snow model driven by a numericalweather prediction model, and is adjusted by remotesensing and surface observations to provide snow

Hydrol. Process. 30, 4899–4916 (2016)

Figure 1. Map of study area: terrain elevation is shaded; areas in yellow have elevations between 2200m and 3000m, and in red higher than 3000m;available ECCC snow depth observation stations are represented by green dots; two AESRD snow course observation sites are marked with magentasquares; the Marmot Creek research basin is marked with a red diamond; and the locations of Calgary and Banff are marked as black and green diamondsand labelled as ‘YYC’ and ‘BNF’ respectively; the Stony Plain upper air sounding site is marked with a black star and labelled as ‘WSE’; the SouthSaskatchewan River Basin is outlined with a thick blue line in the regional map inset. The red box indicates the region where area-averaged snow water

equivalent was calculated

4901CLIMATOLOGICAL ASPECTS AND HYDROMETEOROLOGICAL FEATURES

estimates (National Operational Hydrologic RemoteSensing Center, 2004).Satellite observations from the NOAA series of Polar-

orbiting Operational Environmental Satellites (POES) andthe NOAA Geostationary Operational EnvironmentSatellites (GOES) were used to subjectively examinesurface snow coverage and the evolution of weathersystems. Moisture transport was inferred from satellite-derived Total Precipitable Water (TPW) products re-trieved from the NOAA Office of Satellite and ProductOperations (OSPO) (ftp://www.ospo.noaa.gov/Products/bTPW/index.html). The TPW is created by employingcombined data from GOES and POES satellites, as wellas GPS data (Kidder and Jones, 2007).

Particle dispersion model. Moisture source attributionwas studied using a Lagrangian particle model in whichthousands of particles (representing air parcels) arereleased into a modelled atmospheric flow and followedeither forward or backward in time. The model can beused to estimate the moisture uptake over source regions.For example, Stohl and James (2004) used FLEXPART, aLagrangian particle model in forward mode, to estimateglobal moisture sources and sinks during one year.Drumond et al. (2008) used the same methodology toidentify moisture sources over central Brazil and the LaPlata Basin.


This study utilizes the method of Sodemann et al.(2008). A target grid over the precipitation area (the targetregion) is specified, from which particles are released inbackward mode at a time when precipitation in the targetregion is most significant. Changes in specific humidity(q) are calculated every 6 h along the trajectory todetermine where along the air parcel path moisture wastaken up or released. Moisture taken up later (closer to theprecipitation area) and intervening precipitation events(drops in q) occurring along the trajectory reduce theweight attached to moisture taken up earlier. It is assumedthat if moisture is taken up in the boundary layer, it can beassociated with evaporation/evapotranspiration from thesurface. If the moisture uptake occurs above the boundarylayer, it cannot necessarily be attributed to the surfacebelow, but may be associated with convection, advectionor other processes occurring in the mid-atmosphere. Themoisture taken up in and outside the boundary layer istracked separately.The MLDP0 (Modèle Lagrangien de Dispersion de

Particules d’ordre zéro) particle dispersion modeldeveloped by the Canadian Meteorological Centre(CMC) (D’Amours et al., 2015) was utilized for thiswork. The driving meteorological data was provided bythe Global Deterministic Prediction System (GDPS)model (Côté et al., 1998a, b). The release (target) grid isset along the Rocky Mountain foothills in southern

Hydrol. Process. 30, 4899–4916 (2016)

ftp://www.ospo.noaa.gov/Products/bTPW/index.html

ftp://www.ospo.noaa.gov/Products/bTPW/index.html


Alberta. Altogether 80 000 particles, evenly distributedhorizontally, were released. In the vertical, particles werereleased in equal numbers between evenly spacedpressure levels so that the particles represent air parcelsof equal mass. The trajectories were calculated backwardin time for 168 h (7 days). This length of time was foundsufficient to attribute almost all the precipitation in thetarget region to either above or below boundary layermoisture uptakes somewhere in the model domain. Aparticular trajectory in which the value of q decreases(that is, the excess moisture falls as precipitation) in thefirst 6-h segment after leaving the target region isconsidered to be a precipitating trajectory, along whichmoisture uptake calculations are carried out (Sodemannet al., 2008).

Figure 2. Map of Alberta showing contours of accumulated precip


A CLIMATOLOGICAL VIEW OF THE 19–21 JUNE2013 PRECIPITATION EVENT

The significance of precipitation

The heavy precipitation event began on 19 June 2013and continued for two days, with many stations insouthern Alberta recording substantial (>100mm)amounts of rain. Furthermore, some snow occurred athigher elevations (Pomeroy et al., 2015). The spatialpattern shown in Figure 2 illustrates several regions ofintense precipitation. Over 100mm of rainfall fell in thearea southeast of Jasper; the area extending south ofSundre towards Pincher Creek received more than200mm and the station at Burns Creek measured over300mm. More detailed analysis of the precipitation

itation (mm) from 06 MDT 19 June to 06 MDT 22 June 2013

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features associated with this event can be found in Part 2of this study (this volume, Kochtubajda et al., 2016).The observed precipitation and its variability (in both

amounts and time) at two long-term stations, Calgary andBanff, are briefly discussed to help put the extreme rainevent in climatological perspective. Banff, located in avalley in the mountains, is about 100km northwest ofCalgary, and 500m higher in elevation. The precipitationanalyses, based upon monthly precipitation from 1960 to2013, indicate that in Calgary the month with the mostprecipitation is generally June, but this is not as evident inBanff. The maximum amount of precipitation recordedduring the last 50 years in both Banff and Calgaryoccurred in June 2005 when more than 200mm wasreported. It is typical that, on average, Banff receives lessprecipitation than Calgary. For example, the climatologyfor the month of June yields 79.8mm for Calgary and61.7mm for Banff. Hence this event produced more thanthree times the average June precipitation at Banff andclose to the June average at Calgary. In addition, similarto the majority of previous observed extreme rainstormsin the region (Szeto et al., 2016), the event occurredwithin the month typically receiving the highest precip-itation amounts in Calgary and one of the highest inBanff.

A comparison with similar systems

An overview of long-lived extreme rain events oversouthern Alberta since 1960 is presented in this section,

Table I. Comparison of storm characteristics between the 2013 rai

Parameter

Mean JRA Precip (mm d�1) over P-area during storm periodMean ANUSPLIN Precip (mm/d) over P-area during storm periodFreezing Level of Mean T (hPa) over P-area (First Day of Storm)Mean T (K) at 925 hPa over P-area (First Day of Storm)Mean precipitable water (kg m�2) over P-area (First Day of Storm)Mean soil wetness over southern Prairies during 15Days precedingMinimum sea level pressure (hPa) over 225E–264E, 30N–55N durLongitude (deg E) of Minimum SLP (First Day of Storm)Latitude (deg N) of Minimum SLP (First Day of Storm)Difference in Mean SLP (hPa) Between (245E–260E,63N) and (24(First Day of Storm)Latitude of 500 hPa COL centre (deg N)Longitude of 500 hPa COL centre (deg E)Mean 500 hPa Centre Height of COL (m) (First Day of Storm)Maximum 500 hPa wind speed 240–270E, 30–55NMean Zonal Water Vapour Flux (kg m�1s�1) over 252.5E, 50N–55(First Day of Storm)Mean Meridional Water Vapour Flux (kg m�1s�1) over 255E–262.(First Day of Storm)Ratio of Mean Convective P to Mean Total P over P-areaMean Lifted Index between 925 and 500 hPa (K) over P-area (FirstMean Convective Available Potential Energy (J kg�1) over P-area(First Day of Storm)


while systematic analyses and a more detailed descriptionof methodology can be found in Szeto et al. (2016, thisvolume). The extreme rain events were identified usingthe ANUSPLIN 10-km gridded daily precipitation dataset(Hutchinson et al., 2009) with the condition that the dailyrainfall averaged over southern Alberta and southwesternSaskatchewan (49–53°N, 245–252.5°E; referred to be asthe P-area in Table I) is higher than 12mmd�1 for at leasttwo days. Only 23 such events, including the 2013 storm,were identified for the period 1960–2013. Various stormcharacteristics were diagnosed using the 55-yr Japaneseglobal ReAnalysis (JRA-55, Kobayashi et al., 2015). Asummary of the inter-comparison of the 2013 system withthe ensemble average and extremes of all cases is given inTable I.The mean daily rainfall during the 2013 event assessed

using ANUSPLIN is 13.6mm, which is close to theensemble minimum. On the other hand, the JRA value(22.1mm) is amongst the highest of all, suggesting thatthe gauge observations used in ANUSPLIN wereprobably not able to capture the localized extremeconvective rainfall of the 2013 storm, and under-estimated the rain rate of this event. The area with meanANUSPLIN rainfall>10mmd�1 during the two dayperiod is 233×103 km2, which is also somewhat lowerthan the 302×103 km2 ensemble average.Szeto et al. (2016) found that all the identified

rainstorms were associated with a slow-moving upperlevel COL that typically tracked near the region close to

nstorm and 22 similar events occurring between 1960 and 2013

2013 Max Min Average

22.1 22.6 8.5 15.613.6 27.0 12.2 17.4600 800 600 677.2291.1 292 278 285.423.6 24.3 12.8 17.9

storm 413.7 424.5 289.4 352.7ing storm period 996.7 1005.5 978.2 995.9

248.8 256.25 248.8 253.246.3 52.5 37.5 45

5E–260E, 45N) 18.6 30.1 �1.5 14.2

45 61 43 48.1233 247 226 237.75546 5722 5361 5543.525.6 38.5 19.3 28.1

N �177 154.85 �200.9 �91.6

5E, 45N 205.6 375.4 122.2 219.2

0.71 0.71 0.04 0.3Day of Storm) �5.6 4.8 �5.7 �0.4

714.9 714.9 �18.67 107.3

Hydrol. Process. 30, 4899–4916 (2016)


northwestern U.S. and southern BC during their incipientstage. East of the COL, an accompanying surface lowpressure system was often found over the northern centralU.S. This is also the synoptic situation characterizing the2013 case, except that the surface low pressure centre waslocated somewhat west of the ensemble-average lowcentre position. However, it was still situated east of theupper COL, which maintained a westward-tilted verticalstructure to facilitate baroclinic growth. The strength ofthe surface frontal zone (as measured by the horizontalgradients of low level temperature or equivalent potentialtemperature) was typically weaker than other cases. Inaddition, both the minimum 500 hPa geopotential heightat the centre of the 2013 COL and the lowest surfacecentral pressure were higher than the ensemble averagevalues.Also similar to many other cases, a broad high pressure

area over central and northern Canada was found in theJune 2013 storm. This blocked the northward propagationof the surface system and enhanced the low level north–south oriented pressure gradients and, hence, theassociated synoptic low level easterly flow towards thefoothills. However, the overall low level cyclonic flowwas relatively weak. In particular, the maximum merid-ional wind speeds at 850 hPa and associated northwardmoisture transport over the central Great Plains weresomewhat lower than the ensemble average.The storm characteristics that separated the 2013 event

from the majority of other cases are related to thermalconditions and convective stability. In particular, thestorm environment was much warmer than average andwas very convectively unstable (i.e. exhibiting the largestconvective available potential energy (CAPE) and lowestLifted Index (LI) of all cases shown in Table I). In fact, itsCAPE value was substantially higher than those estimatedfor the three other strong convective events that occurredin 1965, 1973 and 1982, and all other cases occurred inmore stable and cooler conditions (Szeto et al., 2016). Asa result, the June 2013 case had amongst the highestvalues for both the precipitable water and the ratio ofconvective to total rainfall. It also has the highest freezinglevel, which suggests that the precipitation over high-elevation areas was more likely to fall as rain versussnow.In summary, the synoptic scale features associated with

the 2013 rainfall event were not particularly intense whencompared to many other cases, but its storm environmentwas the most convectively unstable. Unlike most previousevents, excessive rainfall from both convective andsynoptic system, contributed significantly to theflooding. The following sections will discuss thecharacteristics for the June 2013 case itself, includingthe evolution of the synoptic patterns, moisture transport,precipitation phase and pre-existing snow conditions over


the mountains. Focused discussions on the fine-scaleconvective precipitation features are presented in Part 2of this study.

EVOLUTION OF THE WEATHER SYSTEM

The evolution of the antecedent large-scale atmosphericflow pattern about two weeks prior to the onset of theheavy precipitation has been studied by Milrad et al.(2015). They found that a train of Rossby waves acrossthe North Pacific Ocean contributed to the developmentof a cut-off anticyclone over Alaska, an upper trough offthe west Pacific coast, and an associated blocking patternin the northeastern North Pacific.During the event, a well-organized surface low

pressure system moved into southern Alberta and keptthe western part of the region in continuous precipitationfor close to two days (00 UTC 20 to 00 UTC 22 June),especially over the eastern slopes of the Rockies. Previousto the heavy precipitation dominated by the synoptic-scale low pressure system, there was also intenseconvective rainfall associated with thunderstorms, whichoccurred during the initial stages of the event, roughlyfrom 00 UTC to 06 UTC 20 June. The analyses in thissection will focus on the evolution of the synopticweather system and convective features during the heavyprecipitation period.At 00 UTC 20 June (Figure 3a) the 250 hPa analysis

chart shows a deep negatively tilted trough along thePacific coast, stretching northwestward from northernCalifornia to southern Alaska with a southeast-northwestoriented upper ridge extending from the central GreatPlains towards central Alaska. Between the trough andridge, a strong southeasterly jet streak was present withmaximum wind speeds close to 130kmh�1, as measuredby the Kelowna, BC (British Columbia) upper airsounding, and a second northwesterly jet streak upstreamof the trough (Figure 3a) was also present. The same deeptrough, with a closed circulation at its base, can also beseen in the 500 hPa chart (Figure 3b). This uppercirculation enhanced the advection of Pacific Ocean airinto the northwestern U.S., and southern BC and Alberta.Southern Alberta was in the right entrance region of aminor jet core at 250 hPa, as well as under strong diffluentflows aloft (Figures 3a and 3b), and these factors tend topromote lift and are favourable for severe thunderstormdevelopment.At 850 hPa (Figure 3c) the air was warm and relatively

moist over the northern Great Plains, which wasadvected northwestward into southern Alberta by a70kmh�1 jet, which acted to destabilize the loweratmosphere and provided low level moisture to supportthunderstorm development. Associated with the upperfeatures was a surface low pressure system centred in

Hydrol. Process. 30, 4899–4916 (2016)

Figure 3. CMC 250 hPa, 500 hPa, 850 hPa and surface analysis charts at 0000 UTC 20 June 2013. (a) 250 hPa chart: geopotential height (solid lines), jetstreaks (shaded), wind barbs and sounding data; (b) 500 hPa chart: geopotential height (solid lines), vorticity (dashed lines), wind barbs and soundingdata; (c) 850 hPa chart: geopotential heights (solid lines), isotherms (dashed lines), wind barbs and sounding data; (d) surface chart: mean sea level

pressure (solid lines) and surface station observations


eastern Montana, with a weak warm front extendingsoutheastward into the central U.S, a cold frontsouthward into central-western U.S., and a troughextending northeastward into southern Alberta and BC(Figure 3d). The convergence associated with the surfacetrough, combined with terrain-induced vertical motionbecause of easterly upslope winds against the mountains,likely acted as the trigger for thunderstorm developmentover southern Alberta.Isolated thunderstorms formed along the foothills of

southern Alberta and Montana on the afternoon of 19June, as shown in the satellite image in Figure 4a.Intensive convective anvil-shaped clouds became wide-spread overnight associated with severe thunderstormsover all of southern Alberta (Figure 4b). More detailed


analysis of the heavy rainfall associated with thesethunderstorms is given in the Part 2 of this study.The 250 hPa analysis at 12 UTC 20 June (Figure 5a)

shows the upper ridge still in place over the southernPrairies, so that only the southern portion of the uppertrough continued to dig eastward, resulting in the jetstream ahead of the trough turning more zonally along theU.S–Canada border. A blocking pattern can be seen at500 hPa (Figure 5b), with a closed circulation around theupper low over northwestern Montana and a highpressure centre over extreme northeastern BC. This typeof large-scale circulation pattern may have slowed theeastward propagation of the system and maintained theupslope flow on the lee side of the mountains. The jetsand diffluent flows ahead of the low in southern Alberta

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Figure 4. (a) POES Satellite NOAA-19 image at 20:28 UTC 19 June2013. The yellow areas depict low clouds, white regions over westernMontana and southern Alberta showing Cb tops from developingthunderstorms; (b) POES infrared satellite NOAA-16 image at 03:55UTC, 20 June 2013, with cloud temperature �40 °C to �70 °C colourshaded (warmest colours map to coldest temperatures) to illustrate detailed

thunderstorm structures


propagated further eastward into southern Saskatchewanand Manitoba, weakening the upper level support forsevere thunderstorm development over southern Alberta.At 850 hPa (Figure 5c) the southeasterly low level jet

also weakened, and the airflow from the central GreatPlains was now advected through southern Saskatchewanand Manitoba before reaching southern Alberta via aneasterly low level jet, which allowed warm moist air tocontinue to be advected against the Rockies. At thesurface, a low pressure system formed over southernAlberta (Figure 5d). Although the low pressure systemwas not particularly intense (central pressure of 1000hPaat 12 UTC 20 June), the pressure difference between thelow and the surface ridge to the northeast was enough tomaintain a strong easterly low level flow. The orographiclift associated with these upslope winds, combined withsynoptic ascent and abundant moisture, contributed to the


significant precipitation. The earlier dominant convectiveprecipitation had, by this time, evolved into widespreadsynoptic stratiform rainfall. Because of the blockingpattern, the low pressure system was quasi-stationary andthe associated rainfall persisted over the next 36 h,gradually weakening with time.As the southwesterly jet streak continued to push the

upper trough northeastward, the ridge over the centralPrairies collapsed, resulting in the end of the blockingpattern by 12 UTC 21 June. The low started to movenortheastward, and the mid-level moisture transportshifted further eastward. As the surface low continuedto propagate away from the lee side of the mountain andgradually fill, both moisture advection and upslope flowdiminished, which led to the end of the precipitation by00 UTC 22 June.The evolution of the low level atmospheric thermo-

dynamic environment over southern Alberta was char-acterized by peaks in both area-averaged surfacetemperature and dew points during the extremeprecipitation (Figure 6a). The atmospheric moisturecontent, as shown in the precipitable water field, alsopeaked on 00 UTC 20 July when the convective rainfallstarted (Figure 6b). The hourly lightning data oversouthern Alberta indicates diurnal convective activitywith vigorous thunderstorms occurring on the 19th and20th (Figure 6c), when surface temperature and moisture,and atmospheric precipitable water over southern Albertawere high.

LARGE-SCALE ATMOSPHERIC MOISTURETRANSPORT

Excessive rainfall requires not only a dynamic weathersystem, but also abundant moisture. In this sectionsatellite observations and model analyses will be usedto examine the large-scale atmospheric moisture transportfor this event.The grey-shades on the 6.7-μm water vapour image at

03 UTC 20 June (Figure 7a) indicate a generally east–west oriented band of mid-level moisture in the southernPrairies being directed against the Rocky Mountains.Well-developed convective clouds over southern Albertaand western Montana are also evidenced by the purpleand green shades. Figure 7b illustrates a band of highTPW with values of >30mm reaching southern Albertaand Saskatchewan. As shown below, this moisture wasadvected directly from the central Great Plains. This bandwas not shown in the water vapor image, which is onlysensitive to mid-to-upper level atmospheric moisture,suggesting that the majority of the moisture was locatedin the lower atmosphere. The moisture axis was also co-located with the low level jet as shown in Figure 3c,

Hydrol. Process. 30, 4899–4916 (2016)

Figure 5. Same as in Figure 3, but at 12 UTC 20 June 2013


which provides more evidence of strong low leveltransport of moisture into southern Alberta.Vertically integrated moisture fluxes from the RDPS

simulation were also employed to study the source andtransport of the moisture (Figure 8). A band of intensemoisture flux from the central Great Plains stretchinginto the southern Canadian Prairies first appeared in the00 UTC 20 June analysis (Figures 8a and 8b), whichcoincided with the convective precipitation that oc-curred from 00 UTC to 06 UTC 20 June. This bandbecame more organized and intense by 12 UTC 20June, when convective precipitation evolved intostratiform synoptic rainfall, and the surface low formedover southern Alberta (Figures 5d and 8c). Significantmoisture flux convergence was evident over southernAlberta at both times. It was apparent that the GreatPlains low level jet had enhanced the transport overcentral and northern Great Plains between 00 and 12


UTC 20 June. Twelve hours later, the moisture fluxweakened and the major moisture path shifted farthereastward (Figure 8d). The model moisture flux analysessuggest that the central Great Plains provided a directmoisture feed for this event.To further investigate the source of the atmospheric

moisture, the trajectory method described in the sectionon Data and methodology was employed with 80 000particles released over southern Alberta at 06 UTC 20June, and the MLDP0 model run 168 h backward intime. This time was chosen because the most intenseprecipitation during any 6-h time interval of the eventoccurred during this period. Figure 9 shows thelocation of particles 168 h before they reached theprecipitation area. High level particles were primarilyadvected in from the Pacific, but those in the low levelstypically originated from the central Great Plains andthe Canadian Prairies.

Hydrol. Process. 30, 4899–4916 (2016)

Figure 6. Time series of (a) RDPS analysed surface temperatures and dew points; (b) vertically integrated TPW, and TPW with precipitation from RDPSanalysis; and (c) observed lightning activity over southern Alberta, averaged over a latitude/longitude rectangle (49.0 to 51.5 N, 110.0 to 114.0 W) for the

period 17–22 June


Figure 10a depicts the regions where the uptake ofmoisture occurred in the boundary layer. The significantarea is over the central Great Plains from the northern U.S.into the southern Canadian Prairies. It is more complicatedto attribute the moisture uptake from above the boundarylayer to specific surface locations, because this moisturecould be derived from many sources, such as convectivevertical transport or advection from other regions. Figure 10b shows that the maxima also occurred along the north–southGreat Plains belt, butwas less dominant than that fromthe boundary layer sources. There was also a wide band ofmoisture directed towards the target area from the PacificOcean. The above analysis suggests that evapotranspirationfrom the central Great Plains played an important role inproviding moisture to support the excessive precipitationthat triggered southern Alberta flooding. While notinvestigated here, the trajectory and satellite moisturepatterns suggest that the Great Plains may have initiallyreceived some of its moisture from the Gulf of Mexico.


SNOWMELT CONTRIBUTIONS

Snow preconditions

The ECCC seasonal precipitation analysis shows thatthe preceding winter had below-normal precipitation overthe mountainous areas of southern Alberta, and the springwas slightly above-normal. Temperatures during springsnowmelt season were near-normal over southwestAlberta, yet May to mid-June was colder than normal,which kept the snow on the ground to above normalconditions. MODIS satellite imagery (Figures 11a and11b) indicates the presence of snow (red shades) in themountainous areas of southern Alberta before and afterthe flooding event. The CMC snow mass analysis (Brownand Brasnett, 2010) also indicates snow over much of thesame area (Figures 11c and 11d). Snow-covered areaswere also confirmed by NOAA’s SNODAS high-resolution snow water equivalent analysis (Figures 11eand 11f).

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Figure 7. (a) GOES water vapour image valid at 03 UTC, 20 June (°C); (b) blended TPW product valid at 04 UTC 20 June


ECCC surface meteorological stations located withinthe boxed area of Figure 1 observed no snow on theground during this period. However most of thesestations are located in valleys below 2000m whereasthe satellite-observed snow is mainly over locationswith elevations above 2200m (detailed topographicalheights are shown in Figure 1). Snow coursemeasurements from two high-elevation AESRD(Alberta Environment and Sustainable ResourceDevelopment) stations, Little Elbow Summit and ThreeIsle Lake (each near 2200m), reported snow waterequivalent values of 428mm and 483mm on 1 June,and 12mm and 62mm on 1 July respectively, alsocorroborating the satellite observations and snowanalysis.


Rain-on-snow scheme

The closest rawinsonde station to the areas affected bythe most intense precipitation is Stony Plain, which isabout 300 km to the northeast (Figure 1). While this is toofar away to draw any detailed conclusions, some generalcomments may be made. The 00 UTC 20 June sounding(not shown) indicates that the atmosphere was close tosaturation above the boundary layer to just below600 hPa, with the freezing level at approximately3300m above sea level (ASL), a relatively high value.Stony Plain was under the northern edge of the weathersystem at this time, so these values could beunderestimated for the study area. An aircraft soundingfrom Calgary near 00 UTC 20 June (not shown) showsthe freezing level was about 3850m ASL (550m higher

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Figure 8. RDPS analysed vertically integratedmoisture flux (kgm s�1) at a) 00UTC19 June; b) 00UTC20 June; c) 12UTC20 June and d) 00UTC21 June


than at Stony Plain), indicating that Calgary was locatedin deep warm air. This sounding also captures thenortheasterly low level jet near the surface, which impliesupslope flow into the Rocky Mountain foothills. Thesounding analysis suggests that the precipitation morelikely fell in the form of rain rather than snow even athigher elevations, which were still snow-covered.To examine the impact of rain-on-snow on snowmelt

and its contribution to runoff, overall snow mass changewithin the boxed areas in Figure 1 was quantitativelyexamined by using snow water equivalent data from theSNODAS analysis. Figure 12a reveals a continuousseasonal snow mass decrease as a result of radiation-induced snowmelt, and does not indicate any unusualsnow mass decrease during the flooding period. However,surface observations from Marmot Creek research basinshow that some precipitation did fall as snow during theevent (Pomeroy et al., 2015). New snow being added intothe snow mass balance calculation could result in anunderestimated snowmelt rate; therefore, biases couldexist in deriving snowmelt from snow mass balances.It is difficult to accurately understand the impact of the

rain-on-snow merely based on a simple mass balancecalculation over a relatively large spatial and temporal


period. This difficulty is highlighted by examiningmeasurements of snow depth and coverage, which areavailable from a high-elevation station, Fisera Ridge,located in the Marmot Creek research basin. Figure 12bgives a record of temperature and snow depth at thisstation during the period 13–26 June. There was, asexpected, a general decrease in snow depth because of theseasonal warming; however, this was interrupted with anincrease of snow depth on 21 June after the major rainfallwhen surface temperature dropped to near 0 °C. Snowdepth data were missing from 19 to 20 June, but itappears there was a significant snow depth decreaseduring this period.A comparison of snow coverage and snowmelt rates for

two nearby stations (ridge-top and south-facing slope) onFisera Ridge during this event show significant differ-ences, which suggests that the rain-on-snow schemecould be more complicated in mountainous areas as aresult of various thermodynamic factors. The rain-on-snow dynamics, examined by Pomeroy et al. (2016),indicate that the melt rates during the rain-on-snow periodwere lower than during high insolation periods, and thatwidespread snowfall at the end of the event provided alarge snow cover that was subject to rapid melting.

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Figure 9. Footprint of air parcels at t0 �168 h, where t0 = 06 UTC 20 June 2013. Colours indicate height (m)


DISCUSSION

It has been shown that a unique feature of the heavyprecipitation event that triggered severe flooding insouthern Alberta was that both convective thunderstormsand the parent synoptic low pressure system significantlycontributed to the heavy rainfall. Detailed precipitationanalyses in Part 2 of this study show that prolongedstratiform rainfall made a major contribution to mostnorthern precipitation areas, such as at Burns Creek,where rainfall amounts exceeded 300mm, whereasconvective rainfall dominated at other locations farthersouth. Although convective rainfall occurred for only afew hours, the precipitation rates were as high as50mmh�1. As addressed in the section on A Climato-logical View of the 19–21 June 2013 Precipitation Event,the synoptic features in this event were not particularlyintense compared to previous similar systems, but had themost convectively unstable environment.The early-event convection analysis (Figure 6c)

suggests diurnal convective activity with vigorousthunderstorms occurring on the 19 June, the day beforethe major event. However, the spatial distribution oflightning places these storms mainly in southeast Alberta,far away from the mountains and foothills. As well,rainfall amounts on that day were not significant. The lackof moisture support, as illustrated in Figure 8a, could be


one major reason for less rainfall being associated withthe thunderstorms on 19 June.The development of a strong convective environment

was assisted in this case by a sustained and uninterruptedtransport of warm moist air. Time series analyses of area-averaged surface temperature and dew points oversouthern Alberta show that on average southern Albertabecame warmer and moister over time before the onset ofthe precipitation (Figures 6a and 6b).A conceptual model as described below was used to

help understand large-scale hydrometeorological processassociated with this event. Before the event, a lee lowdirectly produced extreme pr the east side of theRockies in response to an upper trough along the Pacificcoast. The quasi-stationary low, alongside a high pressureridge that extended into central northern U.S., facilitatedthe sustained transport of warm and moist air from thecentral Great Plains into southern Alberta. The low level850 hPa southeasterly jet acted in concert with thecyclonic flow of the low to enhance the transport of heatand moisture, which was maximized during the eveningand night hours. The strong and sustained upper levelheating associated with thunderstorms reduced the lowlevel pressure over southern Alberta and induced a newlow pressure centre, which together with the diminishinglow level jet, weakened the cyclonic southerly flow, andhence cut off the fuel for convection. This initiated the

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Figure 10. Moisture sources for precipitation occurring between 06 and 12 UTC 20 June 2013 over the target area: (a) where the source was in theatmospheric boundary layer; and b) where the source was above the boundary layer. Units: mm 6 h�1


decay of the system and the precipitation became mainlystratiform thereafter.A similar evolution has occurred in other events.

Gochis and Coauthors (2015) found quasi-stationaryweather systems because of a blocking pattern helped toset up large-scale flow patterns, which triggered severalmesoscale circulation features in the great Colorado flood.Grams et al. (2014) found that a quasi-stationary COLinitiated multiple consecutive low pressure systemstravelling along an abnormal track, resulting in a fewdays of heavy precipitation, which triggered the 2013central Europe flood. It is common to all three events thatthe quasi-stationary synoptic system maintained the large-scale circulation during the event, which provided therequired heat and moisture fluxes, although the individualweather system that directly produced extreme precipita-


tion could be a single quasi-stationary low pressuresystem, such as in the Alberta flood, or a few consecutivetravelling cyclones, such as in the great Colorado andcentral European floods.Moisture analysis using satellite products, model

simulated fluxes and trajectory modelling confirmed thatstrong evapotranspiration from the central Great Plainsprovided the moisture that was advected into southernAlberta. Milrad et al. (2015) also found that the airparcels involved in the precipitation mainly originatedfrom the Great Plains. It is also noted that the moistureband corresponds well to the location of the Great Plainslow level jet. In the conceptual model of Brimelow andReuter (2005), this pattern often transports moisture forsignificant rainfall events into the lower Mackenzie RiverBasin, which is farther north than the area of interest in

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Figure 11. Snow cover (red colour) derived from MODIS Terra satellite data at (a) 1911 UTC 16 June 2013, and (b) 1917 UTC 23 June 2013; snowmass (kg m�2) from CMC snow analyses at (c) 06 UTC 16 June 2013 and (d) 06 UTC 23 June 2013; snow water equivalent (kg m�2) from SNODAS

snow reanalysis at (e) 06 UTC 16 June 2013, and (f) 06 UTC 23 June 2013


this study. Examining station and gridded precipitationobservations for the warm season months over anddownstream of the Great Plains, DeAngelis et al. (2010)found enhanced precipitation over and downstream ofirrigation areas, which supports the moisture transportanalysis for this event.Strong interaction and feedbacks exist between the

Great Plains and the atmosphere above. The moisturefrom the Gulf of Mexico might fall as precipitation as itmoves inland and subsequent evaporation can bring themoisture back into the atmosphere. Dirmeyer et al. (2009)found that the Great Plains has significant moisture


feedback between atmosphere and land surface through-out most of the year. This event occurred during mid-Junewhen evapotranspiration from crops is high, so it is notsurprising that the central Great Plains region would be animportant moisture source.It is also worth noting that the temporal and spatial

patterns of evapotranspiration in the Great Plains couldvary under future climate change. Previous observationaland modelling studies have demonstrated significantevapotranspiration responses to climate changes (Calancaet al., 2006; Cong et al., 2008), and impact to the overallhydrologic cycle (Jung et al., 2010). In general, summer

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Figure 12. (a) Daily 06 UTC SNODAS snowwater equivalent (kg m�2), averaged over the outlined rectangular area as shown in Figure 1, with the red barscorresponding to the period of interest, 16–23 June 2013; (b) snow depth and temperature measurements at Fisera Ridge in the Marmot Creek research site


temperatures in the Great Plains could increase as a resultof global warming, which may lead to an increase inevapotranspiration, and enhanced moisture fluxes andassociated precipitation. Also, changes in land use in theGreat Plains could affect evapotranspiration and thereforeimpact downstream precipitation and flood potentials.Whitfield and Pomeroy (2016) highlighted the importanceof considering land use changes and climate changes forflood risk assessment by investigating hydrological regimechanges in a mountain headwater basin in southern Alberta.Gochis and Coauthors (2015) identified that the long-

range transport of moisture from the Gulf of Mexico wasone of the primary sources for the great Colorado flood,and Grams et al. (2014) argued that continentalevaporation provided significant moisture support forthe central European flood in June 2013. Among thesethree extreme flooding events in 2013, evapotranspirationover land played an important role in the Alberta andcentral European floods in June. Therefore evaporationshould be carefully considered for assessing rainfallintensity and flood risks in summer.Surface and satellite observations, as well as snow

reanalysis products, show there was snow over high


elevations before the flooding event. High freezing levelsenabled a rain-on-snow event, which likely providedadditional runoff and contributed to downstream runoff.However the evidence provided here is not definitive, andmeasurements from Marmot Creek research basin suggestthat the snowmelt evolution can be very different overshort distances. Previous studies highlight the sensitivityof snowmelt processes during rain-on-snow events (Singhet al., 1997; Marks et al., 1998). Garvelmann et al. (2014)observed significant spatial variability of energy balanceterms during different snowmelt conditions in a mountainenvironment. More detailed studies (Pomeroy et al.,2016) employing both hydrological modelling andobservations from Marmot Creek research basin suggestthe snowmelt process during this event was different fromclassic rain-on-snow floods where the energy fromlongwave and shortwave radiation, downward sensibleand latent heat and from the rainfall increases thesnowmelt rate dramatically. In this event snowmelt insome areas might have slowed down because of reducedsolar radiation reaching the snow. The analyses in thisstudy suggest that rain-on-snow did occur and overall itcontributed to the flooding, although the use of enhanced

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observational networks in mountainous areas and high-resolution modelling would improve the understanding ofthe rain-on-snow process.

CONCLUSIONS

Through analysis of data from a variety of sources,including surface hydrometeorological observations, up-per air soundings, satellite data, lightning data andoperational model analyses, this study has revealedimportant hydrometeorological features associated withthis event which, in combination, contributed to theunprecedented flooding:

• The preceding spring had above-normal precipitationand normal to below-normal temperatures, whichhelped to maintain snow over higher elevation areasprime the region for a potential rain-on-snow event.

• A slow-moving upper COL was blocked by a highpressure ridge. The circulation around an associatedsurface lee low kept southern Alberta in continuousprecipitation for up to two days, especially over thefoothills and the eastern slopes of the Rockies.

• The majority of the moisture affecting southern Albertaoriginated with evapotranspiration from the centralGreat Plains and was advected into the region by asoutheasterly low level jet.

• The precipitation event was first dominated byconvective thunderstorms, which evolved into steadyrain associated with the synoptic low pressure system.Strong thunderstorms and persistent upslope windsassociated with the synoptic low pressure systemproduced intense convective and orographic precipita-tion in southern Alberta.

• The relatively high freezing level of the system resultedin rain rather than snow mainly falling over snow-covered high elevation areas. Melting associated withthe rain-on-snow likely helped to increase alreadysignificant surface runoff.

• A comparison with similar weather systems previouslyover the region suggests that, although the synopticscale features and overall precipitation associated withthe 2013 rainfall event were not particularly intense,its storm environment was the most convectivelyunstable.

It is evident from this study that multiple hydromete-orological factors combined to create the severe floodingin southern Alberta. It is also noted that there aresimilarities, but also significant differences, between thisevent and two extreme flood events that occurred inColorado and central Europe (also in 2013) with similartopographic setups. Different combinations of key factors,


such as large-scale synoptic systems, moisture sourcesand transport, snowmelt regime, or evapotranspiration,can all result in devastating floods. Further studiesincluding enhanced observation networks, especially inmountainous areas, and coupled atmospheric-hydrological simulations would improve the understand-ing and future forecasting of such extreme events.

ACKNOWLEDGEMENTS

This research is supported by Environment and ClimateChange Canada (ECCC) and the Changing Cold RegionsNetwork (CCRN) which is sponsored by the NaturalSciences and Engineering Research Council of Canada(NSERC). The authors thank the anonymous reviewersand Dr. William Quinton for their helpful comments andsuggestions. The authors would like to thank RachelMintz and Alain Pietroniro of ECCC for reviewing themanuscript. ECCC provided synoptic analysis charts,precipitation and surface snow observations. The NationalOceanic and Atmospheric Administration (NOAA)provided satellite observation and snow reanalysis data.The authors would like to acknowledge Colleen Walfordfrom Alberta Environment and Parks for providing theAlberta snow survey data.

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