MAY 2017 H A T C H E T T E T A L . 1359 Avalanche Fatalities during Atmospheric River Events in the Western United States a b c,d a,d BENJAMIN J. HATCHETT, SUSAN BURAK, JONATHAN J. RUTZ, NINA S. OAKLEY, e a EDWARD H. BAIR, AND MICHAEL L. KAPLAN a Division of Atmospheric Science, Desert Research Institute, Reno, Nevada b Snow Survey Associates, Bishop, California c Western Region Headquarters, National Weather Service, Salt Lake City, Utah d Center for Western Weather and Water Extremes, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California e Earth Research Institute, University of California, Santa Barbara, Santa Barbara, California (Manuscript received 12 September 2016, in final form 5 February 2017) ABSTRACT The occurrence of atmospheric rivers (ARs) in association with avalanche fatalities is evaluated in the conterminous western United States between 1998 and 2014 using archived avalanche reports, atmospheric reanalysis products, an existing AR catalog, and weather station observations. AR conditions were present during or preceding 105 unique avalanche incidents resulting in 123 fatalities, thus comprising 31% of western U.S. avalanche fatalities. Coastal snow avalanche climates had the highest percentage of avalanche fatalities coinciding with AR conditions (31%–65%), followed by intermountain (25%–46%) and continental snow avalanche climates (,25%). Ratios of avalanche deaths during AR conditions to total AR days increased with distance from the coast. Frequent heavy to extreme precipitation (85th–99th percentile) during ARs favored critical snowpack loading rates with mean snow water equivalent increases of 46 mm. Results dem- onstrate that there exists regional consistency between snow avalanche climates, derived AR contributions to cool season precipitation, and percentages of avalanche fatalities during ARs. The intensity of water vapor transport and topographic corridors favoring inland water vapor transport may be used to help identify pe- riods of increased avalanche hazard in intermountain and continental snow avalanche climates prior to AR landfall. Several recently developed AR forecast tools applicable to avalanche forecasting are highlighted. 1. Introduction average of 28 deaths per year in the United States (Colorado Avalanche Information Center 2015). Slab avalanches are In mountain environments of the conterminous western the most dangerous type of avalanche and result when a United States (wUS), snow avalanches are a dangerous type failure initiates and propagates outward in a weak layer of mass movement that pose significant hazards to life and underlying a cohesive slab of snow, causing the slab to be- property, yielding millions of dollars per year in economic come unsupported (Schweizer et al. 2003). Fatal slab ava- losses (National Research Council 1990; Mock and lanches are commonly triggered by human actions but also Birkeland 2000). Since 1995, avalanches have caused an occur because of natural release mechanisms resulting from loading by newly fallen or wind-deposited snow. Forecasting slab avalanche occurrence is a major challenge because of Denotes content that is immediately available upon publica- the complex interactions among terrain, snowpack, and tion as open access. meteorology (LaChapelle 1980; Schweizer et al. 2003, 2008). Furthermore, avalanches are not always weather related Supplemental information related to this paper is available at the and by nature are multivariate problems (Mock and Journals Online website: http://dx.doi.org/10.1175/JHM-D-16-0219.s1. Birkeland 2000). Prior studies on the synoptic conditions resulting in Corresponding author e-mail: Benjamin J. Hatchett, benjamin. large avalanches showed the presence of an upstream [email protected] 500-hPa trough (Mock and Birkeland 2000; Birkeland DOI: 10.1175/JHM-D-16-0219.1 Ó 2017 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). 1360 JOURNAL OF HYDROMETEOROLOGY VOLUME 18 FIG. 1. (a) Example AR along the wUS identified using SSM/I satellite-derived IWV (filled colors). (b) Total U.S. avalanche fatalities by water year (gray bars) and fatalities linked to AR conditions in the wUS (blue bars). Annual percentages of wUS fatalities associated with ARs are listed as a percent of total U.S. fatalities. (c) Choropleth map of the wUS where colors are created from blending the percentage of AR-derived cool season precipitation in blues (Rutz et al. 2014) and the percentage of avalanche fatalities during AR events in yellows. The Mock and Birkeland (2000) snow climates are represented by overlain cross hatches (continental), single hatches (intermountain), or no hatches (coastal). et al. 2001), although other synoptic patterns can lead to equivalent (SWE; Serreze et al. 2001). While studies avalanches (Esteban et al. 2005; Muntán et al. 2009). linking ARs to avalanches do exist (Hansen and Here we focus on atmospheric conditions characterized Underwood 2012), they are limited to a few regions by narrow plumes of concentrated water vapor flux and a small number of case studies. Anecdotal evidence called atmospheric rivers (ARs; Zhu and Newell 1998; exists for a linkage between ARs and avalanches and is Ralph et al. 2004). ARs originate as a combination of used by avalanche forecast centers and the public, but local convergence along the warm conveyor belt and this connection has not yet been fully documented in cold frontal region of the extratropical cyclone and as detail. Here we attempt to quantify the linkage between direct poleward transport of tropical moisture (Bao ARs and deadly avalanches using readily available and et al. 2006). ARs are often identified via satellite as archived data. elongated regions of enhanced column-integrated water We hypothesize that since ARs are associated with vapor (IWV; Fig. 1a) and have important roles in wUS extreme cool season precipitation, they should contrib- hydrometeorology. They frequently contribute to cool ute to avalanches by favoring snowfall in excess of the season (defined as November–April) extreme precipita- 30-cm storm total threshold required for avalanche ac- tion and flooding events (Ralph et al. 2006; Dettinger tivity established by Atwater (1954) and supported by et al. 2011; Alexander et al. 2015; Swales et al. 2016) Bair (2013) and Perla (1970). To address this hypothesis, and supply large percentages of climatological pre- we illustrate the relationships between AR conditions cipitation (Rutz et al. 2014), with mean largest 72-h and avalanche fatalities throughout the wUS and place events often contributing 10%–25% of total snow water this information into the context of established snow MAY 2017 H A T C H E T T E T A L . 1361 avalanche climate and hydroclimate regimes. We dem- variables including temperature, snowfall, SWE, rain- onstrate the importance of recently identified prefer- fall, and December snowpack vertical temperature ential pathways for inland moisture transport (Rutz gradient. Large snowpack temperature gradients con- et al. 2015; Swales et al. 2016) and show how integrated tribute to the formation of weak layers in intermountain water vapor transport can be used as a tool to identify and continental climate snowpacks and lead to sub- magnitudes of precipitation and the extent of inland sequent deep slab avalanches (Schweizer et al. 2003; penetration. We then demonstrate that heavy to ex- Marienthal et al. 2015). In coastal climates, avalanches treme precipitation and rapid snow loading in excess of often result from failures occurring within storm snow commonly used thresholds for avalanche activity oc- layers or at the old–new snow interface (Bair 2013). curred during a large fraction of the avalanche fatalities coinciding with AR conditions. Finally, we highlight 3. Methods several recently developed AR forecast tools that can be used by avalanche forecasters, emergency managers, The latitude and longitude of each avalanche inci- and the general public. These tools can be used to more dent was estimated from archived avalanche reports. precisely predict timing, location, and inland penetra- SNOTEL observations from stations located within a tion of ARs. 0.58 radius of each incident (eight stations on average) were used to calculate the precipitation percentiles from the period of record (typically 1981–2014) nonzero cool 2. Data season precipitation days. We compared these values Archived avalanche incidents in the wUS between with the maximum daily precipitation percentiles ob- November 1998 and April 2014 were acquired from served between 4 days prior to the avalanche event day Atkins (2007) and the Colorado Avalanche Information and 1 day after. This period covers complete storm event Center (2015). Because of a lack of archived nonfatal precipitation at time scales relevant for avalanche haz- avalanche observations, we were unable to quantify the ard. Changes in SWE (hereafter DSWE) were calculated frequency of nonfatal avalanches during AR events in a as the difference in SWE over this 6-day period to meaningful manner. Atmospheric fields of 500-hPa geo- quantify total new snow loading. We also calculated the potential height, 700-hPa air temperature, meridional greatest 1- and 2-day change in SWE over the 6-day and zonal wind, and specific humidity on isobaric sur- window to estimate the maximum rates of new snow faces were derived from daily 32-km horizontal resolu- loading,