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Hydroclimatic Conditions Preceding the March 2014 Oso Landslide

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  • 1 Department of Civil and Environmental Engineering, University of Washington, Seattle, Washington
  • | 2 Department of Geography, University of California, Los Angeles, Los Angeles, California
  • | 3 U.S. Geological Survey, Tucson, Arizona
  • | 4 Department of Atmospheric Sciences, University of Washington, Seattle, Washington
  • | 5 ** NOAA/National Weather Service, Seattle, Washington
  • | 6 Hydrometeorological Design Studies Center, NOAA/National Weather Service, Silver Spring, Maryland
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Abstract

The 22 March 2014 Oso landslide was one of the deadliest in U.S. history, resulting in 43 fatalities and the destruction of more than 40 structures. We examine synoptic conditions, precipitation records, and soil moisture reconstructions in the days, months, and years preceding the landslide. Atmospheric reanalysis shows a period of enhanced moisture transport to the Pacific Northwest beginning on 11 February 2014. The 21–42-day periods prior to the landslide had anomalously high precipitation; we estimate that 300–400 mm of precipitation fell at Oso in the 21 days prior to the landslide. Relative only to historical periods ending on 22 March, the return periods of these precipitation accumulations are large (25–88 yr). However, relative to the largest accumulations from any time of the year (annual maxima), return periods are more modest (2–6 yr). In addition to the 21–42 days prior to the landslide, there is a secondary maximum in the precipitation return periods for the 4 yr preceding the landslide. Reconstructed soil moisture was also anomalously high prior to the landslide, with return periods relative to the particular day that exceeded 40 yr about a week before the event.

Supplemental information related to this paper is available at the Journals Online website: http://dx.doi.org/10.1175/JHM-D-15-0008.s1.

Publisher’s Note: This article was revised on 29 June 2015 to correct a typographical error in the Fig. 1 caption.

Corresponding author address: Brian Henn, Department of Civil and Environmental Engineering, University of Washington, 201 More Hall, Box 352700, Seattle, WA 98195. E-mail: bhenn@u.washington.edu

Abstract

The 22 March 2014 Oso landslide was one of the deadliest in U.S. history, resulting in 43 fatalities and the destruction of more than 40 structures. We examine synoptic conditions, precipitation records, and soil moisture reconstructions in the days, months, and years preceding the landslide. Atmospheric reanalysis shows a period of enhanced moisture transport to the Pacific Northwest beginning on 11 February 2014. The 21–42-day periods prior to the landslide had anomalously high precipitation; we estimate that 300–400 mm of precipitation fell at Oso in the 21 days prior to the landslide. Relative only to historical periods ending on 22 March, the return periods of these precipitation accumulations are large (25–88 yr). However, relative to the largest accumulations from any time of the year (annual maxima), return periods are more modest (2–6 yr). In addition to the 21–42 days prior to the landslide, there is a secondary maximum in the precipitation return periods for the 4 yr preceding the landslide. Reconstructed soil moisture was also anomalously high prior to the landslide, with return periods relative to the particular day that exceeded 40 yr about a week before the event.

Supplemental information related to this paper is available at the Journals Online website: http://dx.doi.org/10.1175/JHM-D-15-0008.s1.

Publisher’s Note: This article was revised on 29 June 2015 to correct a typographical error in the Fig. 1 caption.

Corresponding author address: Brian Henn, Department of Civil and Environmental Engineering, University of Washington, 201 More Hall, Box 352700, Seattle, WA 98195. E-mail: bhenn@u.washington.edu

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