Fig 1.
Location of 19th century historical fort daily weather observations.
Modern historical comparator sites (Global Historical Climate Network) are listed in Table 1. River abbreviations from north to south are Sn: Snohomish, Is: Issaquah Creek, Du: Duwamish, Ce: Cedar, Gr: Green, Wh: White, Pu: Puyallup, Ni: Nisqually, De: Deschutes, Ch: Chehalis, Co: Cowlitz, Le: Lewis. Snotel stations are ShCa: Sheep Canyon, SkCr: Skookum Creek. Map was developed in QGIS using base layers from the U.S. Geological Survey, National Geospatial Program, downloaded from https://apps.nationalmap.gov/downloader/ and https://elevation.nationalmap.gov:443/arcgis/services/3DEPElevation/ImageServer/WMSServer.
Table 1.
The five largest three-day and four-day precipitation events from forts in western Washington and comparator GHCN-Daily stations.
The 14 Dec 1867 event is highlighted in bold. Data for Fort Steilacoom, Fort Canby, and Fort Vancouver were obtained from the Midwestern Regional Climate Center (see footnote for data pretreatment). Data for American Camp was entered from digital scans of handwritten ledgers. GHCN-Daily stations were downloaded from https://www.ncdc.noaa.gov/cdo-web/ Site locations are shown in Fig 1.
Fig 2.
Cumulative precipitation at 19th century historical forts compared to 20th/21st century GHCN comparator sites.
Cumulative precipitation is restricted to the high-rainfall season from 01 November to 31 March. The thicker line is the 1867/1868 water year.
Fig 3.
Cumulative distribution of wet-day daily precipitation at five historical observation locations (blue) compared to 20th and 21st century GHCN stations (green).
Data were limited to days with precipitation >0.5 mm and to the months of November through March, as in Fig 1.
Fig 4.
Three-day precipitation sums from historical forts recording daily observations between 1850 and 1890 (blue) compared to nearby GHCN-daily stations (green).
Four-day precipitation sum is plotted in gray below the three-day sum. The December 1867 atmospheric river event is shown by points and dashed lines. The right column shows the frequency-magnitude relationship of three- and four-day precipitation from the weather stations. The frequency-magnitude plots use the n largest non-overlapping events from the n years of record. Fort Steilacoom and Fort Canby were compared to composites of nearby GHCN-Daily stations. Fort Canby only reported cumulative precipitation for 12-17 December 1867. See Fig 1 for site locations. Data coverage, pretreatment, and values of the largest five events at each site are in Table 1.
Fig 5.
Frequency-magnitude relationship of precipitation summed over four GHCN locations (Astoria OR, Vancouver WA, Puyallup WA, and Olga WA).
The four-site daily time series was used to compute non-overlapping sums in three-day (green) or four-day (gray) periods, as in Fig 2. The end-dates of the two largest events are labeled. The precipitation summed over the four historical records for December 1867 are shown as dashed lines.
Fig 6.
Weather observations at six military forts in western Washington and northwest Oregon for December 1867.
Temperature and wind force and direction were measured three times daily. Wind force has unknown units. Cumulative rainfall curve in blue shows times of observations. Atmospheric pressure was recorded at Fort Steilacoom (black line, overlayed on precipitation curve). Rainfall at Port Townsend is not plotted because of a missing observation on 14 December. At Astoria, only precipitation data were available. S=snowfall, L=lightning observed.
Fig 7.
Rainfall, snowmelt, and flooding in December 1867 compared to floods in 1996 and 2006.
A) Western Washington and Oregon rivers, showing the locations of four-day rainfall observations for 12-15 December 1867 and mapped newspaper accounts of flood damage from 14-15 December 1867. Letter A-D show locations of reported high-water-levels: A: Issaquah Creek, B: Cowlitz River near Vader, C: White River near Kent, D: Cowlitz River near Longview. E: Skagit River near Concrete; date of the flood on Skagit River is uncertain. See S1 Text for all flooding accounts. Cities shown have newspaper accounts from 1867. B) Peak streamflow rank for the 05-08 February 1996 atmospheric river for USGS streamgage basins. Only streamgages with observations in 1996 and ≥ 40 yr of observations between 1960 and 2020 are shown. Contour lines show modeled four-day precipitation (PRISM 2023). The four-day change in snow-water-equivalent (square symbols) is from the SNOTEL network. C) Same as (B) but for the 04-07 November 2006 atmospheric river. Maps were developed in QGIS using base layers from the U.S. Geological Survey, National Geospatial Program, downloaded from https://apps.nationalmap.gov/downloader/ and https://basemap.nationalmap.gov:443/arcgis/services/USGSShadedReliefOnly/MapServer/WmsServer? and rainfall data from https://prism.oregonstate.edu.
Fig 8.
Four accounts of high-water marks of the 1867 flood and inferred water elevations on valley cross sections.
Lidar elevation data from 2017 to 2021 show modified floodplains (dikes, elevated roads, and ditches) that were absent in 1867. A) Green/White River near Auburn; B) Cowlitz River near Longview; C) Issaquah Creek; D) Cowlitz River near Vader. Dashed line on profile b shows the channel depth. Locations of farms and houses were obtained from General Land Office survey maps and placed on lidar topography. Quotes on maps provide the basis for the inferred high-water elevation (blue areas on cross sections). The FIS 1% flood elevation are from hydrological models in published flood insurance study reports (msc.fema.gov), except for transect d, where only an approximate flood hazard zone was available. Note that dikes constrain the 1% probability floods at sites A (near Auburn WA) and site B (near Longview WA). Maps were developed in QGIS using elevation data from the Washington Lidar Portal (https://lidarportal.dnr.wa.gov) also available at https://apps.nationalmap.gov/downloader/.
Fig 9.
Moisture flux, vertical velocity, precipitation rate, and snow melt during the landfalls of three atmospheric-river floods.
Data are from NOAA-CIRES-DOE 20th Century Reanalysis (V3) provided by the NOAA/OAR/ESRL PSL, Boulder, Colorado, USA at https://www.psl.noaa.gov/data/gridded/data.20thC_ReanV3.html. The magenta square is centered on western Washington. Times are PST. The figure illustrates (left to right) the supply of moisture (moisture flux), and the large-scale atmospheric cooling mechanism (vertical velocity, rising implies cooling), that favors cloudiness and precipitation (precipitation rate), plus the additional contribution to the hydrologic response of snowmelt (change in snow cover).