Seattle City Light Trends

Historical Climate Trends

What changes have we seen so far in the Northwest?

The visualization below shows observed changes in temperature, precipitation, snow, and streamflow for a selection of sites and metrics that are of interest to Seattle City Light.

Direction and Layout of the tool

The interactive historical trends tool was created using Tableau. The tool includes three panels: (1) Disclaimer, (2) Maps and Graphics, and (3) Data Sources. The layout of the tool is illustrated in the figure below. The main features of the tool are a map showing trends for all stations with data on a particular metric, under which is a plot showing the year-to-year changes for one particular measurement site. All user selections can be made on the left-hand side of the figures, while all legends are included to the right.

Menus and Options


The drop-down menu on the top right side of the tool allows you to select the variable you want to consider: Temperature, Precipitation, Snow Water Equivalent (SWE), Streamflow, Degree-days, and Cloudiness (SeaTac only). In order to simplify the options, Temperature and Precipitation are separated into three categories: Seasonal & Annual, Monthly, and Extremes/Other. Results for snowpack (SWE) are listed separately for Snow Course and SNOTEL stations (see “Data Sources” tab above).


Once you have selected a variable, a list of metrics appears below it. We use the term “metrics” to describe different quantities that can be derived from each variable. For example, daily temperatures can be used to calculate the number of frost free days, as well as the number of cooling degree days in summer. Both metrics are included in the tool.

Use the radio buttons alongside each metric to select the one of interest. Once a metric is selected, the map will update showing information on the longest trend that can be computed for each station (i.e.: some observational records begin as early as the late 1800s, while others do not start until after 2000). The legends to the right show both the trends corresponding to different symbol sizes on the map, as well as both the sign and significance of each trend. Some trends are not available because there was insufficient data to reliably estimate a long-term trend; see the documentation for more on how this was determined.

Once a metric has been selected, a plot showing all valid data will appear below the map (or be updated if a previous metric had already been selected). In addition to the observed values for each year, a trend line will be shown if there is sufficient valid data, along with average values for the last 30 years of the record and the 30 years following the selected start year. The legend to the right of this plot shows the colors for the data and 30-year averages, trends are displayed using the same colors as are used in the map.

Start Year

Trends can be calculated for different start years, in order to evaluate how robust a trend is to different starting points. For simplicity we only computed trends on five-year increments, starting from the first 5-year period with sufficient valid data. Move the slider bar to see how the trend – and its statistical significance – change as a function of start year. Note that some start years will not have a valid trend due to insufficient data.

As an example of the importance of the start year for estimating long-term trends, consider Water Year Precipitation at Diablo Dam. For start years before 1940, a few of the long-term trends for this station are statistically significant. However, none of the trends starting in 1940 or later are significant. Although this does not necessarily imply that there are measurement issues prior to 1940, the lack of consistency in trends suggests that greater care must be taken in interpreting the trends observed for earlier start years.

Reference Data


This project was funded by Seattle City Light and the State of Washington.


Mauger, G.S., Bumbaco, K.A., Norheim, R.A., Won, J.S., Bond, N.A. Observed Climate and Hydrologic Trends for Seattle City Light. Report prepared for Seattle City Light. Climate Impacts Group, University of Washington, Seattle.

Download the report here:


Please contact Guillaume Mauger ( with any questions about this project.