About these Maps
On This Page
These maps show how four important parameters of climate – temperature, precipitation, snowpack, and soil moisture – vary over time in the Pacific Northwest (PNW) as a result of natural climate variability and climate change. Climate in the region is significantly affected by two cycles, the El Niño/Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). While the cycles operate on different time scales – 2-3 years for ENSO and 40-60 years for – they each have warm and cool phases, which have distinct effects on the climate of the PNW. Generally, warm phases of the cycles (El Niño and warm PDO) increase the chances for warmer and drier PNW winters, while cool phases (La Niña and cool PDO) increase chances for cooler and wetter PNW winters. The combination of the two cycles is important, however, and there is significant variability across the region. These maps show how these cycles – separately and in combination – affect the climate across the region. See more information on PDO and ENSO and their effects on the PNW.
Reading the Maps
The color schemes for the maps are intended to be suggestive of the climate variable. Specifically, red for warmer, blue for cooler; green for wetter, brown for drier. A given amount of anomaly is represented by the same color on every map for each climate parameter (except that winter and summer precipitation are not comparable with each other). The legend for each map shows the actual range of colors and anomaly values on the map. In some cases, there may be “extreme” anomalies in the dataset that are greater than the largest positive anomaly or less than the largest negative anomaly shown on the legend. In these cases, the “extreme” anomalies are represented on the map by the most extreme colors on the legend bar – but the number on the legend does not represent those extreme values. Restricting the range of colors to non-extreme anomalies allows the maps to show patterns of variability more clearly. Generally, extreme values were considered to be anomaly values in the highest and lowest 1% of a dataset. The table below lists the values for each climate parameter beyond which extremes were ignored.
|Precipitation (Winter)||±71 in||±180 cm|
|Precipitation (Summer)||±10.6 in||±27 cm|
|Soil Moisture||±24 in||±62 cm|
|Snowpack||±65 in||±166 cm|
Time Periods and Anomalies
For the maps, precipitation and temperature data are aggregated to seasons (December/ January/February and June/July/August). Snowpack and soil moisture data are ‘captured’ on specific dates (April 1 and July 1). The data are then aggregated to collections of years characterized by particular climate regimes (defined by PDO and ENSO phases). The anomaly data presented on the maps is the average value over those collections of years versus the average value over the period 1915-2003. Presenting anomaly data instead of averages allows for better visualization of the changes occurring during each climate regime.
The VIC model begins in January 1915. Because of the definition of “winter” as December/January/February, and because the calculation of snowpack and soil moisture is dependent on the beginning of the “water year” (October through September), the winter precipitation and temperature maps, and the snowpack and soil moisture maps, begin with the 1915-1916 winter and do not include data from the 1914-1915 winter.
To see a map of historical average conditions (1915-2003) for any climate parameter, choose “All Years” in section 1b.
Precipitation anomalies are presented in both absolute (inches or centimeters) and relative (percent) terms. We provide two different ways of viewing precipitation anomalies because they each provide different insights.
A certain absolute change in precipitation in a very dry location may be more significant than a similar change in wet region. An extra inch of rain is a bigger deal in Yakima (10 inches per year) than in Forks (100 inches per year). Thus, relative changes attempt to indicate the local significance of the changes. So, an additional inch in Yakima is a 10% change while an additional inch in Forks is only a 1% change.
On the other hand, absolute changes indicate the true impact on the regional water budget, which is integrated over the river basins. While that extra inch of rain in Yakima might be easy to notice, it doesn't really amount to much despite the fractional increase being 10 times the fractional increase of the same inch of rain at Forks.
Comparing variability on different maps is a helpful way of understanding spatial patterns. It is easy to compare maps by using the enlarged versions. If you click on a map on the main map page, a larger version of the map will appear in a separate browser window. If you then use the controls to display another map and click on it, a larger version of that map will appear in the second window. In the second window, you can use your browser’s forward and back controls to toggle through a series of maps you have opened in this manner. This capability makes it easy to compare the spatial patterns of different climate phases or different climate parameters. This feature works best in the Firefox browser.
The source data for all these maps are derived from the Variable Infiltration Capacity (VIC) Macroscale Hydrologic Model for the Pacific Northwest and Columbia River Basin. VIC models the hydrologic cycle for the region using detailed information such as temperature, precipitation, land cover, topography, and soil type. For the Pacific Northwest, the model is run on a daily time step from 1915 to 2003, over a 1/8 degree grid (latitude and longitude) (approximately 7.5 miles square). The data shown on the temperature and precipitation maps are prepared from historical National Climate Data Center records, “gridded” across the region for use as input to VIC. By contrast, snowpack and soil moisture data are model outputs from VIC. You can find detailed technical information about the VIC model and how the historical data is prepared at the VIC web page.
The data for these maps is available for use in resource planning models and similar applications. To download data for a map, right-click on the “Download data file for this map” link and choose “Save Target As”. The data are in a format for use with Geographic Information Systems software. See more information about the data file format.