Air Temperature and Precipitation
Temperatures for the Pacific Northwest demonstrate a clear upward trend in the regional projections (see the top graph in Figure 1 to the right). Scientists expect that the quantity of precipitation that the Pacific Northwest receives will remain constant (see the bottom graph in Figure 1 to the right) while when that precipitation comes is predicted to change.
The temperatures estimated for the Pacific Northwest from downscaled Intergovernmental Panel on Climate Change (IPCC) global climate models agree that temperatures will rise for all seasons under any of the emissions scenarios. Projected annual temperatures for the Pacific Northwest region are between 4.7 and 7.0° F warmer for 2080s than for the 20th century average. Within the Pacific Northwest, projections for the Skagit Basin are moderated by proximity to coast, thus the predicted range is between 4.0° and 5.8° F greater than 20th century baseline conditions represented by average temperatures between 1970–1999 (ENVISION report by Lee et al. 2011).
As shown in the top figure to the right, the projected increases in annual temperature exceed the highest temperatures in the 20th century attributable to natural variability. This means that by the mid-21st century, annual temperatures in the coolest years (5th percentile) will exceed the very hottest years (95th percentile) for the mid-20th century.
Future projections of mean annual precipitation (average number of inches of rain during a given year) for the Pacific Northwest do not differ significantly from 20th century observations; and the projections of mean annual precipitation are within the limits of past variability (seen in the bottom figure to the right). However, an analysis that breaks the projections into seasonal categories, reveals a clear seasonal trend in precipitation: drier summers and wetter springs, winters and autumns compared to 20th century averages (Mote and Salathé 2010).
For the Skagit Basin, precipitation projections indicate increases of 9.8% in the winter, 8.0% in the spring and 19.2% in the fall, but a 27.6% decrease in the summer by the 2080s compared to the late 20th century seasonal averages (ENVISION report by Lee et al. 2011).