Over the 20th century, the Northwest has grown warmer and wetter. The average trend in temperature is + 1.4°F with nearly equal warming in summer and winter. Annual precipitation also increased nearly everywhere in the region, by 11% on average, with the largest increases of about 50% in northeastern Washington and southwestern Montana.
In addition to this trend toward a warmer, wetter climate, the Northwest's climate also exhibits significant recurring patterns of multi-year variability. The predominant pattern is that warm years tend to be relatively dry with low streamflow and light snowpack, while cool years tend to be relatively wet with high streamflow and heavy snowpack. Although the differences in temperature and precipitation are relatively small (differences in monthly-average temperature of up to 2 to 4°F in winter), they have clearly recognizable effects on important regional resources. For example, warmer, drier years tend to have summer water shortages, less abundant salmon, and increased risk of forest fires.
These year-to-year variations in the region's climate are clearly related to two large-scale patterns of climate variation over the Pacific, one more and one less well known. The more well known pattern is the El Niño/ Southern Oscillation(ENSO) and the lesser known pattern is the Pacific Decadal Oscillation (PDO). While both of these patterns have an influence on the seasonal weather, for instance raising the average winter temperature by about 1°F, they do not control it and there are other influences involved.