Recently Brad Coleman of the Seattle National Weather Service Forecast Office presented results of a long-term climate analysis for the Northwest. Historical data collected in and around the Seattle area, including precipitation, temperature, snowfall, and pressure data, showed very definite cyclical patterns, with a pronounced 18-year time period evident in the data. Oregon Climate Service has analyzed Oregon data in similar ways in the past and found cyclical trends, although areas in different parts of the state operate on different cycles.
As might be expected, Portland exhibits historic trends which are similar to those in Seattle. Distinct wet and dry periods, and warm and cool periods, can be seen in the records. Causal mechanisms include shifts in large scale circulation patterns and sea surface temperature changes.
Figure 1 shows annual precipitation, smoothed over a 5-year running average. From 1910 until the late 1970's, wet and dry cycles occurred with a frequency very close to the 18-year period identified by Colman. Peaks were centered on about 1917, 1935, 1952, and 1971. Extrapolation of those cycles would have brought another peak in about 1989. However, that cycle arrived earlier (during the very wet 1982-83 period). Interestingly, the protracted dry period of 1985-92 is the driest of the century.
The evidence of long-term periodicity in the Portland data encourages us to continue to study time histories in the Oregon data, as well as attempt to correlate the Oregon results with changes in other parts of the world. Our long-term goal is to understand the cause-effect relationships between large-scale weather patterns and Oregon's weather, and to use that information to better forecast our weather conditions well in advance.
