Recent evidence suggests long-term changes in the intensity and frequency of extreme wave climate around the globe. These changes may be attributable to global warming as well as to the natural climate variability. A statistical model to estimate long-term trends in the frequency and intensity of severe storm waves is presented in this paper. The model is based on a time-dependent version of the Peak Over Threshold model and is applied to the Washington NOAA buoy (46005) significant wave height data set. The model allows consideration of the annual cycle, trends, and relationship to atmosphere- ocean-related indices. For the particular data set analyzed the inclusion of seasonal variability substantially improves the correlation between the model and the data. Also, significant correlations with the Pacific--North America pattern, as well as long-term trend, are detected. Results show that the model is appropriate for a rigorous analysis of long-term trends and variability of extreme waves and for providing time-dependent quantiles and confidence intervals.