Winter Freezing Level
As winter temperature has increased, the winter freezing level in the Skagit has risen. In fact, on average, the winter freezing level has risen 650 feet between 1959 and 2012. The winter freezing level is the elevation where the temperature is cold enough for precipitation to fall as snow, not rain. This represents about a 12-foot per year increase over the 53 years.
More Rain, Less Snow
Based on projected temperature increases, the Skagit basin will continue to undergo substantial changes in average conditions. Rising freezing levels in winter increase the land area in the Skagit Basin that captures rain instead of snow, resulting in more water entering the river during the fall and winter and less during the summer.
In the maps of the Skagit Drainage Basin (Figure 2), the light blue shading indicates the snow dominated zone and the darker blue indicates the Rain on Snow (ROS) zone. The charts (Figure 3) show that looking into the future, the Skagit region may see a doubling of the acres capturing rain instead of snow between 1991 and 2080.
Dr. John Riedel calculated the changing freezing level using NOAA’s Western Regional Climate center site. Dr. Riedel analyzed the data from the Diablo Dam weather station for the months November to April to calculate the averages. The data on the site goes back to the 1950’s and is available for any weather station in the United States.
One reason people may care about this can be seen in Figure 1: Schematic of a Cool Climate Flood. The freezing level, shown in red, dictates the portion of the basin that has the potential to generate a flood (below the red line) and the portion of the basin that retains precipitation as snow (above the red line).
In Figure 2: Schematic of a Warm Climate Flood, the freezing level has risen consistent with what has already occurred in the Skagit. The area available for flood generation has increased as the freezing level (the red line) moves upward. The larger the basin area that is capturing rain (not retained as snow) the greater the flood potential. As the basin area increases, the potential for flood frequency and magnitude increases as well.
This trend of increasing freezing levels is projected to continue into the future based on predictions of continued warming. This change also contributes to the predicted shifts in the timing and magnitude of stream and river flows.