Why is the diurnal temperature range on the Cascade volcanoes so small? Ski-mountaineering in the Pacific Northwest I am frustrated that the snow pack isn't refreezing at night.

There is about a 20F difference between the day and night temperatures in Seattle. Looking at forecasts for Mt. Rainier and Mt. Baker, the night temperature looks to be the same or sometimes even  warmer. I think I have seen these kinds of forecasts even when the sky is clear, when I would expect the snow pack to release a lot of longwave radiation.

What is going on here?
I think the answer to this question can be split into two parts. First, it is important to know which weather model you are using. Weather models use grids to predict the weather. The finer the grid, the better the topography (mountains) are represented by the weather model. To get a reasonable representation of the mountains that you mention, you would need a horizontal grid spacing of a few kilometers at most.

For the second part of the answer, we will assume that the weather model you are looking at has a fine grid. The only effect that I can think of explaining the strong difference in the daily cycle between Seattle and the mountains, is the wind. At higher altitudes, radiative cooling cannot effectively bring down the temperatures since the air is continuously being mixed with air from the free atmosphere which has a much less pronounced daily cycle.