The amount of snowfall relative to the depth of the snow. And more snowflakes on the lowlands

We have an unusually cool period ahead and, believe it or not, more snowflakes are expected over the western lowlands over the next few days.

But before I discuss forecasting, we need to talk.

There is a considerable amount of confusion about the difference between predictions snowfall and snow accumulation or depth of snow.

This difference is especially true when the temperature is marginal for snow.

Even if the atmosphere is below freezing, the depth of snowfall on the ground can vary widely.

If the ground is above freezing (as it is often around the western temperate lowlands), the snow can melt, resulting in a lot less accumulation (or NO). This is especially true on asphalt or concrete roads that support heat conduction from below.

Snow can also compress under the weight of the snow below.

bottom line: event with cold atmospheric conditions, snow depth may be much less than expected.

But the situation is “worse” than that with temperatures above the threshold of possible snowfall. In marginal situations, when the freezing point is higher than the surface, the snow has melted. Model diagnostics treats wet snow as snow, so the snowfall map can be dramatic (5 inches!!), but is much less presumptive on ground where the wet skid layer melts quickly.

For these and other reasons, I repeatedly note the difference between snowfall and accumulated snow in this blog. Maybe I need to emphasize this more. Maybe I need to show the snow depth forecast!

OK, let me try to do it right in this blog.

Over the next few days, we’ll continue to be unseasonably cold, with a low center forming offshore (see the 7 a.m. low temperature and sea-level pressure forecast map on Saturday). Temperatures will be low for snow on the Puget Sound land near sea level, but some minor flakes in precipitation are possible. Blue indicates areas that are cold enough for snow to reach the surface.

But Portland is in a better place for snow. A significant pressure differential from west to east across the Columbia Gorge (higher to the east), will push colder air north into the Willamette Valley. The air will also be drier, facilitating evaporative cooling.

Here’s the snowfall forecast until Sunday morning at 4am. Snow load in the mountains. But there was only light (snow) around Puget Sound and NW Washington at lower elevations. Much more in Southwest Washington, where more rainfall and higher terrain, and several inches of snowfall around Portland.

But what about the snow depth forecast around western Washington?

There is nothing near sea level around Puget Sound and NW Washington. There is nothing along the coast where the snowfall is essentially greater than zero. QED.

Snow maps are fun to look at, but they can be misleading.