Why are the winds so different between the gaps and peaks in the Cascades?

If you are in one of the Cascade passes, what do you think the prevalent wind direction in the winter should be?

You might argue that in the midlatitudes the winds should be predominantly southwesterly or westerly (from the west), as illustrated by Monday morning's vertical radiosonde sounding at Quillayute on the Washington coast (see blue wind pennants below), since winds are generally westerly in our part of the midlatitudes.

But if you guessed westerly you would be wrong.... easterly winds are far more prevalent in the passes (such as Snoqualmie or Stevens).   To prove this, here are the wind directions and speeds at Snoqualmie and Stevens during the past ten days, secured from the nice Northwest Avalanche Center website.

Snoqualmie Pass

Steven's Pass

Most of the time, the winds are EASTERLY, with occasionally westerly periods (with stronger winds) when fronts move through.

But then what is going on at the ridges, saying at the top of Crystal Mountain at 6830 ft?

Crystal Mountain

VERY different.  Winds are rarely easterly, but are typically southerly to westerly.  Much more like the coastal radiosonde in the free air, which is expected since high crest-level stations have good exposure to the prevailing winds and are far less impacted by terrain or gap effects on wind direction.  There can be some acceleration of the winds on peaks or crests.

So that is going on?   The answer is that winds in gaps in the mountains, such as Snoqualmie and Stevens, tend to reflect the pressure differences across the gaps.  The same is true of the Columbia Gorge.  If pressures are higher in eastern WA than western WA, there are generally easterly winds, the opposite westerly winds.

During much of the winter there is cold, dense air in easterly WA (which causes the pressure to rise).  In contrast,  low pressure centers are frequently offshore or approaching our coast.  This configuration results in higher pressure in eastern than western Washington most of the time.

For example, on Monday a trough of low pressure was approaching the coast and that produced an east-west pressure difference (gradient) over WA state (see pressure forecast for 10 AM Monday below).

In contrast, when a front moves through the situation changes.  The front has a pressure trough associated with it and temporarily reverses the pressure gradient (higher pressure to the west of the Cascades).  Here is a nice example of such a reversal that occurred on January 13th at 1 PM.  The result is westerly flow.

As low centers approach the coast, the pressure difference across the Cascades increase, producing strong easterly flow.  That can draw some of the cool, foggy air in the "bowl" of the Columbia Basin up into the passes.  This happened yesterday, something I experienced first hand while cross country skiing near Easton, WA.    As a trough approached the coast yesterday afternoon, fog and low clouds thickened east of the pass.  Here is my view at around 3 PM.  Beautiful snow, but lots of low clouds!