Fog and Freezing Fog in the Inland Empire: Intense Inversion Aloft

The Columbia River basin is well known for sun and warmth for much of the year, compared to the sodden, cloudy western side of Washington State.  

During the cool season from mid-November through mid-February, the reality is often different, with cool, cloudy air trapped within the basin, often producing fog and FREEZING fog.

The last few days are great examples of this chilly, fog-laden situation.

Consider the visible satellite image around noon today (Friday). Clear over most of western Washington and Oregon, with the Columbia Basin full of stratus and fog.

Checking on the Washington Department of Transportation webcams, on the eastern side of the Cascades, Kittitas was in fog:

And Yakima was not better. Much of this fog was of the freezing variety, a great danger for ground transportation.

Not surprisingly, my colleagues at the National Weather Service had dense fog and freezing fog advisories out last night (see below).

To appreciate the cold and unpleasant nature of the old air trapped within the Columbia Basin, below is a plot of the maximum temperatures today around the state.

Yikes.  Much of the Columbia Basin never got out of the mid-30s.    Some locations never rose above freezing.  In contrast, lots of 50s in Western Washington and some very warm conditions in the mountains (Cascades and Olympics). 66F in the Olympic.

Why so warm in the mountains? 

Because a strong inversion (temperature increasing with height) was created by the high-pressure system aloft (see the upper level map for 4 PM Thursday below).

High pressure aloft causes sinking and warming aloft, with descending air warmed by compression (like when you pump up a tire!).  The sinking decreases as one approaches the surface (air can't move downward through the surface).  Thus, temperatures remain cool at low levels, which is further chilled by the emission of infrared radiation to space.

There is a world-class inversion above us today.    To show this, below are temperatures above SeaTac Airport at 8 PM, measured by aircraft landing and taking off.

Unbelievable!  Low 40s near the surface and nearly 59F at 1400 ft.    Take a hike tomorrow morning.  You will not believe how temperatures warm with height.

Consider the situation on the slopes of Tiger Mountain in Issaquah.  At the Tiger Mountain Family Nudist Park, the temperature was 40F, but up on the slopes below Poo Poo Point (about 1500 ft), it was 57F!   

My pro-tip for the nudists is to take a hike up the hill before removing their clothing!