Fog In the Wrong Season

This morning, fog was widespread over the lowlands of western Washington, with many of the river valleys in the murk (see visible image below around 7 AM)

Several local weather cams showed the fog.  Olympia had almost no visibility.

And it wasn't much better on the Chehalis River Bridge.

All this foggy action is a bit unusual in late April, which is close to the time of minimum frequency of dense fog for most locations in our region. (see below)

Why is dense fog unusual in April and May?

Because days are much longer and the sun is getting quite strong.  Thus, the surface is getting warmer. But the air is still relatively cold aloft.  

That leads to a rapid decrease in temperature with height, which results in instability and vertical mixing.

Vertical mixing is the enemy of fog, which generally forms when cold, dense air is near the surface, and warmer air is aloft.

Exactly, the situation that was occurring this morning (see the temperature above SeaTac Airport this morning below).

So why the fog this morning?  

A strong high-pressure area aloft developed to our northwest (highs of the 500 hPa pressure surface are shown below...think of pressure around 18,000 ft).  Red indicates the high pressure.

Such an offshore high-pressure aloft causes sinking, which warms the middle atmosphere.  Furthermore, high pressure causes clear skies aloft, which allows the lower layers to cool by emitting infrared radiation.

Warming above and cooling below is perfect for fog formation and the development of a low-level inversion.