High Pressure Danger: Fog and Roadway Collisions

This winter, persistent high pressure has frequently developed over the western U.S.

High pressure that has often brought dry conditions and afternoon sun.  But such high pressure also results in extensive fog, particularly in valleys and basins.

Fog is a serious threat with such high-pressure conditions, often resulting in multi-vehicle collisions, injuries, and loss of life.

For example, yesterday there was a 49-vehicle chain collision in the Central Valley of California between Bakersfield and Fresno as a result of dense fog.

Below is a visible satellite image from yesterday morning, which clearly shows the fog. 

 Such fog in the Central Valley is known as Tule Fog (named after the Tule grass that is extensive over the region).

Fog accident events often progress in a similar way.   

A collection of vehicles is driving too fast for conditions.     The first vehicle in the "pack" enters the fog and hits the brakes suddenly, setting off a chain reaction behind that car that can involve dozens of vehicles.

Washington State is home to many of such fog related accidents associated with high pressure.

Low-level fog is frequently present during high-pressure periods during winter in eastern Washington, with a long list of vehicle pile-ups during the "fog season" from November to February (see satellite image example below on January 17)

For example, a 38-vehicle pile occurred on I-90 near Kittitas on December 7, 2022 (see below).  I could list a dozen other such chain accidents associated with fog over the Columbia Valley.

The danger of such fog events are inceased during cold weather, when freezing fog makes roads slippery and dangerous (according to the 2022 event).

As mentioned earlier, these west coast events are often associated with high pressures aloft, as shown by the upper level (500 hpa, about 18,000 ft) upper level map for yesterday morning.  In meteorological parlance, we call this a ridging situation.

High pressure aloft reduces upper-level clouds, allowing the surface to radiate heat to space, and allowing the surface to cool to saturation (producing fog).   The high pressure aloft is also associated with sinking air that warms by compression aloft.  

Warming aloft and cooling near the surfae produces stable, inversion conditions, which are favorable for fog.

Finally,  the Northwest is one of the foggiest locations in the U.S., so being cognizant of the threat of fog while driving is important.