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.

Mid-Winter Precipitation Review

We are now solidly into mid-winter--a good time to check on the status of precipitation over the region.

Let's start with the total cool-season precipitation since October 1, the beginning of the "water year" (see below).

The majority of the western U.S. has received above-normal precipitation, with the North Cascades and the Los Angeles area being substantially wetter than normal.

There is one major dry area:  western Oregon.

There has been enough precipitation east of the Cascade crest so that the Yakima Reservoir levels are not only WAY above normal, but GREATER THAN THE HIGHEST AMOUNTS of last year (see below).  Wow.

Temperature is a contrasting story (below).  

Much warmer than normal over the Rockies,  and near normal over western Oregon and the interior valley of California.  Modestly warmer than normal over much of Washington.

The key driver of these patterns of precipitation and temperature has been persistent high pressure aloft over the western U.S, and the persistent trough of low pressure aloft over the eastern U.S.    This is illustrated below, which shows the difference from normal at 500 hPa (roughly 18,000 ft).  Higher than normal pressure is indicated by the yellow;   blue and purple indicate the oppoosite

High pressure is associated with sinking and warming aloft.  It also contributed to dry conditions.  

The complex distributions of precipitation and temperature have created an even more complex distribution of snowpack over the western U.S.

The southern Sierra Nevada is fine, and eastern Washington and the Rockies are just a little below normal.

But the real problem area is found over Oregon and northern CA, where the combination of a lack of precipitation AND warm temperatures has resulted in snowpack areas below 40%.

Looking forward, the latest European Center forecasts are predicting substantial precipitation along the Washington Coast, over southwest BC, and the north Cascades.

But far less to the south, where the real deficiencies exist.

Oregon folks should not panic yet.  

I can remember a number of years that were dry until early February, but made up most of the deficiency in late winter and spring.