December 16, 2019

Freezing Fog Caused Mayhem in Eastern Washington

In eastern Washington, some of the most dangerous conditions in winter occurs when high pressure is overhead.

The cause?  Frequent freezing fog within the "bowl" of eastern Washington.

Today was unfortunately a good example, with DOZENS of accidents on icy roads and the Umatilla Bridge to Oregon shut down for hours.  I-82 was blocked for nearly 5 hours.

During mid-winter (really mid November through mid-February), cold air settles into the lower elevations of eastern Washington, and in this cold air their is sufficient moisture to allow temperature to drop to the dew point, allowing saturation.

The result?  Fog.

The satellite image today around 11 AM showed the cold, foggy layer quite well (see below).  Such fog layers have a relative uniform, white look to them.

Such fog layers are often topped by an inversion, after which the air warms and dries for several thousand feet.  This warmth is usually associated with sinking air associated with high pressure over the region (see schematic).  High pressure also is associated with little cloudiness aloft, that allows good radiational cooling at the surface.

Such trapped foggy air is not unusual over eastern Washington during mid-winter and is not only depressingly persistent, but when temperatures are below freezing (as they frequently are), major trouble can occur:  freezing fog.

Even when temperatures are below freezing by several degrees, the fog can still be made up of liquid water droplets.  This is called supercooled water.  When such supercoooled droplets in the fog strike a cold surface (such as a cold roadway or a cold vehicle), they can freeze on contact.  That is what happened today and has happened several times during the last month.  The fog may look ordinary (see below), but the dangers are real.

And in a few minutes, freezing fog can put down a substantial coating of ice that makes driving treacherous (see a view of Walla Walla this morning, below).

Bridges are usually the first place to ice up quickly, because they lack the upward conduction of heat from the soil below.

The bottom line: you can save your life and those of others by remembering to slow down and defend a substantial following distance when fog is around and the temperature on your car thermometer or the reading reported a local airport drops to around 35F. 

Why not freezing?   Because the temperature of the ground can be cooler than the air temperatures a few feet above!

The National Weather Service has freezing fog warnings out for parts of eastern WA be careful.


  1. This 'fog bowl' phenomena is a very interesting geographically. It would be very useful to see a mapping distribution of the areas generally affected. Is it typically below a certain elevation?
    The coastal fog, and tule fog, zones in California are well mapped. The coastal fog zone is in very close approximation to the native distribution of Coastal Redwoods.
    Is there any similar marker for this 'fog bowl' distribution?
    Any mapping source on this would be appreciated.

    1. I want to know too. Is Yakima a fog problem spot as it has a gradual rise in elevation going west.

  2. cliff i would recommend a blog on the pass weather ASAP because stevens pass is supposed to get hit hard core WednesdayS now. High near 29. East wind 5 to 7 mph becoming south in the afternoon. Chance of precipitation is 80%. New snow accumulation of 2 to 4 inches possible.
    Wednesday NightSnow. Low around 25. East wind around 9 mph becoming south after midnight. Chance of precipitation is 90%. New snow accumulation of 3 to 7 inches possible.
    ThursdaySnow. The snow could be heavy at times. High near 30. East wind 7 to 10 mph, with gusts as high as 20 mph. Chance of precipitation is 90%. New snow accumulation of 7 to 11 inches possible.
    Thursday NightSnow before 2am, then snow, possibly mixed with freezing rain between 2am and 3am, then snow after 3am. The snow could be heavy at times. Low around 29. Chance of precipitation is 100%. New snow accumulation of 12 to 18 inches possible.
    FridaySnow. The snow could be heavy at times. Snow level 4200 feet. High near 33. Chance of precipitation is 100%. New snow accumulation of 10 to 16 inches

    1. Pass weather is already available via the WSDOT website

  3. I've driven through that freezing fog before in past winters and always at night for some reason. :)

    Just gotta drive slow, and we had to pull over occasionally to unfreeze the headlights and windshield wipers, they would both ice over and stop working while driving.

  4. I drove eastward, down vantage hill 2 days ago at 5pm. I have driven this road hundreds of times and know every curve by heart. I got passed by people who were probably going close to 80 mph, in dense, thick freezing fog, 28°. I was white-knuckled going 62 mph. What is wrong with people?

  5. I agree with the previous post. Komo is calling for near record rainfall and huge mountain accumulations. Would love to see your predictions.

  6. Cliff, Your weather warnings are very helpful! I wasn't aware of the fog bowl issue. Hope everyone was safe.


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