December 28, 2019

The Deadliest Weather Phenomenon in the Northwest

What is the deadliest weather phenomenon in the Pacific Northwest?

A major Pacific cyclone, with gale to storm force winds?

Heavy rain and flooding associated with intense atmospheric rivers?

Thunderstorms or lightning?

I don't think so.  From my experience, the biggest threat is from a weather feature that most folks would think benign:  FOG.  And particularly its freezing version.

Consider some the headlines in the news this fall.

Two weeks ago there was a 15-car pile up in freezing fog on the Umatilla Bridge on the WA/OR border.

Plus,  multiple accidents in Richland at the I-82/182 interchange the same day.

Two days before, 4 cars were in accidents  in fog on the Palouse Highway.

I could give you dozens of other examples.  Early in my career I did some forensic meteorology, providing information for legal cases.   The number one issue for such cases was freezing fog resulting in serious accidents.  And some accidents occur without freezing fog--just folks driving to close and fast.  A pack of cars enters a fog bank and  the first car slows down, followed by chain collisions of cars behind.

Fog is particularly a problem during cold weather.  Frost can make a roadway slippery, but generally doesn't create a think layer of ice on the roadway.   But fog is a different story.  Around here, fog is generally made of liquid droplets, even when temperatures are below freezing.  The water is supercooled and when the fog goes over a roadway that is chilled below freezing, the fog droplets can freeze on contact, producing a significant ice layer.

So anytime temperatures shown by your car thermometer or the local weather reports get below 35F, and fog is around, you should slow down and maintain a generous following distance with the car in front of you.  Why 35F?  Because in winter road temperatures at night are often cooler than in the atmosphere immediately above.  Freezing fog obviously has a double threat---visibility can be very poor, so when one car hits slippery conditions and slows down, other cars, driving too close and fast, slams into it.

Eastern Washington is particularly vulnerable to freezing fog this time of the year, as cold, foggy air tends to pool into the Columbia River Basin.  The Willamette Valley is also a problem location, with the south Sound and SW Washington often have issues (see satellite picture below).  So during foggy conditions, conservative driving is good idea.


  1. I drive public transit in an eastern Washington "bowl" valley. We sock in very often, and you are quite right. I can safely handle almost anything, but the glare ice from freezing fog has me telling dispatch: "If you direct me to continue, it is over my professional judgement, and I will file a grievance if I crash."

  2. I remember back in 1964 driving a school bus when I was going to UPS. January could become very cold a foggy on the morning drives. I remember one foggy during January I clipped with my rear tires one of the lane dividers, It was cold morning with fog. The tarmac appeared dry, but suddenly my bus with 34 kids started sliding, which ended up as a 180. I was very lucky at the time to be in light traffic pattern.

  3. Do you have any statistics on how many people are killed by freezing fog compared to being killed by falling trees/limbs in our bigger windstorms? I am terrified to drive in a windstorm, especially living in a forested rural area on the west side of the mountains.

    1. This would be interesting to know. As a PNW transplant, one of my earliest weather memories from the area was of a couple being killed when a tree fell on their vehicle while driving around Lake Whatcom.

  4. Also, when driving in fog, please turn on your headlights! Don't rely on the small lights these new cars have on whenever you drive because tail-lights are needed too! The number of people around Ellensburg who drive in thick fog without lights is huge. I don't understand that and hope we survive it.

  5. Maybe exacerbated by the freezing fog forming after the sun sets, in the evening hours when there are more drivers driving under the influence of alcohol and/or pot?

  6. Tailgating while texting is an American right! We don’t need no stinkin classical physics. MAGA!

  7. Following distance is probably the single factor that can prevent these crashes. Both in good and adverse weather. That means 2 seconds between you and the next car in good weather and light traffic conditions. Add one second for each of: heavy traffic, reduced visibility, precipitation or frost. 5 seconds may not be enough in snow/ice. Some people are hyper vigilant and may be able to drive closer without issues for fair weather short drives in Town, but it can become a dangerous habit and false sense of security when the weather is bad and/or you're out on the open road for hours between rest stops.

  8. The state should do a public interest documentary on safest methods of driving in fog. Things like turning on your lights, slowing down, actually paying attention to your cars outdoor thermometer temperature, adjusting your schedule so you don't drive in fog at all. Another thing I don't understand is why our speed limits don't go down slightly after dusk. Your vision is greatly reduced and many states DO have a nighttime speed limit.

  9. Fog lamp headlights don't seem to work very well because you just illuminate the dense fog, and cannot really see distant objects any better.


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

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