December 28, 2012

Why are so many trees falling?

During the past few weeks the stories have been incessant.  

 One major highway after another has been closed for extended periods recently as hundreds of snow-encased trees have fallen over the roadways, including  SR-2 across the central Cascades, the Mount Baker Highway, RT 6 and 101, on the Olympics Peninsula.  Large deposits of snow weighted the trees down until they toppled over.

Mount Baker Highway: WSDOT Photo
A number of cars have been hit, nine people injured and tragically two were killed on December 21, just east of Leavenworth.   The Mt. Baker ski area was inaccessible for days and folks had to go the long way around (I90, 97) to get to Leavenworth and vicinity.

Mount Baker Highway:  WSDOT Photo

SR-101 Closed
SR-2 Trees Leaning Over:  WSDOT Photo
Long-time Washington Department of Transportation maintenance personnel say they have never seen a situation like this, with snow causing such extensive and long-lasting tree fall periods over state roadways.  I have been around for a while and can't remember an analogous situation.  The threat was so large that WSDOT began flying over the road with helicopters, in the hope of blowing off snow.

So were the trees falling?  We have had periods of more snow, of more wind, of more rain.  Periods that were colder. What is different this time?

Let's play detective!
When you have eliminated the impossible, whatever remains, however improbable, must be the truth....S. Holmes

The last month has been a wet one over the region and the mountains have gotten large amounts of snow.   But other years have had as much or more snow in the mountains (e.g., 98-99) without so many trees coming down.  So snowfall alone is not the answer.

What about the locations and altitudes?  The SR-2 tree falls occurred about 10 miles west of Leavenworth at an altitude of roughly 2400 ft (see mapP

The Mount Baker Highway tree falls occurred east of Maple Falls (see map, east of red A).  Elevation about 1400 ft.

 U.S. 101 on the Olympic Peninsula was closed between Shelton and Brinnon with trees downed not far from sea level.   So the elevations varied.

I think there is an answer.   There is something that all these elevations had in common during the period, something that I believe caused massive tree falls.  So let me give you my hypothesis.  Let's consider the facts, my dear Watson.

Fact 1:  We know that the trees fell when they were weighted down with snow.
Fact 2:  Snow is most "sticky" near and just below freezing (say mid-20s to freezing)
Fact 3:  During really cold periods (our Arctic outbreaks) snow tends to be lighter, less sticky, and more easily blown off trees.
Fact 4:  Major warm up periods (e.g., pineapple express atmospheric river periods) have very high freezing levels and rain at most elevations, resulting in  melting and washing off snow from trees.

There WERE some unusual aspects of the past few weeks that allowed large amounts of snow to stick on trees until they fell over.

Thinks about it.  This month we did not have any major Arctic, cold-air, outbreaks.  So no really cold, light snow that doesn't stick well.  We have NOT had any pineapple expresses or warm up periods that would melt the snow at a wide range of elevations.  In fact, recent temperatures in the mountains have been AMAZINGLY steady.   Let me demonstrate this.

Take a look at the temperatures (red daily high, blue daily low) at the Winton weather station run by the WA DOT.  Winton (MTWINT, roughly at 2000 ft MSL)  is several miles north of Leavenworth on RT 2...not far from the tree falls.  Or Nason Creek (a bit farther north at 2000 MSL as well).

Only minor temperature variations.  Temps never got above 40F and only below 20F on the 18th and 19th.  Most of the two weeks (Dec 7-21st) before the major tree falls on RT 2  had highs in the low 30s and lows in the mid 20s.  Perfect temperatures to deposit snow on trees.   And winds were generally light...and thus not blowing snow off the branches.  This is a very unusual situation.

What about the Mt. Baker Highway tree falls?  Lets consider the Maple Falls station (a bit lower--670ft--than the section with the tree falls).  AGAIN, amazingly constant temperatures in mid-December, with highs in the mid to upper 30s and lows around 30F.  Go a little higher, to where the trees were falling, and temperatures were surely cooler..right in the range of maximum stickiness and deposition.

But you ask, what about the trees (less numbers) falling near the Hood Canal?  Surely, it was much warmer there?  The answer is no....that area can be unusually cool for the lowlands, as southeasterly flow banks cool air over that area. Consider Lake Cushman, which is close to sea level, and on the SE side of the Olympics, not far from 101.  A number of day had highs in the lower to mid thirties, starting December 19th. And yes, temperatures were relatively constant.

So my hypothesis is that we had a wet, snowy period with consistently cool, but not super cold, temperatures in a range that promoted sticky snow.  No major wind events to blow off snow.  No pineapple express warming.   It all game together in a very unusual way, causing massive snow loading on trees.    Another contributor might be the mild temperatures that have left the ground unfrozen and thus less able to hold the trees in place.  Perhaps one of you have an alternative theory...if so, I would like to hear it!

Certainly, not "elementary."


  1. I agree, Cliff. The temps have also been stuck in a narrow range in West Seattle, also. So much so, that my 91 year old neighbor complained to me that his outdoor thermometer was broken. He said it was stuck on 40 degrees.

  2. By jove, I think he's got it! Seriously though, nice work.

  3. Cliff, one thing different this year is that forests have been stressed by beetle kill for several summers in a row. Hiking east of the crest shows vast swathes of brown conifer trees. They are either dead or dying. Could that be what is bringing the trees down when they are further loaded by heavy wet snow?

  4. Cliff, great information.

    I can't state enough how great it is to have you take your personal time to post on this blog. I have a buddy in Denver who is jealous that we have you doing a blog like this in the PNW.

    Thanks for all the hard work and great information!

  5. Something else worth pointing out is that above-average rainfall has left the soil saturated, and above-freezing temps have left the soil soft. Trees in super-saturated soil fall over with minimal provocation. Saturated soil + heavy snow loading = trees falling over.

  6. Might the amount of rain saturating the soil also be a factor? I would think that large amounts of rain soaking the soil before the snowfall might make it easier to uproot the trees.

  7. Well perhaps Doctor Mass, but rarely does mere deductive science have all the answers. I propound that it is the evil genius of Professor Moriarty that is behind these series of tree falls through the use of his new temperature flattening device. By flooding the lumber market with cheap state timber he can then go long on the overall construction industry thereby reaping tremendous unscrupulous profits to further his nefarious schemes. “Eliminate all other factors, and the one which remains must be the truth.”

  8. I think the location you were referring to on the map is west of Leavenworth, not east.
    Great blog though!

  9. Wow, first off great post. I love the variety on this blog.

    I had some thoughts regarding the issue. I think something else that should be considered is that all three of these areas are not places where the trees are used to having heavy snowloads for long periods of time - we're not seeing this issue at the heavily forested lcoations say at Snoqualmie Pass level or near the Mt Baker Ski Area parking lot. At the Baker location you mentioned it's at about 1400' so snowpack there traditionally is limited and doesn't stay on branches for more than a week at a time, same goes for the Hood Canal location near sea level. In regards to the US-2 location, that area is east of the crest and the trees aren't accustomed to getting the quantity of snow that we've had over the last couple of weeks. Additionally, that area usually gets lighter/drier snowpack like you have mentioned.

  10. SnowGirl's and Mike's comments seems to be the most plausible explanation. Another factor that is a likely influence is the lack of health of the tress involved. Extensive root rot or other rot from fungal infections can significantly weaken tree structure.

    Pine beetle infestations mostly occur in lodgepole pine that is found mostly east of the Cascade crest. Other than Leavenworth the fallen trees are on the west side of the crest and most likely Douglas, true firs or red cedar.

  11. Could it be we will have no snow this winter on the Sammamish plateau?

  12. Thanks for this. We were travelling from Seattle back home to Penticton (BC) and had to take I-90 rather than Hwy 2, and I was curious about what caused the treefalls that closed the road.

    I really appreciate your blog, I use it in my Weather and Climate class at Okanagan College.

  13. I guess my hypothesis that it was lost Oregon Beavers looking for a home isn't correct.

  14. The question that needs to be answered is the tree fall as great away from the road in those locations and thus more systemic or is it mainly along to road in which case other causes may be implicated?

  15. We had lots of trees down on the Clear Creek Trail in Silverdale. Sticky snow plus lots of rain and a beaver dam that has created quite a marsh softening up the trees all add up.

  16. I have a cabin in Plain just south of Lake Wenatchee which is east of Highway 2 through Tumwater Canyon and ground zero for all this mess. We had many more tree fall issues than Highway 2 and the roads have only just officially opened up, although there has been Local Access Only for a while now.

    What happened was not just the sticky wet snow but that was indeed the first event:

    We had about 12-20 inches of snow in the space of a couple of days and that snow was sticky and wet. But then it cooled down, as it does at night, and FROZE to the trees as a compact snow/ice mix. Then it snowed more, gradually and not a lot, and that just added more weight.

    Additionally, it was not sunny really at all for about a week. On Christmas eve there was a nice sunny day that was warmer than expected (40F) which melted and shed a large amount of of the snow/ice. After that the situation was much improved but Chelan County mentioned in one update that trees shaded from the sun were still loaded with snow, but by that time a lot of the danger trees were already down either by the force of nature of the hard work of the road crews in removing the leaners.

    A final and important factor noted by a WSDOT worker or spokesperson is that the ground is not frozen yet which means that trees have less of an anchor. A lot of the trees that fell were on slopes. Snowgirl mentioned this in the comments too.

    I can't thank enough Chelan County PUD, Chelan County Public Works, and all the other PUD teams and road works teams that helped. For several days there were over 100 people on the job almost day and night. This was over the Christmas holidays where these workers could have been at home with their families. The power situation is much better now but the fiber infrastructure (yes, fiber to the home in rural Chelan County!) is still being rebuilt.

    BTW, the trees out in Plain/Lake Wenatchee are a mix of Pine and Fir. I don't recall seeing a cedar out there.

    I'm heading out to my cabin today (Saturday) to assess any damage.

  17. There's likely a Fire Ecology component here as well, equally complex as the moisture/temperature factors: As Chris and Bill have offered here, tree damage from beetles and rot cannot be overestimated. Trees growing too close together become increasingly susceptible to bug larvae and especially, laminated root rot. Resultant tissue damage weakens the tree above ground (trunk snaps in icy/windy conditions) and below ground (damaged roots can no longer anchor tree, causing uprooting). But historically, periodic naturally occurring fires provided healthy thinning.

  18. I would add one more fact that played a part in this mystery at least here at Lake Wenatchee. The majority of trees that fell were Ponderosa pine and most broke off near the top. Pines branching structure isn't designed to shed snow as readily as fir and alpine species whose branches droop more readily. Pine also is not as strong as fir and is more brittle thus being subject to breaking under stress.
    What's interesting is how all these factors can come together and create such historical events. Power's back on now so Happy New Year!

  19. Consider soil composition as the main factor.

    Our great NW coniferous forests grow mostly on Glacial Till, a mixture of varying soils plus water-worn riverbed gravels. The gravels and the soil can remain as homogeneous even when saturated, but past a point, the excess water acts to free the soils from the rounded gravel, causing those stones act like ball-bearings. The soil then has very little internal cohesion left, and the root ball of the tree fails it's job of anchoring the whole organism.

    I've watched about every film clip available of these tree-blocked highways, for the past several weeks, and I have invariably seen entire trees down, not tree-tops, in the film offered.

    NOAA could sponsor some research into the transition point of the soils from just a damp but cohesive amalgam to the failed-cohesion part, with an eye towards issuing tree-fall alerts when models show the right combinations of water, temperatures and snow-loading. In fact, I wouldn't be surprised if there hasn't already been a lot of that research done. After all, trans-mountain road-building has been done in these parts for a full century now, and the science to discover the super-saturation point of glacial till is not something we would need computers to discover in the laboratory.

  20. Cliff,
    Nice little analysis. The relevant question however is WHY do the trees fall towards the roads and not towards the forest. Please give that some thought and post your results.

    snowing in Toledo Ohio...excellent snow removal here in NW Ohio. Lot's of salt!

    1. Lots of trees were down in the Fraser Canyon where there is an usual amount of snow.

  21. The same superglue snow has been around here in Spokane. The total accum is about average for this point in the season, and it's come gradually, with no huge storms. But each new fall sticks firmly to branches and wires, and the snow is 'glutinous' for lack of a better word. It pulls like dough instead of crumbling like bread.

  22. I live in Plain.

    Most of the trees did not start falling until the very heavy snow began falling Monday afternoon into Monday night..

    granted, alot of the snow fell before that, but the trees were not too bad until Monday PM.

    anyway, I was out in the mess trying to get home Monday night and while the moon was out in Leavenworth, about 5 miles south of Plain, we hit the heavy snow all at once...I looked at radar, and followed it all evening on my iphone... this is when a small by persistent PSCZ set up over the area.

    I think it's a combination of factors many listed above, but I think the 'icing on the cake' was the intense PSCZ that dumped roughly 10-15" in about 6 hrs on top of the 1-2 ft already on the trees.


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