Tuesday, March 25, 2014

The Oso, Washington Landslide

Monday, March 24, 2014
1:54 a.m.

Retrieved from WSDOT Flickr page

Sorry about the delay in postings. End-of-quarter finals unfortunately usually take higher priority over weather posts. This isn't always the case during the end of fall quarter, as we are in the thick of the storm season and lowland snow is a very real possibility, but in the middle of March, there's usually not too much in the way of meteorological mayhem happening. Our storm season has passed, and the threat of a major lowland snow storm is long gone. I'm afraid you'll just have to wait until November.

The weather's been pretty nice since I got out of finals. But it was rainy as hell earlier in the month, as I'm sure you are all aware. November, December, and January posted a paltry 9.15 inches of rain at Sea-Tac, whereas March has put on 7.71, and we still have a week to go! We could very well pick the 1.44 inches we need to tie the sum of these three months before March is up, and if that happens, that'll have to set some sort of record. When you factor this on top of a February that was also much wetter than average, you begin to really see how delayed our rainy season has been.

All of this rain has saturated the soils, and unfortunately, this saturated soil contributed to a massive landslide on the Stillaguamish River near SR 530. Outside of the Mt. St. Helens landslide (which was not due to soil saturation), this is the largest landslide I can recall ever seeing in the state of Washington. Take a look at the photos below... the size of this thing is colossal.

Aerial photo of the Oso mudslide taken by the Washington State Patrol on 3/23/2014. Retrieved from  http://en.wikipedia.org/wiki/2014_Oso_mudslide

Dave Norman, a state geologist, said that this is "one of the largest landslides he's seen." After surveying, the slide was found to be 4,400 feet wide, 1,500 feet long, and 600 feet tall, with a 30-40 feet debris field. In addition to wiping out a neighborhood of 30 homes and blocking Highway 530, the slide dammed the North Fork of the Stillaguamish River. There were serious concerns about the dam suddenly collapsing and unleashing biblical-like floods downstream, but the river eventually gradually poked through the dam without having any catastrophic collapse take place.

Take a look at the hydrograph below for the Stillaguamish River, and look how the flow increases sharply right after the slide, then decreases. I would think that this increase would be due to the material in the slide pushing a bunch of water downstream as the slide surged down the hill. Of course, once the river was dammed up, the flow decreased dramatically.

As I mentioned earlier, this dam finally gradually let some water through. The following pictures are from the Washington State Department of Transportation's Flickr page and were taken on Sunday the 23rd.

In the midst of the horror of this event, we can at least take solace in the fact that we did not have any flash flooding associated with a catastrophic dam failure, which would have certainly caused more death and destruction, especially if it had occurred at night when everybody was sleeping. 

By Monday, the dam was continuing to erode away and flows downstream were continuing to increase. The photo below is from the Associated Press (bad, I know...), but it was such a great picture that I had to show it here.

So why did this slide occur in the first place? Well, the saturated soil certainly contributed to the slide. The North Cascades have been the wettest place in the state over the last month, with some locations picking up as much as 30 inches of rain. That's pretty insane for March. But the thing is, when it comes to sheer precipitation amounts, 30 inches of rain in one month isn't all that unusual for some locales. I remember in November 2006 when Mt. Rainier National Park got 18 inches of rain in 36 hours. THAT, my friends, is extraordinary. 30 inches of rain in three weeks is wet, but not unprecedented. This hill has survived much wetter spells, so why did it decide to give during this one?

It turns out that this hill has actually had quite a history of instability. In fact, according to a recent Seattle Times article, it is known by some simply as "Slide Hill." The article mentions four major slides (1949, 1951, 1967, two slides... one in 1949, and one 1951 . The 1949 slide was 1000 feet long, 70 feet tall, and affected a 2,600-foot stretch of the Stillaguamish, and the 1951 slide formed a small dam, giving rise to creeks in the area known as "Slide Creek" and "Mud Flow Creek."

The main cause of these slides was found to NOT be soil saturation, but to be that the river is constantly eroding away at the hill. The more it erodes the hill, the less support there is at the bottom to hold the rest of the hill up, and you get a large landslide. Saturated soil no doubt speeds up the process, and it may have been the "straw that breaks the camel's back," but this slide was not due to soil saturation alone.

In 1999, the Army Corps of Engineers issued a report warning of "the potential for a large catastrophic failure" on Slide Hill. Risk mitigation efforts began, but these were delayed when a 2006 slide moved the course of the river 730 feet. During the summer of 2006, a 1,300-foot wall of boom logs and concrete was created to prevent the river from eroding, and although this was to protect fish from sediment erosion, it also acted to stabilize the hill and hopefully lower the risk of any landslides happening. Of course, it was no match for Saturday's disaster.

Here are some more pictures of the disaster from WSDOT's Flickr site.

Below is one of the most powerful and terrifying pictures of the disaster. Comparing the slide with the size of the trees really gives you an idea of its immensity.

This is one of the most powerful and terrifying pictures of the disaster. Comparing the slide with the size of the trees really gives you an idea of its immensity. 

Here, you can see how SR 530 is completely submerged under a pile of mud, and you can see a few lucky homes that just escaped the slide.

These fallen trees bear a resemblance to those that were felled due to the pyroclastic flows from the Mt. St. Helens eruption of 1980.

Finally, the picture below was not retrieved from the WSDOT, but from Twitter. I forget who specifically I got it from. It shows Oso before and after the slide, and it is a haunting photo.

There were 14 fatalities and 176 people unaccounted for last time I checked. I can only hope the latter goes down and the former doesn't go up. If you want to help, you can donate money to the Red Cross specifically for the Oso slide (they have enough food and clothes and don't need any more) by calling 1-800 RED-CROSS or visiting redcross.org. A Disaster Relief Account for the victims has been set up at Union Bank in Arlington (525 N Olympic Ave), and you can send checks addressed to the Cascade Valley Hospital Health Foundation or donate online at http://www.youcaring.com/nonprofits/cascade-valley-hospital-foundation-disaster-relief-fund/154422.

The biggest thing you can do to help right now outside of donating a million bucks is not going to the site of the slide. These people are working their tails off to find anybody who might be alive, and we don't want to interfere with them and put anything else on their plate.


Monday, March 3, 2014

A Good Soaking

Monday, March 3, 2014
9:57 p.m.

I don't want to say winter is over. Heck, Snoqualmie pass was closed from North Bend to Ellensburg today, which I believe is a 70 mile stretch or so. Bellingham got clobbered with snow twice in the past two weeks. But, we face an unfortunate reality - it is March, and we are warming up. In fact, we are warming up fast. Highs this week are expected to be in the mid-50s down here, and there will be plenty of times in which the passes will see liquid precipitation instead of the white stuff for a change. These temperatures aren't just affiliated with warm, subtropical patterns; as the sun gets higher in the sky, surface temperatures warm up further during the day. Our December lows average around 36, and our current lows average around 38... a two-degree difference. Our highs, on the other hand, average 52 degrees for this time of year as compared to 45 for much of December... a seven-degree difference. The pool of arctic air to our north is becoming warmer and weaker, and it is getting harder and harder for us to cool down to a point where we can get sea-level snowfall. In Seattle, snowfall chances tend to drop off dramatically after mid-February.

Highs this week will be in the mid-50s, but these temperatures will not principally be because of increased solar insolation. In fact, our lows will hover around the mid-40s all week long. What we have is a fairly continuous flow of moisture flowing off the Pacific coming into our area interspersed with brief periods of sunshine. I talked about this in my previous post. But now I want to focus on two specific storms that could be quite a doozy,

The ironic part is that these cyclones are actually relatively weak. They are dissipating as they approach the coast, and as you can see below, by the time the Wednesday night storm makes landfall on the coast Thursday afternoon, the low will have degraded to 996 millibars. That is a very weak low considering how much rain is involved with this system. However, it goes to show that you don't need a deep low pressure system to get flooding rains. On the other hand, you need sharp pressure gradients to get wind associated with a windstorm, and these sharp pressure gradients are more common with deep, powerful cyclones than small, weak ones.

Valid 01:00 pm PST, Thu 06 Mar 2014 - 69hr Fcst:   www.atmos.washington.edu/~ovens/wxloop.cgi?mm5d3_wssfc+///3

I should mention that there is a fair amount of disagreement between the models in terms of the timing of the rainfall. The GFS model brings periods of rain in Tuesday night and Wednesday night, with the cold front sweeping through on Thursday. The EURO gives us a similar picture but also gives us some rain Tuesday morning. The NAM on the other hand is different entirely, bringing periods of rain Tuesday afternoon and Wednesday morning, with the cold front sweeping through Thursday afternoon. Right now, I'm sticking with a blend between the Euro and the GFS. Majority rules, right? Also, these models are generally more accurate than the NAM.

Take a look at the picture below. It shows the precipitation predicted over the past 72 hours from 4 a.m Tuesday to 4 a.m. Friday. If you look closely, you can see a region of 10-20 inches on the windward side of Mt. Rainier. That is a tremendous amount of rain. This won't be a record flooding event by any means, but any river in Western Washington could flood, with rivers off the Olympics and the Snoqualmie Basin at the highest risk. The Skokomish will flood. I would bet my life on that.

Valid 04:00 am PST, Fri 07 Mar 2014 - 84hr Fcst:   www.atmos.washington.edu/~ovens/wxloop.cgi?mm5d3_wa_pcp72+///3

We'll need to keep an eye out for landslides. The National Weather Service actually issued a Special Weather Statement highlighting this risk. We've had quite a bit of rain recently, and this additional heavy rain on saturated soil. I remember when there was a massive landslide near our house not too long ago. It seems unlikely that an entire hill would give way, but it does happen. If you live on a hill or steep bluff, take note of this. The landslide risk will diminish Thursday onward as the rainfall lightens up.

Stay dry! Or at least make a decent attempt. :)


Sunday, March 2, 2014

Another Wet Week Ahead

Sunday, March 2, 2014
5:55 p.m.

And the hits just keep on coming.

After our not-so-close brush with snow (it's easy to get excited looking at models beforehand, but when push comes to shove, this is Seattle, and we live right next to Puget Sound and the Pacific Ocean and are protected from cold air by the Rockies AND Cascades. Cliff Mass actually has actually has an excellent post on our protection from cold air here), it's time to get back to a warmer, milder, and, dare I say, wetter pattern. We actually had some brilliant sunshine over the past week, but alas, that is coming to an end. Let's take a look at the current upper level flow, and how it is expected to develop over this week.

Valid 04:00 pm PST, Sun 02 Mar 2014:   http://www.atmos.washington.edu/~ovens/wxloop.cgi?mm5d1_x_500vor+///3   

As you can see, the flow we have coming in is pretty zonal; that is, it is coming directly off the Pacific and is in a more-or-less straight line as opposed to being curvy with lots of ridges and troughs everywhere. When we get a pattern like this, we get storms, and if the jet stream has sufficient velocity, some of these storms can be quite strong. This was the pattern we were in for much of February when we were piling up snow in the mountains, and it looks like we will pile some more snow up there this week. However, snow levels will be higher, so Snoqualmie will likely alternate between periods of rain and snow.

Speaking of snow, it is snowing like no other up there. It's 19 degrees at the summit with chains required on all vehicles except all-wheel drive. The picture above is at Hyak (Summit East) @ milepost 54 on I-90, which is two miles east of the crest. Too bad we didn't see any snow like that down in Seattle.

Valid 01:00 am PST, Thu 06 Mar 2014 - 93hr Fcst:   www.atmos.washington.edu/~ovens/wxloop.cgi?mm5d1_x_500vor+2014030212///3

As we go forward through the week, a trough will approach our coast and affect us Thursday. See that slight ridge behind the trough? That'll come in on Friday.

Valid 10:00 am PST, Fri 07 Mar 2014 - 126hr Fcst:   www.atmos.washington.edu/~ovens/wxloop.cgi?mm5d1_x_500vor+2014030212///3

This will be a very progressive pattern, meaning that storms will come through pretty quickly, and the ridges between storms will swing through the area instead of setting up shop and shunting the jet stream to the north. After this ridge sweeps through, we'll be back in our ol' trusty zonal flow for the foreseeable future.

Throughout the period, highs will be in the mid-50s. As I said before, the snow level will unfortunately be higher for this series of storms. In fact, levels are predicted to skyrocket to 6-7000 feet in the Central Cascades with a rather strong storm that is predicted to come on Wednesday. I'll have more information on this guy in a post tomorrow. There is a possibility that the passes may be insulated from the warming at times due to cool air from eastern Washington flowing westward through them. Snow levels should be significantly lower - 3-4000 feet - in the North Cascades.

It may not seem like it, but spring is definitely in the air! I tend to get my allergies really early in the season, and I was definitely feeling them this past week. Hopefully the rain this week will make my days a little less miserable. :)