Why all the landslides this winter in western Washington?

The reports of landslides, slope failures, and blocked train tracks has attracted a lot of attention by the news media this winter.    Burlington Northern officials suggest this is one of the worst slide seasons in one-hundred years, with a particular focus on their tracks between Edmonds and Everett.    Nearly 200 slides have occurred this winter, with 56 of them large enough to reach the tracks.  On Sunday another slide stopped trains service between Seattle and Everett, after a very rainy day on Saturday.  A huge slide on southern Whidbey Island has isolated dozens of homes and even Governor Inslee came to take a look.   Yesterday, I got a call from a reporter (from KUOW of all places!) wanting
to know whether this was a symptom of global warming.

So what is going on?   Is something unusual going on or is this some random bad luck?  Could human intervention on the landscape be partially to blame?

Let's give this a look.   Using the Western Region Climate Center's website, we can plot the percentage of normal precipitation for various periods.  First, for the past 90 days, we see that most of the state has been quite dry, except for around Everett, where 100-120% of normal has fallen.  A bit wet, but quite frankly, nothing very unusual

 If we go back 6 months, the region around Everett stands out more, reaching 130-150% of normal.  Yes, a bit wet, but I suspect we could find other periods to equal it.
 

As a next step, I plottedt the cumulative precipitation at Paine Field in Everett.  For the last twelve weeks, it has been drier than normal (by about an inch).

We can plot the daily precipitation during that period against the daily records (see plot).  Only one day (last Saturday), slightly beat a daily record.  Not impressive.

Plotting the last six months, we see that the cumulative precipitation is substantially above normal (about 5 inches), with the big contributor being in November and December).

 Plotting the daily amounts against the daily records, we see something interesting--

There were a lot of big rain days in November and December, with several daily records.  We can plot the precipitation at Paine Field since 1 January 2006 (see below).  November/December 2012 was clearly an unusually wet period.

At nearby Everett Junior College 8.61 inches fell in December, the most since 1933 (when 10.81 inches hit the gauge).

I am not a geomorphologist, but it appears that the heavy precipitation in the fall set up the area for slides, even though the winter precipitation was not that unusual.  Perhaps one o you can explain how this can happen.

Now there has been a lot of loose talk about all these slides having something to do with a trend in precipitation, perhaps associated with global warming.   Well, let's use a tool on the Washington State Climatologist  site to plot the trend of precipitation at Everett (see below).  The trend is downward...it is getting a bit DRIER at this location over the past century.   So no reason to expect increasing precipitation trouble in the near future for that area and no reason to place the blame on human-induced global warming.

Another factor that should be considered is non-meteorological:  the substantial disturbance to the slopes by development on its crest and folks mucking about on the slopes.  Do vibrations from the trains have an effect or the drainage from impermeable surfaces on the top?  Here is an example of exactly that (click on image).

 

http://www.youtube.com/watch?v=-yBr-H2yskM

All these slides provide another good reason not to send a lot of coal trains up that route.  Can you imagine the increased vibration load of a massive increase of coal trains?