A Damaging and Deadly Convergence Zone

Most Puget Sound convergence zones are benign affairs that produce a band of cloud or showers between Seattle and Everett, and during the winter there is usually some enhancement of snow in the central Cascades.  But yesterday's convergence zone was particularly intense, enhanced by very unstable air aloft and a persistent flow approaching the coast from the northwest.  It resulted in the deaths of two individuals, the temporary closure of Snoqualmie Pass, and power outages in Seattle.

As many of you know, Puget Sound convergence zones usually occur when the flow from the northwest approaches the Olympics Mountains; some of it goes through the Strait of Juan de Fuca and the other portion goes around the south side (the Chehalis Gap.  An area of low pressure tends to develop on the leeward (eastern) side of the Olympics and the result is an area of convergence, in which the two air streams are forced to rise--producing the clouds and precipitation.

If the air is less stable (tendency to rise vigorously when lifted), the convergence zone is really juiced up.   Such "juicing" is more prevalent in spring when the atmosphere is less stable, since temperature tends to decrease more rapidly with height in this season.  The atmosphere is still cool from winter, but the strengthening sun warms the ground and the air next to it.  Big changes of temperature with height can result in instability and convection...just like your cereal pot when you turn on the burner.   A visible satellite picture yesterday clearly shows convective showers offshore (cold air over relatively warm water) and the solid clouds over the mountain and in the convergence zone.

The convergence zone is also clear in the radar--here is a sample from yesterday.  You can see the enhanced precipitation extending ESE towards the Cascades and I90.

We can get the total estimated precipitation using our weather radar for the entire "storm" and here is the estimate from the NWS Camano radar.   Suggests 2-3 inches of liquid water content over the central Cascades..multiply this by 5-15 for snow depth, depending on how wet the snow is.

Here is the measured snow depth at Snoqualmie Pass (Alpental).  Snow increased from roughly 91 inches to 115... TWO FEET of new snow.   And the snow was relatively wet.  No wonder there were avalanches and road closures.

 The  convergence zone produced some unusually intense thunderstorms over central Puget Sound, particularly in Seattle.   I counted 4 lightning strikes at my home...my dog and cat headed for the hills!  Here are the lightning strikes in the middle of the most intense part of the event in Seattle, the half-hour ending at 5 PM.

The intense thunderstorms brought heavy rain and small hail.   Here is the proof at Shilshoe Marina in Seattle taken by Michael de Haan.

 Power was lost to nearly 15,000 city light customers (see map below) and resulted in loss of power to portions of the University of Washington, including my department (and its computers).

But the most serious aspect of this event was the tragic loss of life of two individuals that were snowshoeing in the mountains.  It can't be stressed enough how important it is to check the forecasts and particularly those provided by our excellent Northwest Weather and Avalanche Center. The forecasts were for heavy snow in the mountains, as illustrated by the UW WRF model 24-h forecast from 5 AM on Saturday.  1-2 feet were predicted in the central Cascades.

 The avalanche center had warnings of considerable danger (orange colors on the figure below)

 Please check with the Avalanche center before heading out to the back country and stay away from steep slopes if the danger is considerable or higher.  If you do, make sure you do not go alone and are equipped with an avalanche radio beacon and probe poles.

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?