Is West Coast Weather Getting More Extreme?

A question that is often asked is whether Northwest or West Coast weather is getting more extreme.  More heavy rain and flooding events?  More windstorms?  Extreme losses of snowpack in the mountains?   Some groups have answered these questions in the affirmative and have stated, suggested or hinted that such extremes are connected with anthropogenic global warming.  They often provide hand-waving explanations of such extreme behavior--for example, it is accepted that the atmosphere will gain water vapor as the planet warms (by roughly 7% per degree C).  Won't that cause extreme precipitation to increase?

My students and I have looked into this issue, as have other weather/climate scientists.  As I shall discuss, this is not a simple issue and uncertainties abound, but the bottom line is that there is little evidence for area-wide increases in extreme weather due to human-induced global warming during the past several decades over our region.  That is not to say there won't be any in the future, but the impacts are anything but clear.

Lets take extreme precipitation. With UW graduate students Mike Warner and Adam Skalenakis, we looked at the trends of heavy precipitation over the past 60 years at coastal stations from southern CA to British Columbia. Specifically we looked at the trends for the top 20, 40, and 60 events of two-day precipitation.  Some of the results are shown in the figure below. Southern/central CA is a mixed bag, but northern CA and southern Oregon show a DECLINE in heavy events. On the other hand, northern Oregon and WA have seen an increase in the big rainstorms.  BC show weak upward trend.

Now heavy precipitation, of course, has a big influence on rivers...what do big river discharge events look like?  Well, we looked at the trends over the same period of maximum annual discharge of unregulated rivers (see figure below).  Importantly, we got the same pattern! Decline over southern Oregon and northern CA, increase to the north.

Up arrows indicate increasing trend, down arrows, declining trend for max annual river discharge.

So there is no uniform change in extreme precipitation along the West Coast:  some places are up and some are down.  Some nice work by members of the Climate Impact Group at the UW, looking at the output of a variety of global climate models, suggests that this pattern is the result of natural variability and not the result of any kind of global warming signal.
    What about big windstorms?  Storms such as the Inauguration Day Storm and Chanukah Eve storm?   Fascinatingly, the pattern is the same over the past half-century--- upward trend over Washington and northern Oregon and decreasing frequency of such storms to the south.  We are trying to understand exactly what is causing this configuration.

The Chanukah Eve Storm of 2006

   What about the next fifty years or so?  Will global warming change extreme weather events over our region?   The honest answer---we don't know.   The computer models do not agree.   And one can easily think of scenarios where there will be a decline of extreme precipitation and storms.   Extreme weather events are closely associated with the jet stream.  Most climate models indicate the jet stream will move northward under global warming.   Will the most extreme events gets displaced northward?

   In short, the changes in extreme weather along the West Coast has been non-uniform and don't suggest a human-induced global warming signal.  Global warming due to our greenhouse gas emissions will be very significant, but the local implications regarding extreme weather are quite uncertain.