A Tale of Two Decembers

An image stolen from Cliff Mass's blog post about Seattle's December 17, 2012 high water event

Do you remember last December? Likely you do, but perhaps not for the reason that I am thinking of. Last December (2012) was notable for the high water and coastal flooding that wet the shoreline of most of the Puget Sound region. That high water event was due to a convergence of really two factors - high astronomical tides (also known as "king" tides around these parts) and big "non-tidal residuals", or the component of water level that is not due to predictable tidal forces. Its worth noting that the non-tidal residual is really made up of a bunch of processes (that Cliff Mass describes here, I have described here, and others have described a bunch of places)...but for simplicity's sake I'm just going to treat collectively.

Water level data from Seattle from the December 17, 2012 high water event, which matched the previous record high water event from 1983. This plot includes the predicted water level (the blue line), the actual water level (the green line), and the difference between the two, which I term the "non-tidal residual" (the red line).

This December has been pretty mellow from a coastal flooding standpoint...so what is the difference? Last December the highest predicted tide during December in Seattle (as an example) was 3.91 m above MLLW, which is just about identical to this year's highest predicted high of 3.907 m above MLLW on Decmeber 6. Furthermore, the average predicted water level for December 2012 was 2.11 m above MLLW, while for 2013 it was the same. In other words, there is not a huge amount of difference in the astronomical tidal water level.

Seattle water levels for December 2012 (top) and December 2013 (bottom). The red line in each plot is the predicted water level, and the blue line is the actual water level.

The big difference between the two years was in the non-tidal residual, or the part of water level that is not predicted by the astronomical tide. In Puget Sound the non-tidal residual is most closely linked to atmospheric pressure, though it can also be due to other processes. Just a few days ago, for example, La Push, WA on the outer coast experienced large non-tidal residuals that were likely due primarily to wind piling water up against the coast.

The plots above, which show the predicted and actual water level from December 2012 and 2013 are sort of hard to interpret, so the plot below should help to clarify the different between the two months - this shows the difference between the predicted and actual water level for the two months:

So what should be clear is that in 2012 December water levels were much higher than those in December 2013, not due to "tides" (at least in the way that we usually think of them as being driven by astronomical forces), but rather due to very high average non-tidal residuals that lasted pretty much all month. In fact, the average of all measured water levels in December 2012 was 2.28 m above MLLW, while in December 2013 it was only 1.92 m above MLLW, a difference of over 14 inches. And when it comes to coastal flooding, 14 inches of water level makes a big difference...