East vs. West Precipitation Beat Down

"Extreme precipitation events, associated with landfalling atmospheric rivers in California have been historically comparable, in magnitudes and frequences (for 3-day precipitation totals), to those mostly associated with landfalling hurricanes and tropical storms in the southeastern US and are among the largest historical storms in the conterminous US."

- Dettinger et al. (2011)

In the wake of the Hurricane Isaac deluge (see previous post), I thought it would be interesting to compare extreme precipitation events in the eastern and western United States.  To do this, I'll be taking advantage of a recent paper by Dettinger et al. (2011), which appeared in the open access journal Water and should be freely available here.

Let's begin the East vs. West precipitation beat down by examining the number of nonoverlapping 3-day precipitation totals that exceed 40 cm at cooperative weather stations in the contiguous United States from 1950–2008.  Many stations in the southeast United States have observed one such event, and a few that have observed more than one.  In the western US, California also sees its share of events, including several sites that have observed more than 5.

Number of events with nonoverlapping 3-day precipitation totals
of 40 cm or more from 1950–2008.  Source: Dettinger et al. (2011)

Dettinger et al. (2011) argue based on this figure that the magnitude and frequency of extreme 3-day precipitation events in California is comparable to that found in the southeast United States, which are associated primarily with landfalling hurricanes and tropical storms.

Making this all the more interesting is that the precipitation climate of California is far more variable than that of the southeast United States.  Dettinger et al. (2011) illustrate this using the coefficient of variation of water year (October–September) precipitation.  The coefficient of variation is the standard deviation divided by the average, with larger values indicating greater variability relative to the average.  As illustrated by the plot below, California observes the largest coefficient of variations in water year precipitation in the United States, with values at some locations exceeding 5.  

Source: Dettinger et al. (2011)

All of this tells us that California has the ultimate feast-or-famine precipitation climate. 

Many of California's biggest precipitation events are produced by atmospheric rivers, narrow filaments of moisture that are frequently found ahead of cold or stationary fronts.  Many of these atmospheric rivers originate in the tropical moisture reservoir found in the low latitudes near the equator.

Source: Dettinger et al. (2011)

Unlike hurricanes and tropical storms, which occur primarily from June through November, atmospheric rivers can affect California throughout the October to April cool season.  Further, precipitation during atmospheric river events can be strongly enhanced over California's coastal and inland mountain topography.  

There are many ways to examine extreme precipitation events, so Dettinger et al. (2011) does not yield a knock out in the East vs. West precipitation beat down.  It does, however, show that precipitation events in California produce accumulations at individual stations that are comparable in magnitude and frequency to that of landfalling hurricanes and tropical storms in the eastern United States.