The water year is a natural precipitation measure along the U.S. West Coast because we typically get very little precipitation over the summer and precipitation generally is not significant until October.
Thus, there is a hydrological reset each summer, with the soils dried, the snowpack melted, and the rivers dropping to low early fall levels. So October 1 is a good date to start the new water season.
By late September there is generally very little snowpack left in the Cascades
Want proof? No problem. The National Weather Service has some wonderful water year plots, with dark green showing actual accumulated precipitation this water year and light green indicating normal values (shown below).
Here is the plot for Seattle-Tacoma Airport for water year total precipitation and snow. Amazing... Sea-Tac passed the normal water year amount during the past week...and the show is not over yet. The late winter has been very wet--particularly early February and the last few weeks. More snow than normal too...mainly from a crazy local snowfall in early February. Note how the light green curve (normal year) plateaus out after May 1. And in normal year, the bulk of our precipitation falls between Nov 1 and April 1---our wet season.
Seattle Tacoma Airport Water Year Plot
But let's not stop there. Let's head to the Washington coast at Hoquiam, where the total amount (63.25 inches) is just behind the normal water year. They will cross the line this coming week.
Hoquiam Water Year
But why only look west of the Cascade crest? Consider Spokane, where the total (17.66 inches) is already well past the normal water year.
Spokane Water Year
Another way to look at the water year situation is in map form. Here is the percent of average precipitation precipitation for the current water year (again, since October 1) for the entire West (courtesy of the Western Region Climate Center). Virtually the entire West Coast has higher than normal precipitation, with California, Montana, and eastern WA being particularly wet.
Now, a question many of you are asking is whether this anomalous year is an indication of a trend towards wetter winters and thus might be the sign of some cause (like global warming). To answer, here is a plot of the long-term trend of water year precipitation (from the NOAA/NWS ESRL website). Bottom line: no significant trend since 1948. Thus, what we are experiencing is probably just natural variability.
This plot is consistent with our best climate models, which suggest that global warming will have only a small impact (slight increase) in regional annual precipitation by the end of this century. Climate models project that Northwest will not lose our substantial precipitation as the earth warms, but more of it will fall as rain rather than snow.