With the first month of the water year in the books, observed precipitation remains quite dry—ranking as the ninth driest October in the past 45 years (1980–present, plotted below). Is this a big deal? Not necessarily. When precipitation comes in October, it tends to be sporadic, and a wet or dry October isn’t a reliable indicator of how the rest of the water year will unfold. The long and short of it—don’t judge a book by its cover, and don’t judge a water year by its October. What matters most is what happens during the historically wet winter months.

California Water Watch (CWW) provides precipitation estimates utilizing PRISM (Parameter-elevation Regressions on Independent Slopes Model) data. Developed by Oregon State University, PRISM data provides high-resolution spatial estimates of climate variables such as precipitation, temperature, and snowpack. However, its limitations include potential inaccuracies in capturing small-scale variations in precipitation and reliance on interpolated data, which may not reflect local conditions.

As such, we treat measurements from the NWS cooperative observer network as ground truth. But for the purpose of visualizing precipitation patterns, the CWW map is helpful. The image below shows the spatial distribution of precipitation through the end of October, with most of the state at or below 20% of average through this point of the water year (gray shading).

A preliminary analysis of observed precipitation through mid-November reveals an intriguing pattern. In the Northern Sierra 8-station index plot below, the precipitation received through November 18 falls between the observed values for the 2017-18 and 2018-19 water years—analog years identified in our forecast release. We will continue to monitor the water year’s trajectory in relation to these analogs as the season progresses.