PRECIPITATION
There is no significant trend in how much rain California gets. That is the finding, not a failure to find one.
Why October
THE WATER YEAREvery precipitation figure on this site is summed over the water year — 1 October through 30 September — and not the calendar year. This is not a formality. A California wet season runs from late autumn through early spring, straddling the New Year, so a calendar total cuts every single one of them in half and files the two pieces under different years.
The consequences are not subtle. On calendar years, the drought of 1976–77 is smeared across two unremarkable-looking totals. On water years it is the second-driest year in the whole record, which is what it actually was, and what every reservoir operator in the state experienced it as. California's own agencies use the water year for this reason and so does this site.
Water year 1895 cannot exist — the record begins in January 1895, so the preceding October is not there to sum. The precipitation series on this site therefore starts at water year 1896 and is one year shorter than the temperature series.
Every water year since 1896
AS A PERCENT OF THE 1991–2020 NORMALShown as a percent of normal rather than in inches, because the interesting quantity is the departure. Bars above the line are wetter than the modern normal, below it drier. The first thing to look for is a tilt in the whole field from left to right — and the second thing to notice is that there isn't one.
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It means this record cannot distinguish a real long-term change in the mean from the enormous natural year-to-year variability California already has. That variability is the largest of any state in the country, which makes it very hard to detect a slow drift inside — a real trend could be present and still be invisible at this sample size. What the data does not support is a confident claim in either direction, and this site will not make one.
The wettest and the driest
THE EXTREMES OF THE RECORDCalifornia's precipitation distribution is not symmetric. The wettest year on record is roughly four times the driest, and the median year is drier than the mean — a handful of very wet years pull the average up above what a typical year actually delivers. This is why "average rainfall" is a misleading way to describe this state.
Whiplash
THE SWING BETWEEN YEARSIf the total is not moving, the shape of the record still might be — and this is where something does show up. Define a whiplash event precisely: a water year in the driest fifth of the entire record immediately followed by one in the wettest fifth, or the reverse. The quintiles are taken over the whole record, not a moving window, so the count is directly comparable across eras.
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Fourteen events across 130 years is a small sample, and a count of rare events in three unequal bins is a weak test — this is a described pattern, not a significance result, and it is not presented as one. It is included because the clustering is large enough to be worth showing and because the definition is stated precisely enough that anyone can check it. A stronger claim would need the daily record and a longer one.
Where the rain falls
DIVISIONAL NORMALS, NORTH TO SOUTHThe statewide average of 22 inches describes almost nowhere in California. The state's precipitation gradient is one of the steepest in the country, and it runs almost entirely north-to-south — which is why a statewide percent-of-normal figure can be near 100% in a year that was a disaster in one half of the state.