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The atmospheric river storm of Saturday through Monday was among the most powerful ever to impact California. According to state hydrologists and the National Weather Service, it is tied for the third most powerful in the past 71 years. The low pressure center that powered the storm off the Washington coast was measured at 943 millibars. That’s the lowest pressure ever at this latitude and the equivalent of a strong hurricane.

But, importantly, although a good start to the rainy season, the storm barely made a dent in addressing the state’s reservoir storage shortfalls.

The immediate gains in Lake Sonoma, the main reservoir for the North Bay region, was about 15 thousand acre feet with runoff continuing. Lake Mendocino received around 4 thousand acre feet with 8 inches on average falling in its drainage. But both reservoir are still far below average levels of past years:

Current: 119.2 thousand acre feet
Average on this date: 194.2 thousand acre feet
Last year on this date: 174.9 thousand acre feet

One key contribution of the storm was soil saturation. The recharge of 7-10 inches across the Russian River drainage means additional rain that falls in the coming weeks will result in immediate runoff into storage reservoirs.

In the main stem of the Russian River, the storm brought big flows that receded quickly after the rain ended. (Images 1, 2, 3)


By any measure, the storm was an impressive rainmaker across the North Bay in particular.

The entire region saw heavy rain, generally over 6 inches. This graphic is for the past 7 days. (Image 4) along with rain for the month across the region (Image 5), and the percent of average rain for the date. (Image 6)


All of Northern California benefited, with tens of thousands of acre feet entering the state’s reservoirs. And all of the northern half of the state is running well ahead of average precipitation. (Image 7)

Still, we have a long way to go to refill the state’s main reservoirs. This graphic tells the storm story. Although runoff continues, levels are still near historic lows.

One example is Lake Oroville, the state’s 2d largest which went from 23% of capacity on Friday the 22nd, to 27% on Monday the 25th.

And Folsom Lake, which went from 22% on Friday the 22nd, to 30% on Monday the 25th. (Images 8 and 9)


A very good start for certain, but we will need an average winter or better to approach normal storage levels. And, at least for November, current forecasts hint at a return to relatively dry conditions. (Image 10)

But, major storms like this past AR are notoriously hard to predict in advance. And La Nina years can, and often do bring near average rain to NorCal. Already some models point to rain returning by November 5th to November 7th. (Image 11)

At this point, many long range forecasts point to a slightly below average winter, with 80-100 percent of normal rain. Let’s keep crossing our fingers that will happen.

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