Saturday, December 26, 2015

Weather Forecast, and the Travails of Folsom Lake

Lake level this winter at Folsom Lake could at first best be described as a dead cat bounce. The lowest lake level ever recorded in the reservoir's history occurred on December 4th: 135,561 Acre Feet. The increasing inflow due to rain had very little initial effect.

Weather-wise, California is now heading into a quiet period. Our dear friend, the Really Resilient Ridge, is in control. That means the NW Pacific is amazingly stormy, but things are pretty quiet in the NE Pacific. Thus, the small system coming in Monday is likely to be a dud. Precipitation, which had almost reached normal levels, will slip behind again.

The next systems of interest are forecast to arrive on Sunday January 3rd, and Wednesday January 6th.

Folsom Lake received a big influx of water on Dec. 22-23 (there was an article about it in the Sacramento Bee):
After falling to historic low depths, Folsom Lake came back with a vengeance this week, adding nearly 18 billion gallons on Tuesday and Wednesday, state figures show.

Earlier this month, Folsom Lake levels fell to roughly 135,000 acre-feet, the lowest point since officials began keeping records – lower than even the 1977 drought. Since then, the region has seen a few storms, and the lake level has crept upward.

After the latest batch of storms, flows into the lake increased dramatically. On Tuesday, the lake rose 35,000 acre-feet. On Wednesday, it rose another 20,000 acre-feet.

In practical terms, the increases this week almost certainly mean that the lake will not fall below the level at which cities have trouble drawing drinking water from it.

The surge was a bit surprising. It had been raining/snowing from Dec. 18-22, but at first there wasn't much inflow. What made the difference was that as the small storm systems kept rolling through, the temperature kept rising, so by the end, precipitation upstream was falling more as rain than snow, and was at last available to help refill the reservoir.

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