The level of anxiety around Northern California’s Lake Oroville has waned from “extreme” to “cautiously optimistic.” Residents are returning to their homes after the mandatory evacuation was lifted Tuesday afternoon. The water level in the reservoir has receded more than 20 feet since the nearly catastrophic spillway failure Sunday night.
The California Department of Water Resources says it will continue releasing an enormous amount of water from the lake to further reduce its volume. But two storms are in the forecast for Northern California over the next seven days, which will put the department’s plan to the test.
The first begins Wednesday night, lasting through Thursday. The watershed that drains into Lake Oroville will get precipitation, but this storm is expected to be more manageable than what caused the overtopping late last week.
“It’s considerably weaker, and it’s colder” than the storms late last week that caused Lake Oroville to spill over, said Tom Dang, a meteorologist at the National Weather Service in Sacramento. “We do have a fair amount of confidence that snow levels will be in the 5,000- to 6,000-foot range in this storm.”
To put that in perspective, snow levels were around 8,500 feet last week, which means all precipitation below that elevation fell as rain. The ideal storm for Lake Oroville and the communities downstream is cold with low snow levels, which will reduce the amount of inflow to the reservoir — more precipitation falling in the form of snow and less as rain.
Even though the weather will be wet through the weekend, it will be good enough to allow the California Department of Water Resources to continue fortifying the damaged spillways. That effort may prove to be critical, because storms early next week look much rainier.
The next storm, forecast to begin Monday, is another atmospheric river event that will stretch from Hawaii across the Pacific Ocean to arrive at Lake Oroville’s doorstep. Atmospheric river storms are something California has become accustomed to this winter, and they are why some cities have already doubled the amount of precipitation they normally receive by this point in the year.
Over two to three days, the storm could drop more than six inches of precipitation on the Oroville drainage basin. That’s what the European forecast model has been predicting over the past couple of days, and now the other models are coming around to the idea.
More importantly, this storm looks warmer given its origins in Hawaii, which means snow levels will be much higher, and precipitation will fall mostly as rain in the Oroville drainage basin.
If we assume widespread rainfall totals of six inches in the Oroville runoff area, that would mean 400 billion gallons, or 1.23 million acre-feet, of water over the next five to 10 days. In context, the volume of water in Lake Oroville on Wednesday morning was about 3.2 million acre-feet. Six inches of rain is far from insignificant.
Chris Orrock, a spokesman for the California Department of Water Resources, said it will continue releasing water at 100,000 cubic feet per second to head off the effect of big storms, such as what’s predicted for Monday. That output is as high as they are willing to go with a damaged spillway.
“We want to put plenty of room in the lake so we can absorb future inflows,” Orrock told The Washington Post. “848 feet is flood-control level. We’re going to surpass that.”
On Wednesday morning, the water level was 897 feet and falling. The department was forecasting inflows of 40,000 to 60,000 cubic feet per second over the next week. Assuming that forecast is correct, the outflow should exceed the inflow and the water level will continue to drop.
Orrock said he doesn’t know exactly how low the water level will get, and there’s always a possibility they could need to use the (also damaged) emergency spillway again, but they are doing everything they can to prevent that.
Through Feb. 15, not a single climate-monitoring station in Northern California is reporting below-average rainfall since the middle of 2016. At least three stations — Sacramento, Blue Canyon and Santa Rosa — have at least doubled the amount of precipitation they usually receive since July 1.
In fact, by one important measure, there’s been more rain and snowfall in the 2016-2017 water year than any other season on record, to date. The California-Nevada River Forecast Center uses an eight-station index in the northern Sierras to quantify the region’s precipitation. As of Feb. 12, these eight stations have received 68 inches — 226 percent of normal.
Geographically, the swath that has received the majority of rain and snowfall is about 60 miles wide with Interstate 80 along the southern edge. It happens to be exactly the region that drains to Lake Oroville.