Between historic drought and a lively fire season, California has experienced a troublesome year. Now, the state may have received the first sign of major relief: 8 trillion gallons of rain.

Like an epic fire hose, a long, narrow band of water vapor located in the lower atmosphere — known as an atmospheric river — doused California with record-setting rains Sunday and Monday. The event unloaded upward of 12 inches of rain on the northern Sierra Nevada mountains, almost a quarter of the annual average precipitation for the region.

The deluge comes only days after a record-breaking dry spell — what some scientists call precipitation whiplash. Sacramento had its wettest calendar day on record Sunday, just six days after the conclusion of its longest precipitation-free streak observed. Northern California and much of Nevada are experiencing their wettest October in decades.

Source: NOAA Global Systems Laboratory. (The Washington Post)

“It is great that we received this much rain early in the season, which helped with wildfires and air quality,” wrote Helen Dahlke, associate professor in Integrated Hydrologic Sciences at the University of California at Davis, in an email. “The more these precipitation events are spread out over the rainy season the more chance the rain has to infiltrate and replenish soil moisture storage.”

The animation above shows the liquid moisture in the air and clouds from Sunday through Monday falling over several burned areas and active fire regions in northern California. The rain helped quell fire conditions, probably putting an end to the fire season.

However, the intense rain occurred about a month or so earlier than in recent years. As a result, burned lands had little time to regrow vegetation and were more vulnerable to flooding, erosion and landslides. At least one landslide in the Dixie Fire zone blocked a highway.

The National Weather Service on Oct. 24 warned of “potentially historic” rainfall for parts of Northern California as heavy rain produced dangerous mudslides. (The Washington Post)

The precipitation triggered flooding, particularly in coastal areas. The Navarro River reached 21.36 feet, about two feet shy of flood stage. Dahlke said the flooding meant quite a bit of the rain became runoff and flowed out to the ocean before it could seep into the ground.

“Longer storms of lower intensity would be better for groundwater recharge since dry soils often have a hard time infiltrating water quickly,” Dahlke said. “But luckily many reservoirs were empty and therefore most of the runoff will be captured in reservoirs.”

The storm managed to increase water levels in many reservoirs. Millerton Lake and Lake Perris have both exceeded their historical average for this time of the year, but those proved to be exceptions. On the whole, most are still below the historical average for this time of the year even with the bump.

Lake Tahoe received about 61,000 acre-feet of runoff in 48 hours, rising about six inches above the rim since Sunday. Last week, water levels on Lake Tahoe sank to a critical low at an inch below its natural rim. The lake is still about five-and-a-half feet away from capacity.

Earlier this summer, water levels at Lake Oroville dropped to such low levels that officials shut down a nearby hydroelectric plant. After Sunday and Monday’s storm, the lake rose 26 feet. The lake is still more than 130 feet below water levels from two years ago.

California’s largest reservoir, Lake Shasta, is at 41 percent of its historical average for this time, as of Oct 26.

The storm also brought feet of snow to higher elevations. Some locations in the northern Sierra Nevada mountains reported more than 30 inches from Saturday to Tuesday: Northstar and Homewood received 36 inches each, Donner Pass received 33 inches, and Palisades Tahoe received 29 inches. Mammoth Mountain Ski resort, which opens Friday, also received two feet of snow and will be the first major ski resort in the area to open for the season.

The snow will probably not last through the winter, Dahlke said. Warm air temperatures and residual summer heat stored in the soils are likely to melt the snow in upcoming weeks. However, the snowmelt could seep into the soil and replenish some soil moisture lost during the spring and summer.

“This storm also helped immensely with reducing potential drought effects to natural vegetation,” Dahlke said. She said the storm occurred before the senescence (deterioration) of most trees, which were able to soak up some of the water and are likely to survive the drought effects.

While this one event helped provide some short-term relief, it won’t fix the moisture deficits accumulated over the past two years. Only about 56 inches of rain fell over the past two years in the northern Sierra, which is around half of the normal precipitation. This year would need to be above average to make up for the recent deficits.

“California will need at least another three storms of similar magnitude (or better 5-7 storms of lesser magnitude) to achieve the long-term average precipitation,” Dahlke said. “Ideally we do need more than the annual average to make up the deficit of the last two years.”

Long-term patterns show more atmospheric rivers could be in store in upcoming months. While the magnitude of those events is unknown, studies show climate change could increase the intensity of atmospheric rivers. One study showed that atmospheric rivers would be about 25 percent wider and longer at our current rate of greenhouse gas emissions.

Duane Waliser, an atmospheric scientist at NASA’s Jet Propulsion Laboratory, said the key would be if these Category 5 atmospheric rivers tended to happen more and more over time, which is not evident yet from the limited observation records. One Category 5 atmospheric river in a time period — akin to one Category 5 hurricane in a given season — is not necessarily alarming, but multiple occurrences could tell a different story.

“If a big event like this happens near the end of the season, particularly a normal or wet one, then the flood hazards are more likely,” Waliser said.