New atmospheric river to raise flood risk, drop more mountain snow in California
A storm system with milder air that was brewing over the Pacific Ocean at the start of the week will impact California from Friday to Saturday and generate a high risk of flooding, including in areas accustomed to receiving heavy snow, AccuWeather meteorologists warn. This storm will follow an ongoing winter storm that was producing heavy snow across the Sierra Nevada and is forecast to keep affecting the region into Wednesday.
The high-impact storm could potentially unleash flooding rainfall, mudslides, feet of mountain snow, and very strong wind gusts between Thursday night and Saturday.
"People and businesses are already cleaning up from previous storms, meaning that the latest storm has the potential to become a disaster upon previous disasters, further compounding any impacts," AccuWeather Chief Meteorologist Jonathan Porter said. "Additional storms are expected into next week - which also may tap into a 2,000-plus-mile feed of deep subtropical Pacific moisture yielding heavy rain at lower elevations and heavy snow in the mountains."
While these latest installments in the series of moisture-laden storms this wet season across California will once again bring a variety of life-threatening impacts while promoting the risk of property damage, the storms will also provide additional drought relief, building on the relief from previous storms.
"The stormy pattern this winter hasn't only put a dent in the drought, it will eliminate it by the time spring evolves into summer," AccuWeather Chief On-Air Meteorologist Bernie Rayno said, noting that the drought's reduction has already been significant across the state.
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"By the time this week comes to an end, San Francisco may have more than 30 inches of rain for the wet season," Rayno said. From October 1 through March 7, San Francisco International Airport picked up 24.61 inches of rain. The rain year lasts from October 1 to September 30 in California, and the historical average annual rainfall for the airport is 19.64 inches.
"This is a staggeringly positive development, especially given the severity of the drought across California as recently as a few months ago," Porter added. "Some longer-term drought impacts, including the recharging of deep aquifers, can persist despite the major short-term drought turnaround."
Multiple atmospheric rivers unloaded hundreds of inches of snow on the Sierra Nevada high country and feet of rain to some low elevations in the state this winter, and a new atmospheric river is poised to strike by the end of the week.
An atmospheric river is a long plume of moisture that originates from the tropics and extends into the mid-latitude region. This plume can act like a giant conveyor belt or a fire hose and result in excessive rainfall and snowfall. Multiple atmospheric rivers developed from late December into February in California. The latest will set up from Friday to Saturday and focus on Northern and Central California, forecasters say.
"While Thursday may bring calm weather following one storm and before the next storm arrives, the period from Friday to Saturday is set to bring a surge of heavy rain, feet of mountain snow, and gusty winds," AccuWeather Meteorologist Andrew Johnson-Levine said.
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The greatest problem from the upcoming storm and its atmospheric river may not be due to the low-elevation rain and mountain snow it brings but rather the temperature profile in the atmosphere.
A surge of warm air at the onset of the storm will cause the freezing level to climb thousands of feet above the ground, AccuWeather Meteorologist Brandon Buckingham said.
On Friday, rain may first fall on the passes between 7,000 and 8,000 feet in the central and northern Sierra before snow levels start to dip, according to Buckingham. A change to snow will happen quickly over the passes, but warm air may linger and rain may pour down for an extended period in locations farther below.
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"It is at the intermediate elevations, starting around 2,000 feet, up to about 5,000 feet, where enough rain will fall to cause some of the existing snowpack to melt and add to the runoff," AccuWeather Senior Meteorologist Adam Douty said.
At that intermediate elevation, there is an estimated 2-6 inches of water locked up in the snow. Should 2-4 inches of rain pour down with temperatures well above freezing, the snowpack could liquefy in some locations, with the equivalent of up to 10 inches of rain washing through streams and rivers in a matter of hours.
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This image shows areas at greatest risk for rapid snowmelt flooding from the late-week storm. |
There is the potential for 4-8 inches of rain to fall over some of the Coast Ranges and intermediate elevation, west-facing slopes of the Sierra Nevada, which all by itself will lead to flash flooding of small streams and urban areas, as well as debris flows.
The fluctuating snow levels, gusty winds and different consistency of the snow (powdery versus wet and clinging) will raise the potential for avalanches over the high country.
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"Unlike the big storm from late February, the heaviest rain and snow is likely to aim toward Northern California, leaving the Los Angeles and San Diego areas with only light precipitation," Johnson-Levine said. Most of the rain and high country snow will fall from about Point Conception, located in Santa Barbara County, to areas farther north Friday and Saturday.
"This will allow many mountain communities in Southern California, such as Big Bear Lake, to continue cleanup and recovery after the recent record-breaking snowfall," Johnson-Levine added.
Above 7,000 feet or so, several new feet of snow will pile up over the northern and central Sierra Nevada and potentially cause road closures at pass level along Interstate 80. This time, however, snow may avoid the high ground along I-5 in Northern California.
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The storm that rolled in this past weekend will finish up by the middle of the week over Northern and Central California.
Winter storm warnings were in effect throughout parts of the Sierra Nevada and other sections of Northern California Monday. From Sunday evening through Tuesday evening, 38 inches of snow piled up in Soda Springs, 31 inches of accumulation fell in Donner Park, and 30 inches of snow came to Ruth.
This storm, which was not considered to be an atmospheric river, will end up dropping several feet of snow on top of the staggering amounts already on the slopes of the Sierra Nevada into Wednesday.
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The above graphic shows predicted snowfall totals from a storm that reached California this past weekend and will depart the state by Wednesday, March 8 |
In the Sierra Nevada, the continued stormy weather may result in a record-breaking winter in terms of snowfall, Johnson-Levine said.
"At Mammoth Mountain, California, 555 inches of snow have been recorded this season as of March 6," Johnson-Levine said, "With the upcoming atmospheric river and the likelihood of more storms to follow, the all-time record of 669 inches for the mountain, set in 2010-11, could be in jeopardy by the season's end."
AccuWeather's long-range team of meteorologists, led by Senior Meteorologist Paul Pastelok, believes that the California drought will be wiped out by the early summer.
Runoff from melting snow has not even been factored into the drought index, and there has already been a considerable rollback in drought severity in California this winter, Pastelok said. Once melting snow from the high country of the Sierra Nevada kicks in, further rollback in drought classification is likely in the state.
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"The storms this winter have already considerably slashed away at the drought and the runoff from the Sierra snowpack hasn't even kicked in yet," Rayno said.
Within the hundreds of inches of snow on the ground over the Sierra Nevada already on the ground this week, data suggests more than 80 inches of water could be locked up in that snow as of Monday morning, according to Douty.
AccuWeather meteorologists remain concerned that flooding problems may expand over California into this spring due to the excessive amount of snow locked up in the mountains.
"Our meteorologists have seen this over and over during recent decades where drought ends in a flood in various parts of the U.S. and the setup going into this spring could be one of those situations for California," Rayno said. The theory behind the drought-to-flood scenario is that it takes such a tremendous weather pattern to break the drought that it can also lead to major flooding.
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Drought conditions will stubbornly persist farther inland over the Southwestern states, according to AccuWeather forecasters.
"There is no doubt that the tremendous amount of snow on the slopes of the mountains in the Southwest will have a positive effect on the Colorado River and Lake Mead for a time well into this spring," Pastelok said, "But, longer-lasting relief may depend on how active the North American monsoon is this summer."
Lake Mead's water levels this past summer were at their historically lowest levels since Hoover Dam was completed and the waterway began to be filled in the mid-1930s. The massive hydroelectric dam supplies power to 1.3 million utility customers in Nevada, Arizona and California, according to the U.S. Bureau of Reclamation. Water levels dipped to 1,040 feet this summer. A level above 950 feet must be maintained for the dam to produce electricity.
Another storm or series of storms is likely to roll into California next week, and more heavy rain and mountain snow will mostly target the northern counties.
However, there is a likelihood that at least some drenching rain will reach Southern California with the risk of snow at pass levels in the region from the middle to the latter part of next week.
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