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 long-term drought spanning many years in California has been erased in some parts of the state, thanks to record-challenging amounts of snow over the mountains and feet of rain at low elevations. However, the stormy pattern that began in late December is far from over.
“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.
“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 July 1 through March 5, San Francisco International Airport picked up 24.68 inches of rain. The rain year lasts from July 1 to June 30 in California, and the historical average annual rainfall for the airport is 19.64 inches.
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.
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.
“It is at the intermediate elevations from 3,000 to 6,000 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.
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.
“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 said.
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.
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.
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.
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.
“In Mammoth Lakes, California, 538 inches of snow have been recorded this season as of March 5,” Johnson-Levine said, “With the upcoming atmospheric river and the likelihood of more storms to follow, the all-time record of 669 inches [in the state], 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.
“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.
“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.
Produced in association with AccuWeather