earth.nullschool.netComputer model projection showing total precipitable water and atmospheric pressure across the Pacific on Feb. 17, 2017.
Something is going to happen in Southern California on Friday that hasn’t happened in at least six years, possibly longer. It’s going to rain a whole, heckuva lot, and that rain is going to be accompanied by a wide range of other hazards.
It’s all part of one of the most intense storms to strike the region since before the state’s epic drought began in 2012, which is likely to bring several inches of rain on average to areas from Santa Barbara southward to San Diego.
Higher rainfall totals, likely into the double digits, will occur in mountainous areas, with heavy snow falling in the higher peaks of Los Angeles and San Diego counties, among others.
This will lead to landslide and mudslide concerns as heavy rain runs off already saturated hillsides, and flash flooding issues even in urban areas. The highest mudslide risks will be across areas that have burn scars from wildfires.
A river of moisture pours into Southern California over the next couple of days (animating water vapor from GEOS computed at @NASA_NCCS) pic.twitter.com/ftCM8T7q9D
— Bill Putman (@wmputman) February 16, 2017
The storm, which combines an unusually intense low pressure area with a firehose of moisture whose hose stretches back for more than 2,000 miles, way out to near Hawaii, will rage throughout the day on Friday and into Saturday in one of the most populated and storm-averse areas of the country.
At its peak, winds are likely to gust greater than 50 miles per hour in the Los Angeles and San Diego metro areas, which will cause extensive air travel delays and down trees and power lines. Some areas could see winds approach or exceed 100 miles per hour, particularly in the higher elevations of San Diego County.
High wind warnings have been issued for higher elevation areas around Los Angeles and all of San Diego County, where officials are bracing for sustained winds of tropical storm force, or 39 miles per hour or greater, along with higher gusts as the storm center nears the coast.
A look at how the forecast rainfall for southern California stacks up to long-term records for two-day totals over the region. #SoCal pic.twitter.com/H0RKFFzXbJ
— NWS WPC (@NWSWPC) February 16, 2017
The National Weather Service has issued a dizzying array of watches and warnings to cover all the storm impacts, which can be found on weather.gov.
According to the agency’s predicted rainfall amounts, some spots could see a two-day rainfall total that ranks within the top 10 all-time heaviest two-day rain events, but this is not expected for most spots.
NWS forecasting 4"+ rainfall thru mid-Saturday for SoCal w/maximum of 10-inches along coast as strong storm unleashes fire hose. pic.twitter.com/6myX6VQcrw
— Ryan Maue (@RyanMaue) February 16, 2017
According to the Weather Service’s San Diego forecast office, a storm of this intensity at such a low latitude in the state is extremely rare, or "off the charts when looking at the past 30-year record," the agency said in a forecast discussion.
How rare is deep surface low pressure along the southwest or west central California coastline, this map shows that its very rare #CAstorm pic.twitter.com/tZhJCTpYaX
— NWS San Diego (@NWSSanDiego) February 16, 2017
Fortunately, the heaviest precipitation from this particular storm is not hitting Lake Oroville in north-central California, where nearly 200,000 people were ordered to evacuate earlier on Sunday due to fears that a part of Oroville Dam’s infrastructure — the nation’s tallest — could fail. More rain is expected in that area though during the next five days.
However, the footprint from the larger barrage of storms associated with that moisture pipeline, which is known to meteorologists as an "atmospheric river," will have a multi-hazard footprint that would make any national political candidate envious — sweeping states from California northward to Washington and east to the swing states of Colorado and New Mexico.
The Friday through Saturday storm is just the latest in a string of Pacific weather systems that have beaten back the state’s epic five-year drought in many areas, although groundwater supplies that take many years to recharge remain depleted.
Parts of the state have seen record rainfall within what’s known as a "water year," a period of time that includes both a wet and dry season between Oct. 1 through Sept. 30. This is remarkable considering there is no El Niño present, and such climate phases are typically associated with the truly blockbuster winters in the Golden State.
Image: scripps institution of oceanography
Image showing the atmospheric river (yellow and red) extending from the West Coast to near Hawaiit.
Droughts typically do end in deluges in California, but global warming is also increasing the odds of precipitation extremes at both ends of the scale, creating a climate characterized by feast or famine. This is already putting more strain on antiquated infrastructure, such as the Oroville Dam, that was not designed with human-caused climate change in mind.
"Where we really butt up against the climate system is in the extreme events," said Noah Diffenbough, a climate researcher at Stanford University.
"I think the evidence is clear that global warming has increased the probability of these kinds of conditions," he said, referring to the swing from drought to flood, and increasing tendency for atmospheric rivers to be milder than they used to be, bringing rain at many higher elevations rather than snow.