Thunderstorms with torrential downpours bubbled up on Thursday afternoon across southern California, under a low pressure system that has been lingering over the Southwest for two weeks. This system, known as a cutoff low, detached from the jet stream around Oct. 7.
“While lows decouple from the jet stream quite frequently, they typically don’t last this long over the United States,” writes the National Weather Service in Hastings, Neb. “Much of the time, dips or ‘troughs’ in the jet stream will exert enough influence to break them up and lift them to the north, where their remnants are absorbed by the jet stream.”
As rain fell on drought-stricken land, flash flooding became an immediate threat. As much as five feet of mud swept over parts of northern Los Angeles County and the Grapevine, a mountainous stretch of I-5.
Helicopters were used to rescue people and their dogs, reports the Los Angeles Times. Mud surrounded homes in the area around Elizabeth Lake in northern Los Angeles County. Drivers were stranded as mud flowed freely across the heavily trafficked Interstate 5. Bulldozers plowed mud on the highway like it was snow.
Cleanup of the interstate was expected to continue through Friday morning, and officials expect I-5 to open at 2 p.m. on Friday. In the meantime, motorists have to find another route in lieu of the major thoroughfare in and out of Los Angeles.
As the low drifted eastward over southern California this week, it was to take advantage of a very moist environment to produce thunderstorms with rainfall rates up to four inches per hour. “These storms were moving very slowly and dumping tremendous amounts of rain,” Joe Sirard, a meteorologist with the National Weather Service, told the Los Angeles Times. “Very unusual factors have played a role in the flooding today — lots of humidity and just the right amount of instability in the atmosphere.”
The unusually ripe conditions for heavy rain and flash flooding are due in no small part to the ongoing El Niño, and the extremely warm ocean temperatures just off the coast of southern California.
El Niño, which is a measure of how abnormally warm the tropical Pacific Ocean is, can be classified as “very strong” if surface waters are running at least 2 degrees Celsius warmer than average for at least three months in a row. While this can be a difficult metric to achieve — it’s only happened twice before — it appears this El Niño will not only jump that hurdle, but it might even surpass the old record.
Though the effects of El Niño can vary across other parts of the U.S., there’s arguably no stronger known connection in large-scale weather patterns than the one between a very strong El Niño event and an extremely wet winter in the Southwest.
This is due to El Niño’s affect on the jet stream, which carries low pressure systems along its path. During a very strong El Niño winter, the jet stream path is usually aimed directly at southern California.
“Very strong El Niños will cause the trough to shift further south with the average storm track position moving into Southern California,” says the National Weather Service. “During these times, rainfall in California can be significantly above normal, leading to numerous occurrences of flash flood and debris flows. With the storm track shifted south, the Pacific Northwest becomes drier and drier as the tropical moisture is shunted south of the region.”
During the previous very strong El Niño in 1997-1998, southern California, most notably the coastal region including Los Angeles and San Diego, saw more than double its average rainfall, according to El Niño statistics kept by Jan Null, California weather expert and owner of Golden Gate Weather.
Early that winter, mudslides destroyed hillside homes, and major roads were made completely impassable, writes the Los Angeles Times: “And that was just the beginning. El Niño-fueled rains began striking Los Angeles in January and intensified. Over the next few months, a relentless string of storms caused havoc, washing away roads and railroad tracks, overflowing flood control channels, causing 17 deaths and more than half a billion dollars in damage in California.”
It’s not just the ocean surface temperatures off the California coast are running way above average — between 8 and 10 degrees warmer than normal according to recent analysis by NOAA. If this warmth continues into winter it will provide even more moisture for storms to draw from as they track over the coast.
Wildfire burn scars will make matters even worse this winter. California has seen over 5,700 wildfires through Oct. 10, which is 1,700 more than normal, to-date. These fires have charred the landscape, wiping clear all of the beneficial vegetation that can help to keep flooding and mudslides in-check during heavy rain events. It also takes loose soil and bakes it hard, so instead of absorbing into the ground, heavy rain rolls right down the hills which can lead to deadly flash flooding that comes on in an instant.
The California Department of Forestry and Fire Protection is trying to prevent as much post-wildfire destruction as it can by laying straw on the burn scars after the fires are contained, which they hope will act as a “speed bump” to water that rolls downhill in the coming months, reports CBS Sacramento. “Cal Fire has been busy removing burned material from homes, too,” they write. “The worry is heavy rain will wash toxins into streams that eventually drain into the reservoirs we drink out of.”