Storm next week should bring “significant rain and snow in the mountains” to Southland, meteorologists say

A storm system arriving during the second half of next week has the potential to bring moderate to heavy rain to southern California, the National Weather Service said.

Precipitation rates can be high at times, increasing the possibility of debris flows in areas of recent fires, as well as mud and stone landslides on mountain roads. Significant snow accumulations are possible with the storm, mainly above 5,000 feet.

The biggest storm, the third in a series, is expected to incorporate an atmospheric river. Atmospheric rivers are a concentrated stream of water vapor in the middle and lower levels of the atmosphere. Because they are lower in the atmosphere, they tend to be hotter, even when they are not a direct pipeline to the tropics.

This atmospheric river will come farther west, said Eric Boldt, an alert coordinating meteorologist at the National Weather Service in Oxnard. “It is not a direct tropical connection, but [has] above-normal water content that can bring constant moderate rain for many hours. “

The change in pattern comes after a summer that had record heat, the worst fire season in California history and a dry autumn and early winter swept away by the Santa Ana winds.

Although the Pacific Northwest and the intermediate North Pacific region have enjoyed heavy rainfall from a series of North Pacific storms, the central and southern parts of the West are experiencing a drought that is the worst possible. With areas already classified as experiencing extreme and exceptional drought, “there cannot be much deterioration,” reported the US Drought Monitor on Thursday.

The only way for such a dry and burnt landscape to get worse would be to release heavy rain from an atmospheric river – rain that is like a fire hose – washing mud and debris down steep slopes. Rain is necessary, but not all at once. If heavy rain rates occur, there is a risk of debris flows from scars from recent burns, such as those left by the Lake, Bobcat and Ranch2 fires, and other fires in Orange County and the Inland Empire.

Meteorologists do not expect an epic rain scenario, but warn that the public should pay attention to updates and follow the instructions of the authorities if instructed to evacuate.

Large amounts of rain are not necessary to create a dangerous situation. For example, the Montecito landslide in Santa Barbara County on January 9, 2018, after the Thomas fire, occurred after just half an inch of rain. But that rain fell in about 5 minutes, and the debris produced flow 4.5 meters high.

How did we get to that point?
La Niña is in effect in the equatorial Pacific and is expected to persist for the rest of the winter. La Niña usually results in a dry winter in southern California and the southwest, although this is not always the case. El Niño, La Niña’s wettest cousin, usually means a rainy winter in southern California, based on history, but again, there are no guarantees.

Southland has seen a storm in the past nine months, and the drought pattern has so far been consistent with a typical La Niña winter – which also generally has a wetter than normal climate in the Pacific Northwest and across the northern layer of the United States.

Although the Los Angeles area had almost normal rainfall last winter, the excessive summer heat and dry Santa Ana winds in the fall and early winter left dry fuels. Several major fires burned over 4 million acres in California in 2020, sometimes involving almost the entire state in smoke.

In a Mediterranean climate like California, rain falls mainly during the winter. Even without the influence of El Niño and La Niña, southern California depends on five to seven significant storms for its rains each year.

“In LA it doesn’t rain or drizzle every day during the winter like in Seattle,” said climatologist Bill Patzert. “The rain comes in five, six or seven events. There are usually less than 20 days of rain and less than 10 days of significant rain throughout the season. “

The Los Angeles Basin is a floodplain. After a season that started out dry in 1937-38, two storms in late February and early March 1938 abruptly brought down as much rain in the region in five days as it usually falls over an entire season. Catastrophic floods from the foothills of Long Beach followed, causing the equivalent of a billion dollars in dollar losses today. The angelenos were fed up with so many repetitive and damaging floods. These events led authorities to finally tame the Los Angeles River and other waterways, coating them with concrete for flood control.

As Patzert points out, these concrete channels do not flow significantly 99% of the time.

A reminder of what could happen before these water courses were turned into concrete channels happened in the new year 1934.

In November 1933, a fire stripped an area of ​​7.5 square miles in the foothills above Montrose and La Crescenta. Then, on December 31, 1933 and January 1, 1934, an atmospheric storm in a river dropped 25 to 40 centimeters of rain in the foothills and 50 to 30 centimeters in the foothills and mountains. Downtown Los Angeles received 8.27 inches of rain, as The Times reported.

“The Montrose flood, as the calamity soon came to be called, took at least 45 lives, destroyed about 100 houses and transformed the small community into an arid and mud-filled landscape, said local historian Art Cobery, who became an expert on the catastrophe and its aftermath, ”reported The Times in 2009.

Singer-songwriter Woody Guthrie recalled the tragedy in a song about the “Los Angeles New Year’s Flood”.

Atmospheric rivers are plumes of extremely humid moisture. They can pour rain on the plains when they land, but when they rise and over steep and rugged hills and coastal mountains, their moisture load is squeezed. This means that they are dropping heavy rain, possibly at an intense rate, in the same areas that were stripped of vegetation by forest fires during the previous summer and fall.

“All of this serves as a cautionary tale for those who live under burnt areas, like the Bobcat fire in the San Gabriel mountains. Landslides cannot be predicted, but they can be anticipated, ”said Patzert. “The best protection for landslides is to stay informed and evacuate when the authorities issue alerts.

“Major droughts, fires, floods and landslides after heavy rains, decade after decade. It is a story told for thousands of years, ”added Patzert. “It’s a California classic.”