A significant atmospheric river event is set to hit California this weekend, posing a considerable risk of flash flooding, mudslides, damaging winds, and heavy mountain snowfall. With nearly 40 million people under flood watches, including major cities such as San Francisco, Sacramento, Los Angeles, and San Diego, authorities are urging residents to take precautions and stay informed about the evolving weather conditions.

The Los Angeles office of the National Weather Service has issued a warning, stating that there are increasing chances of a prolonged period of heavy rain starting late Saturday night and lasting through most of Sunday and Monday. This storm has the potential to generate unprecedented amounts of rain across a widespread area, posing a significant threat to the region.

The Weather Prediction Center has declared a rare Level 4 out of 4 risk of excessive rainfall for Santa Barbara and Oxnard on Sunday. Additionally, a Level 3 risk, which is still significant, exists for much of coastal California, from San Francisco down to Los Angeles. These high-risk designations emphasize the severity of the storm, as they are issued on fewer than 4% of days per year, yet they are responsible for a majority of flood-related damage and fatalities.

The forecast anticipates rainfall to begin as early as late Saturday night for central portions of coastal California, where a few inches of rain by early Sunday morning could lead to flooding. In Southern California's Transverse Ranges, there is a cautionary note from the prediction center that at least 8 inches of rain could occur within 24 hours, with some locations experiencing maximum totals just over 10 inches due to repeated storms in a short period of time.

Central and southern California are expected to receive widespread rainfall totals of 3 to 6 inches, which is more than a month's worth of rain for most areas. Los Angeles, in particular, typically averages 2.99 inches of rain in February, making it their wettest month of the year on average. In the mountain regions and foothills of Southern California, the weather service is forecasting up to a foot of rain.

The Ventura County Sheriff's Office has issued evacuation orders for certain communities in anticipation of the storm. The orders will be in effect from 5 p.m. local time Saturday until 5 p.m. Sunday, with concerns about landslides or debris flows due to the expected heavy rain.

Mayor Karen Bass of Los Angeles warned that this storm may be as strong as Tropical Storm Hilary from last August, urging residents to take common-sense precautions. She advised people to stay at home, remain updated with the latest information, and ensure they are prepared with emergency supplies.

This atmospheric river event follows closely on the heels of a recent storm that brought record rainfall to various parts of California. However, this upcoming storm is expected to be slower and stall as it moves onshore, resulting in a longer duration of rain compared to the previous system. The ground is already saturated from the previous storm, increasing the risk of flooding and mudslides.

In terms of wind, high winds are forecasted across much of California, with widespread speeds of 40 to 60 mph and gusts as high as 75 to 80 mph. These strong winds could lead to downed trees and power outages. The National Weather Service office in San Francisco emphasized that this storm is likely to be both a wind event and a rain event.

The storm is not only a rain and windmaker but it will also bring significant snowfall. The weather service office in Reno, Nevada, which forecasts for parts of California, has warned of major travel impacts in the Sierra due to heavy snowfall from Sunday through Tuesday. Mono County, along the Nevada border, including popular tourist destinations like Mammoth Lakes and Yosemite National Park, can expect notable snowfall amounts, potentially exceeding 4 feet above 8,000 feet.

Authorities are urging residents to stay informed and prepared, especially in higher-risk areas prone to flooding, mudslides, and heavy snowfall. It is advised to stay updated with the latest weather information and follow any evacuation orders or warnings issued by local authorities.