Forecasters are warning of an increased risk for a significant atmospheric river event that could impact Southern California next week, with potential for heavy rainfall leading to debris flows and mudslides, particularly in areas affected by recent wildfires. The National Weather Service has raised the likelihood of "high amounts" of rain to 30% for the period between February 12 and 15, which is a notable increase from the previous 10% forecast.
The anticipated storm could bring moderate to heavy rainfall across Los Angeles, Ventura, Santa Barbara, and San Luis Obispo counties. In addition to the 30% chance of high rainfall, there is also a 30% chance for moderate or low amounts of rain, while the chance of no rain stands at 10%. Meteorologist Ryan Kittell from the National Weather Service in Oxnard emphasized the uncertainty in the forecast but noted that the potential for a strong, wet system remains.
This forecast raises concerns due to the typically low probabilities associated with such intense rainfall events. Kittell indicated that the risk this time is higher than usual for storms in the region. If the predicted heavy rainfall occurs, it could result in rates of half an inch to an inch per hour, which are sufficient to trigger mudslides and debris flows, especially in areas where vegetation has been burned away.
The potential for significant rainfall could lead to a slow-moving storm that lasts for 12 to 24 hours or more, with predictions of 2 to 4 inches of rain in coastal and valley regions, and even higher amounts of 4 to 8 inches in mountainous areas. Kittell warned that this scenario would create a "high risk for flooding and debris flows," particularly in areas where the ground has been destabilized by fire.
The risk of landslides is particularly acute in burn areas, where the soil lacks the vegetation necessary to absorb water effectively. Fires can also create conditions that impede water absorption, with ash and debris clogging the soil. As a result, water may flow over the surface, potentially leading to dangerous mudflows and debris flows that can carry rocks and other materials at high speeds.
Historical context underscores the danger of such events; the devastating debris flow in Montecito in January 2018 resulted in 23 fatalities and extensive property damage, occurring shortly after the Thomas Fire. Investigations following that disaster revealed failures in local government preparedness, including inadequate debris basins.
As Southern California braces for the possibility of this storm, residents are advised to remain vigilant and prepared for potential flooding and road closures. The current weather patterns follow a weaker atmospheric river system that has been affecting the region, with rainfall expected to peak between Thursday evening and Friday midday.
Despite the impending storms, the drought situation in Southern California remains severe. The latest drought monitor indicates that much of the region is experiencing "extreme drought," with downtown Los Angeles receiving only 1.02 inches of rain since the start of the water year on October 1, far below the average of 7.8 inches for this time of year.
As the weather system approaches, officials are monitoring conditions closely, emphasizing the need for preparedness in the face of potential hazards associated with heavy rainfall and its aftermath.