West LA Times

UCLA and UC ANR climate scientist Daniel Swain will be monitoring a strong storm over California, with the heaviest rain expected during his YouTube Office Hours session on Thursday, February 13. The event is scheduled for 4:00 p.m. PT/7:00 p.m. ET.

Swain's blog post highlights the dual nature of the upcoming weather system. "Southern California desperately needs more rain this season, and the good news is that this system will certainly bring some," he states. However, he warns of potential risks: "The bad news is that it may fall too hard, too fast, to avoid a substantial risk of debris flows and perhaps flash flooding in, near, and downstream of recent wildfire burn areas in SoCal."

Recent wildfires in western and central Los Angeles County from January and others from September are areas of concern due to their vulnerability to heavy rainfall. "While the whole region should see a good soaking, the SoCal mountains will see widespread 3–6 inch totals (locally higher), and that does include most of the major recent fire footprints," Swain notes.

Debris flows are particularly concerning in these areas. Swain explains that they involve a higher ratio of water to solid material than deep-seated landslides, making them more mobile. He attributes their likelihood in recent burn areas to several factors including vegetation removal, creation of hydrophobic soil layers due to intense fires, and destabilization of larger debris on slopes.

"There will be two ‘types’ of precipitation during the Thursday storm that could produce debris flow," Swain explains. Initially, an atmospheric river will transport moist air into Southern California's Transverse Ranges causing heavy rainfall through orographic lifting. This could lead to rainfall rates exceeding half an inch per hour in mountainous regions.

Following this phase, as the cold front arrives, convective showers and isolated thunderstorms might develop due to atmospheric instability. These conditions could result in localized rainfall rates up to or exceeding one inch per hour within recent fire footprints—posing significant risks for flash floods and debris flows.

Swain emphasizes that while downpours of such intensity won't occur everywhere, they present a considerable threat where they do align with recent fire zones.