Flood advisories expire across SoCal; chance of ‘atmospheric river’ remains for next week

The National Weather Service allowed flood advisories to expire early Friday for parts of Los Angeles and Ventura Counties, but said that stronger storms remain possible through Feb. 15 as a result of a second atmospheric river. This follows the expiry of similar warnings in Santa Barbara and Ventura Counties Thursday night.

Rain is still projected through Friday, with half to one inch expected to fall across the flatlands and up to two inches expected in the hills. Minor road flooding and rockslides on canyon streets are possible, according to the NWS.

The chance remains for a significant storm next week, generated by the second of a pair of atmospheric rivers which have been drifting across the Pacific for more than seven days.

The first of these events hit California Tuesday evening, and dropped about two inches of precipitation, measured at downtown L.A.

The NWS projects a 30% chance of a heavy storm from Feb. 12 to Feb. 16, which would deliver two to four inches of rain over the course of more than twelve total hours. If that came to pass, forecasters expect a high risk of flooding and debris flow.

The Service’s forecast also projects a 30% chance of moderate rainfall, a 30% chance of light rain, and a 10% chance that no precipitation will fall next week.

Atmospheric rivers are narrow channels of moisture which stretch through the atmosphere, dropping rain along their lengths. The most common source of such phenomena in L.A. is the ‘Pineapple Express,’ which brings moisture from the vicinity of Hawaii to the West Coast.

Increased rainfall could help to prevent the outbreak of future fires. The blazes which engulfed parts of the hills in January were able to spread so rapidly in part because the vegetation surrounding L.A. was exceptionally dry after eight months without measurable rain.

However, the areas burned by the fires are still liable to generate floods and mudslides. The roots of plants, which would normally hold the soil in place, are weakened or destroyed in burn scars, leaving the earth loose and mobile.

Fires also clear away obstacles to the movement of mud, while residual ash can clog the soil, preventing it from absorbing excess water.

The most costly mudslide in the state’s history occurred in 2018 near Montecito, in an area which had been ravaged by wildfires a month prior. 23 people died, and over 500 residences were destroyed or damaged in the wealthy enclave.

Ongoing land stability issues in Rancho Palos Verdes have also been exacerbated by rain. Land there has been observed to move at a rate of up to one foot per week. As a result, the Portuguese Bend neighborhood has been under an evacuation order since September.

Still, all of Southern California remains in worsening drought conditions, as recent rainfall has not delivered enough water to replenish vegetation, soil moisture, or snowpack.