Two storms are lined up to arrive from opposite directions in Southern California this week, and areas in Santa Barbara, Orange and Riverside counties are warned of possible debris flows and flash floods in recently burned mountains, the National Weather Service said.
Thunderstorms left over from decaying Hurricane Rosa, were expected to hit eastern San Diego County mountains and the Coachella Valley region by early Monday and Tuesday, but largely miss the Los Angeles area.
Right after that, however, an unusually early low pressure wave will bring winter-style rains in from the northwest. The two could combine for six days of rain in mountains above Palm Springs, the NWS said.
The Riverside County Emergency Management Department warned residents near Idyllwild and Lake Elsinore that rain in the Cranston and Holy fire areas, that six days of rain could trigger mud and debris flows near locations where the vegetation was consumed by the fires.
“Powerful mixtures of mud, rocks, boulders and trees” could flow into streets, backyards and homes, depending on the speed and mass of the runoff, the department warned.
Much of the hurricane’s moisture was dissipating Sunday as it moved into colder waters off the Baja California peninsula, NWS forecaster Ivory Small in San Diego said.
Rosa was a Category 4 storm on Thursday, but had been downgraded to Category 1 by Sunday.
But there will still be enough moisture left to prompt the NWS to issue a flash flood warning for the deserts and mountains of southeastern California and western Arizona.
Meteorologists say the storms will dump one to one and a half inches of rain in mountain areas, one half to one inch in the Coachella Valley through Tuesday.
Periods of localized heavy rain could cause flooding in poor drainage areas and normally dry washes and arroyos in the flash flood warning area, which includes eastern Riverside County and all of Imperial County, forecasters said.
Orange County Fire Authority Capt. Steve Concialdi said their agency was preparing for heavy rain in Upper Trabuco and Holy Jim canyons, which burned two months ago. Contingency plans for debris flow or mudslides in the steep canyons east of Irvine are drawn up, and heavy equipment operators and hand crews are available if needed.
Concialdi said all fire stations in Orange County have sandbags, which are available to residents. But not all have sand to put in them.
Forecasters said the second storm, an early-season cold front coming off the Pacific, is giving them “timing issues” and said the front will move through Los Angeles Tuesday, Wednesday or even Thursday.
One forecast model predicts 4-6 inches of rainfall across the Ventura County mountains and around 2-3 inches across the eastern Santa Barbara Mountains, including the Montecito area, where flash flooding in burn areas claimed 22 lives last winter.
But both forecasts agreed that the storm has enough energy to trigger thunderstorms.
Brooke Federico, the Riverside Emergency Management Department’s Senior Public Information Specialist, said the county is monitoring the weather ”closely” and coordinating with first responders for portions of Lake Elsinore and Temescal Valley, as well as south of the Idyllwild area.
“We want residents to register for `Alert Rivco” and that’s how we will call or text people, when its time (for any evacuations).”
That alert system is posted at www.rivcoready.org .