The remnants of Hurricane Rosa plus a winter storm were expected to drop a half inch of rain in the Coachella Valley beginning Sunday evening, and an inch and a half of rain over six days in the mountains west of there, a National Weather Service forecaster said.
The Riverside County Emergency Management Department on Saturday 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 around 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 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.
Small amounts of rain were expected to fall Sunday night, with the real activity expected Monday afternoon.
Meteorologists say the storm will dump one to one and a half inches of rain in mountain areas, one half to one inch in the Coachella Valley and one quarter to one half inch in some high desert areas, such as the Morongo Basin, 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.
“After (Monday’s) activity in the afternoon, it should be weakening,” Small said. “On Tuesday, the sun starts to become more dominant and the clouds from the dissipating hurricane push east to Arizona.”
The rain from Rosa isn’t all that’s in store for the region, though. A separate, more winter-like system could bring more rain into the area on Wednesday, Small said.
According to the EMD, if evacuation orders are given because of flooding or mud flows next week, residents in the affected areas should immediately leave. Alerts will be posted to www.RivCoReady.org/AlertRivCo .
Evacuation warnings — which are advisory but involve recommendations that people take precautions and be prepared to go — generally will be issued 24-48 hours before a storm, officials said.
Evacuation orders will be issued 6-12 hours before impacts.
Riverside County and some municipal fire departments will have sandbags available, in limited quantities, this weekend and next week. More information is available at www.RVCFire.org, or by contacting individual fire stations, officials said.