LOS ANGELES — Rain will fall on the Southland from tonight through Saturday morning, including unusually heavy rainfall on Friday as a result of the region’s most powerful storm so far this season, forecasters said, warning of flash flooding and cascades of mud and debris down slopes stripped bare in wildfires.
What will make Friday’s rains so heavy — up to an inch of rain per hour is expected — is the fact that the approaching storm system is tapping into a so-called atmospheric river, sucking up volumes of warm moisture, forecasters said. Early estimates are that the region will receive 2-6 inches of rain in coastal and valley areas and between 5 and 10 inches in the San Gabriel mountains and foothills, they said.
The first wave of rain will begin along California’s Central Coast today, then move south and into Los Angeles and Ventura counties tonight, with a stronger wave generated by a second storm system slamming into the Southland Friday morning.
“Strong southerly winds combined with ample moisture will lead to periods of very heavy rain, with rain rates at times of around 1 inch per hour,” an NWS statement said, adding that the rain would taper off Saturday morning, causing the threat of flash flooding to disappear around noon, although scattered showers are possible through Saturday evening.
“There will be the threat of flash flooding with mud and debris flows in and around the recent burn areas, especially with Friday’s storm,” warned an NWS statement.
A flash flood watch will be in effect from Friday morning through Saturday morning, with the greatest threat of flash flooding developing in areas below slopes denuded in wildfires, according to the NWS. Widespread urban roadway flooding is also possible, along with flooding in creeks and small streams, and rock and mudslides, especially near canyon roadways, warned an NWS statement.
Officials with the city of Duarte said they plan to declare a yellow alert at noon today in neighborhoods below the Fish Fire burn area. The warning would require residents to keep streets clear of vehicles, trash bins and other objects that could obstruct emergency vehicles or be washed away in a flood. City officials said they could move to a red alert — meaning mandatory evacuations — by Friday night, depending how the storm develops.
Should such an alert be issued, an evacuation center would be established at the City Hall Community Center, 1600 Huntington Drive.
Similar issues could arise in Glendora and Azusa beneath the Colby Fire burn area. Azusa officials have been distributing sandbags to residents at the City Yard, 809 N. Angeleno Ave.
The National Weather Service is forecasting up to 7 inches of rain for the Inland Empire between Friday and Saturday, the days the storm is expected to be the strongest.
Local authorities advise those who live in locations ravaged by fires including the Blue Cut and Pilot fire burn areas to prepare for possible flooding and mudslides.
Los Angeles County Public Works crews, meanwhile, worked frantically to clear debris from catch basins and storm drains in hopes of preventing flooding.
High surf is expected along the coast through Sunday as a result of a large storm-generated westerly swell, with the biggest surf expected Saturday. A high surf advisory will be in effect from 7 tonight until 7 a.m. Sunday, said the NWS, warning of surf of 6 to 9 feet today, increasing to 8 to 13 feet on Saturday before diminishing that night.
“Large waves and strong rip currents will increase the risk of ocean drowning for swimmers and surfers,” according to the NWS statement. “Large breaking waves can cause injury, wash people off beaches or rocks and capsize small boats near shore.”
Also in effect and affecting sailors is a small craft advisory that will be in effect from 9 tonight to noon Friday and a gale watch — issued in cases of winds of between 34 and 47 knots — from Friday afternoon through late Saturday night. During that period, there could be combined seas of 14 to 17 feet and inexperienced mariners should stay off the water, the NWS said.
In the mountains, heavy snow is expected at higher elevations, along with damaging 70-mile-per-hour gusts, the NWS said. The snow level will vary from as low as 6,000 early Friday to 8,000 feet Friday night, then fall back to 6,000 feet. A winter storm warning will be in force in the San Gabriel mountains from Friday morning through Saturday morning.
Between 1 and 2 feet of snow are possible above 8,000 feet and between 6 and 12 inches above 6,000 feet.
“Heavy snow, gusty winds, low visibility in blowing snow, and icy roads make for dangerous driving conditions,” according to the NWS. “Only travel in an emergency,” an NWS statement said, adding motorists traveling in mountain areas should have a flashlight, food and water, and extra clothing and blankets.
The region has been rained on a great deal more than usual. Normally, downtown L.A. would have received 8.96 inches of rain by this time of the year in the season that runs from October to April, but it already has had 16.25 inches, according to NWS forecasters.
The NWS forecast partly sunny skies in Los Angeles County and mostly sunny skies in San Bernardino County today with the following high temperatures:
Avalon: 63
Big Bear: 53
Burbank: 68
Downtown L.A.: 68
Lancaster: 65
LAX: 63
Long Beach: 65
Mount Wilson: 61
North Hollywood: 67
Northridge: 68
Ontario: 71
Palmdale: 68
Pasadena: 68
Rancho Cucamonga: 71
Redlands: 72
San Gabriel: 67
San Bernardino: 72
San Pedro: 65
Santa Clarita: 67
Torrance: 64
Van Nuys: 67
West Covina: 69
Whittier: 68
Woodland Hills: 70
Wrightwood: 56
Friday’s temperatures will be between 2 and 12 degrees lower, then will fall slightly on Saturday amid showers. More rain is expected Tuesday.
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