A cold weather system out of the Gulf of Alaska generated showers in the Southland Friday, triggered a precipitous temperature drop and is expected to lower the snow level to around 3,000 feet.
The cold snap, which is forecast to last through Saturday, results from a cold upper-level trough of low pressure, according to the National Weather Service. Temperatures will be 10 to 20 degrees below normal, and highs in L.A. County valley areas today will be in the 50s and low 60s, according to NWS forecasters.
Showers starting Friday afternoon will douse the region through Saturday morning, then taper off, forecasters said, adding they expect only a quarter of an inch of rain or less, although up to three inches is possible any area where a heavy shower occurs.
The snow level, around 5,000 feet at the start of the day, will drop to near 3,000 feet by late Friday night, according to an NWS advisory. Only light accumulation is expected, although up to 3 inches is possible above 3,500 feet, especially on northern San Gabriel Mountains slopes in Los Angeles and Ventura counties.
"The snow should affect travel along Interstate 5 in northwestern Los Angeles County, including over The Grapevine, especially late today and tonight," according to the advisory. "Motorists planning to travel into the mountains through Saturday should be prepared for the possibility of snow showers, reduced visibility, sharply colder temperatures and icy road conditions."
The CHP uses temperature sensors built into the pavement to keep track of hazardous conditions along the Grapevine.
"When the time comes to shut it down, it's very easy," CHP officer Brent Townsend said. "To anticipate that time is very difficult."
That means if a closure becomes necessary, drivers likely will not get much notice.
Forecasters warned there also could be light snow showers in the foothills above the Antelope Valley, although no accumulation is expected.
On Saturday night, the skies will clear and a cold air mass will appear over the region, plunging overnight valley temperatures to between the mid 20s and low 30s, according to the NWS. Forecasters said conditions may warrant frost advisories or freeze warnings.
The NWS forecast highs Friday of 42 on Mount Wilson; 56 in Lancaster; 57 in Palmdale; 59 in Saugus; 62 in Burbank and Woodland Hills; 63 in Pasadena and San Gabriel; 65 in Avalon, Anaheim and Newport Beach; and 66 in downtown L.A., Long Beach and at LAX.