High-gusting Santa Ana winds will combine with low humidity levels Wednesday to create wildfire conditions in bone-dry mountain and forest areas of the Southland and, perhaps, the valleys as well, forecasters said.
The National Weather Service said a red flag warning denoting wildfire conditions would be in effect from 3 p.m. Wednesday until 3 p.m. Thursday in the mountains of Los Angeles and Ventura counties, including the Angeles and Los Padres national forests.
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"While this warning is in effect for only the mountains, heightened fire-weather concerns will exist for the valleys of Los Angeles and Ventura counties as well, especially the Santa Clarita and eastern Ventura valleys," according to an NWS advisory.
Wind gusts in the valleys are expected to range between 30 and 40 miles per hour, according to the NWS.
Describing the conditions behind the wildfire risk, the NWS said high pressure will form Wednesday over the surface of the Great Basin, churning up gusty offshore northeast winds of between 15 and 30 miles per hour in mountain areas beginning Wednesday morning, gusting to 40 mph and perhaps 50 mph through some passes and canyons.
Those winds will then sweep into the valleys Wednesday night and persist through early Thursday afternoon before weakening, according to an NWS advisory.
"These winds will also bring a very dry air mass to Southern California, which will result in widespread humidities below 10 percent by late Wednesday or Thursday, lasting through the end of the week," it said. "If fire ignition occurs, there could be rapid spread of wildfire that would lead to a threat to life and property."
Dry vegetation also will be a factor in creating dangerous conditions, forecasters said.
Ironically, some of the same conditions that are creating a risk of wildfires will bring the Antelope valley the first frost of the season, according to forecasters.
"While the daytime temperatures will be on the rise due to the offshore winds, the influx of very dry air will cause much cooler conditions Wednesday and Thursday nights across interior portions of Southwest California," said an NWS advisory. "There is the potential for temperatures to reach the freezing mark in the coldest wind-sheltered portions of the Antelope Valley."
A frost advisory will be in effect from 1 a.m. to 9 a.m. Thursday in the Antelope Valley, where temperatures will generally run between 28 and 32 degrees overnight, and a less serious freeze watch from late Thursday night through Friday morning, the NWS said.
"The cold temperatures could result in possible damage to crops and sensitive plants," according to NWS advisory.
The weather service also issued a wind advisory, indicating that winds of 35 mph or greater are expected, which will be in effect from 3 p.m. Wednesday until 3 p.m. Thursday in the Los Angeles County portion of the San Gabriel mountains and from 8 p.m. Wednesday until 3 p.m. Thursday in the San Fernando and Santa Clarita valleys and the Santa Monica Mountain Recreational Area.
The NWS forecast highs Wednesday of 59 on Mount Wilson; 65 in Newport Beach; 66 at LAX; 67 in Avalon; 68 in Palmdale; 69 in Long Beach and Lancaster; 70 in downtown L.A.; 71 in Burbank, San Gabriel, Newhall and Anaheim; 72 in Pasadena, and 74 in Woodland Hills.
Wednesday's partly cloudy skies will give way to more widespread sunshine Thursday, when temperatures will be 8-9 degrees higher in some places. They'll be a few degrees higher than that on Friday.