Red flag warnings denoting a high danger of wildfire will be in force Monday and through at least Thursday in Los Angeles and Orange counties because of strong Santa Ana winds and low humidity, according to the National Weather Service.
''This will likely be the strongest and longest duration Santa Ana wind event we have seen so far this season,'' which runs from late September through April, according to an NWS statement. "If fire ignition occurs, there will be the potential for very rapid fire spread ... and extreme fire behavior.''
The strongest of this week's winds will be Monday and Tuesday, NWS said. Santa Ana winds could persist into Friday or Saturday, which would requite that red flag warnings to be extended, the NWS said.
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A red flag warming went into effect in the San Gabriel and Santa Monica Mountains at 10 Sunday night and will remain in force at least until 6 p.m. Thursday. At the same time, red flag warnings will be in force in the forests that blanket the San Gabriels -- the Angeles National Forest in LA County and the Los Padres National Forest in Ventura County.
Over both mountain ranges, the wind will blow Monday at sustained speeds of 20 to 40 mph, gusting to between 45 and 65 mph, said the NWS, adding that wind gusts during the week could reach 75 mph in the Santa Monicas and 80 mph in the San Gabriels. Humidity levels will be very low today through Thursday -- 5-10 percent in the Santa Monicas and 5-15 in the San Gabriels, according to the NWS.
Additionally, red flag warnings went into force at 3 Monday morning and will last until 6 p.m. Thursday in the San Fernando, Santa Clarita and San Gabriel valleys and the Los Angeles coastal zone, which consists of beach cities, metropolitan LA, the downtown area and the Hollywood Hills. Northeast winds of between 20 and 35 mph gusting to between 40 and 55 mph are forecast for those areas, although 65-mph gusts are possible in the valleys tonight.
Also forecast are humidity levels of 15 percent, later today falling to between 5 and 10 percent, forecasters said.
A red flag warning will also be in force until Thursday night in both coastal and inland Orange Country, where winds of 20-30 mph gusting to 55 mph are expected, although gusts of 70-80 mph are possible Tuesday and Thursday, according to the NWS. Humidity levels in Orange County will fall to 10 percent Monday but dip to around 5 percent between Tuesday and Thursday.
Wind advisories of varying severity were in effect along with the red flag warnings.
In the Antelope Valley, "a cold Santa Ana wind event'' is expected, lowering temperatures to the upper teens and lower 20s tonight and Tuesday night, forecasters said.
"...Temperatures may be cold enough to cause pipes to burst,'' warned an NWS statement.
But in most of LA County, temperatures should be mild this week amid consistently sunny weather. Downtown L.A, for example, is forecast to hit 73 degrees today, 77 Tuesday, 79 Wednesday, 82 Thursday, 81 Friday, then 80 Saturday and Sunday. Roughly similar temperatures are expected in inland Orange County.
Temperatures will be about normal in the early part of this week but around 10 degrees above normal by Thursday, Kaplan said.