With exceptional fire weather conditions forecast starting Monday evening, the National Weather Service office issued a rare “Particularly Hazardous Situation” alert for Los Angeles and Ventura counties over the coming days.

The last time a particularly dangerous situation warning was issued for the area was just a month ago, on Election Day, when forecasters warned of extreme fire weather behavior. A day later, on November 6, the Mountain Fire ignited in Ventura County and, buffeted by strong winds, destroyed more than 240 structures. It was the third largest wildfire in Southern California since 2013.

Before November, the last time the National Weather Service office issued a particularly hazardous situation warning for Los Angeles and Ventura counties was in 2020.

National Weather Service meteorologist Robbie Munroe said warnings of particularly dangerous situations should only occur every three to five years on average. But just like in 2020, it has now happened twice in the same fire season – once in October and another time in December.

Munroe warned that the “particularly dangerous situation” was expected to begin around 10pm on Monday and last until 2pm on Tuesday. He said the extreme fire weather could be “similar in magnitude” to the howling winds in Santa Ana on Nov. 5 and 6 that fueled the Mountain Fire.

The windy, dry and vegetation-drying conditions are also similar to those that sparked the Thomas Fire of 2018, Southern California’s second-most destructive blaze since 2013, which destroyed more than 1,000 structures in Ventura and Santa Barbara counties. (The region’s most destructive fire during this period was the Woolsey Fire in 2018, which destroyed more than 1,600 structures in Los Angeles and Ventura counties.)

What makes this week’s weather forecast so dangerous?

(National Weather Service)

A primary focus of forecasters during this wind event will be the traditional Santa Ana wind corridors in Los Angeles and Ventura counties.

This time of year, Santa Ana winds form when a high pressure system builds in the Great Basin – over Nevada and Utah – and then seeks a release valve offshore where low pressure prevails.

Under these extreme conditions, valleys and ravines can turn into wind tunnels. On the way to the beaches, a descending wind blows at high speed, generally in a northeast-southwest direction. With gusts of up to 80 miles per hour, a small fire could turn into an inferno, whipping out embers at astonishing speed and leaving firefighters little choice but to try to get people to evacuate.

(Paul Duginski / Los Angeles Times)

One wind corridor to watch in Santa Ana will be that of the Santa Clara River Valley, which essentially funnels air sinking from a high-pressure area over the high desert to the Ventura County coast via Santa Clarita.

“With any new fire in these wind corridors, rapid fire spread is expected,” Munroe said. “The damaging winds will also likely result in isolated downed trees, power lines and power outages.”

Other areas exposed to Santa Ana winds include the San Fernando Valley and the beaches between Ventura and Malibu.

(Paul Duginski / Los Angeles Times)

Boaters should be aware of extreme winds expected to be strongest in the area from Ventura to Malibu and offshore to the Channel Islands. Strong winds are also expected in the San Pedro Channel and Santa Catalina Island. Waves of 3 to 6 feet and coastal flooding could hit east-facing harbors like those on Catalina Island, Munroe said.

Criteria for issuing a Red Flag Warning, constituting a particularly hazardous situation, include forecasting sustained winds of at least 35 mph and/or gusts of at least 60 mph, single-digit relative humidity, and extremely dry vegetation.

There could be peak gusts of 40 mph in Northridge, 44 mph in Oxnard, 48 mph in Fillmore, 52 mph in Westlake Village, 53 mph in Santa Clarita, 55 mph in Pyramid Lake and 60 Enter miles per hour in Acton. Munroe said there is a possibility over the next two days that airports in Southern California will be forced to postpone flights due to strong winds.

Minimum relative humidity is expected to reach very low levels – just 5% in Oxnard, Fillmore, Westlake Village and Northridge; 6% in Ojai; and 7% in downtown LA, Santa Clarita and Pasadena.

A regular red flag warning is expected to be issued for much of Southern California late Monday morning through Wednesday, including the San Fernando and San Gabriel valleys, the mountains of San Luis Obispo County, the Inland Empire and large portions of San Diego County.

Warning signals are expected across much of Southern California between Monday morning and Wednesday.

(National Weather Service)