Such winds can spread fires at astronomical rates.
When the first round of winds fanned the Thomas Fire, which has now scorched 90,000 acres, the blaze engulfed the region from the mountains around Santa Paula, Calif., to near Ventura and Ojai in just 24 hours. “It’s an incredible rate of spread that I don’t think we’ve ever seen,” said David Sweet, a meteorologist at the National Weather Service office serving Los Angeles.
“There will be no ability to fight fires in these kinds of winds,” state fire chief Ken Pimlott told the LA Times.
Hundreds of thousands of residents received a text alert Wednesday night that warned of the extreme fire danger. “Stay alert. Listen to authorities,” the message said.
The Weather Service placed much of southwest California in its critical risk zone for fire weather Thursday, including Los Angeles and San Diego, encompassing a population that exceeds 10 million. “Dangerous and extremely critical fire weather conditions are expected across a broad portion of southern CA today,” it said.
The interior portions of this region were declared an “extremely critical” risk area, the most severe classification, due to explosive combination of the howling winds and humidity values of just 5 to 10 percent. Isolated wind gusts near and above 90 mph were even possible in this zone.
Late Wednesday and early Thursday, winds were already sustained at 30-40 mph with gusts to 50-85 mph in the region’s higher terrain. Such winds were forecast to persist into Thursday afternoon.
Active fires will rapidly grow in such winds while the region becomes vulnerable to new fires. “If fire ignition occurs, very rapid spread and extreme fire behavior is likely,” cautioned the Weather Service office serving Los Angeles. “Everyone needs to exercise extreme caution when handling any potential fire ignition sources … such as campfires, cigarettes, welding and brush clearing equipment.”
By Friday morning, winds are forecast to relax some, allowing the fire risk to ease, but the Weather Service warns that winds could increase Sunday through Tuesday next week.
The dangerous fire conditions are being driven by Santa Ana winds, which develop when high pressure systems form east of the Sierras. The clockwise flow around these high pressure systems force winds to blow from east to west over the Southern California mountain ranges before they crash down through canyons. As the air is blown down the mountain slopes, it warms up and dries out. The result is a devilishly perfect atmosphere for fire growth.
This Santa Ana wind event is the longest and strongest of the year, Sweet said. While not in the same league as an event in 1951, which lasted 24 days, “this one is looking to have the potential to be about 10 days long, which is definitely impressive,” he said. Sweet said it was particularly notable to have produced two severe wind impulses in three days.
Weather satellites have captured stunning imagery of the massive smoke plumes ignited by these explosive fires so far.
And, on Wednesday, the International Space Station captured these views:
The tinderbox conditions which have hastened the spread of these fires were likely intensified by abnormally warm and dry conditions in coastal Southern California during October and November. It was the warmest and second driest October-November on record in the region, tweeted Daniel Swain, a climate scientist at UCLA.