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This is how Southern California wildfires exploded into a critical disaster overnight

Intense Santa Ana winds are creating a critical wildfire situation in Southern California. (REUTERS/David McNew)

Thousands of people were forced from their homes on Monday night in Southern California when a brush fire went from 0 to 30,000 acres in just a few hours. The so-called Thomas Fire, which pushed into the Ventura in the early morning hours of Tuesday, was stoked by what forecasters have been calling the most intense Santa Ana wind event of the season.

To firefighters’ dismay, the hot, dry wind is expected to continue through the end of the week, with more wildfires popping up across the region.

Tuesday will probably be the worst day for wind in Southern California. Gusts are peaking around 70 mph in the L.A. County mountains and Santa Monica Range. Humidity is nearly-nonexistent at around just 10 percent. A fire weather warning is in effect for Los Angeles and Ventura counties through Thursday.

The wind and dry air is due in part to an area of intense high pressure is building over the West Coast. The ridge in the jet stream is forecast to push far north into northwest Canada and southern Alaska — which means not only is it going to be dry, but temperatures are going to be warmer than normal from Southern California through western Canada.

Latest news updates on the Ventura and Los Angeles wildfires

The other part of this weather pattern is the Santa Ana effect — air flowing from the high-elevation California interior to the ocean. These winds are notoriously hot and dry. The National Weather Service says these Santa Anas could be the most intense of the entire season.

Given that forecast, the National Weather Service is using strong language to convey the severity of the situation in the region, which includes Ventura, Los Angeles, Orange, San Bernardino and Riverside counties.

“This will likely be the strongest and longest duration Santa Ana wind event we have seen so far this season,” the Weather Service wrote in its fire weather warning Monday. “If fire ignition occurs, there will be the potential for very rapid fire spread, long range spotting, and extreme fire behavior.”

Usually, the Santa Ana season runs from spring to late fall, with October being one of the worst months. But sometimes the season extends into late fall and early winter, as we’re seeing this week. The vegetation in California is already parched, since summer months tend to be the region’s driest.

Santa Ana winds blow from east to west, from high elevation to low elevation. As the air flows down to sea level, three things happen: It gets warmer, winds get faster and humidity plummets.

The sinking air is compressed because of a pressure increase — something you can feel when your ears pop on a drive out of the mountains. As air is compressed, it heats up. This is the first law of thermodynamics at work.

During the sinking process, the amount of moisture in the air isn’t changing much, but the temperature is still rising. This creates a large disparity between temperature and moisture, and pushes the relative humidity very low.

The wind also gets faster as it’s compressed, forced over mountains and pushed through canyons. This is how 80-mph winds are in the forecast for the mountains around Los Angeles this week.

Heat, low humidity and wind aren’t enough to create wildfires, but if one does start, the growth could be wild and rapid. These factors proved to be deadly earlier this year when a fire ignited and raced across the counties north of San Francisco Bay in the overnight hours. At least 36 deaths were attributed to that fire, all in regions that did not receive wireless alerts.