Southern California’s devastating wildfires are fueled by numerous factors: dry vegetation, drought conditions, dense residential areas, and potential ignition sources (e.g., cigarettes, campfires, power lines, arson). Gravity also plays a significant role.
A crucial factor is the movement of air—specifically, cold air masses approximately 1,200 meters high in California’s mountains and the southern Oregon and northern California ranges. These descending air currents, known as Santa Ana winds, are far stronger than the warmer, less dense air at sea level. This powerful downdraft is ideal for rapidly spreading wildfires.
Alexander Gershunov, a research meteorologist at Scripps Institution of Oceanography, University of San Francisco, explains these downslope winds as similar to water flowing over obstacles, accelerating as they descend, creating an “airfall.”
These powerful winds have significantly exacerbated the wildfires, resulting in numerous fatalities, widespread displacement, and substantial property damage in the Los Angeles area. Last week, particularly strong gusts, especially in the Santa Monica Mountains, were recorded. While winds lessened over the weekend, they were predicted to reach 65 mph by Wednesday, January 15th and Thursday, January 16th – speeds comparable to Category One hurricanes.
The frequency of Santa Ana winds is somewhat predictable; Los Angeles typically experiences five wind events in December and four or five in January. These are primarily caused by seasonal changes in pressure gradients and the jet stream over the Great Basin and Four Corners regions.
However, Santa Ana winds aren’t limited to December and January. October also sees wind events, and the current situation is unusual because the strong winds arrived before the typical winter rains (usually starting in June or July). The lack of rain—the latest start to the wet season in 150 years—with no rain currently forecast, exacerbates the wildfire risk with the continuing Santa Ana wind events.
Gershunov anticipates additional Santa Ana wind events, potentially two to three before the arrival of rain. This mirrors the 2017-2018 situation, where wildfires burned extensively until an atmospheric river storm in January 2018. The resulting debris flows in Montecito, Santa Barbara County, tragically caused 22 deaths.
Fighting wildfires in dry, windy conditions is extremely challenging. Dry vegetation acts as accelerant, and wind both intensifies the flames and spreads embers. Gershunov likens the wind’s effect to a giant, uncontained air blower directed at a fireplace.
While weather forecasts provide some predictability regarding Santa Ana wind resurgence, local terrain features create unpredictable turbulence. Furthermore, upper atmospheric conditions can be even more erratic. Gershunov notes an unusual jet stream pattern last week, directing wind directly into the L.A. basin.
Low humidity is another critical factor. Hot, dry winds from the Sierra Nevadas, White Mountains, and Klamath Basin deplete atmospheric moisture, drying out vegetation. Low humidity, coupled with dry fuels, creates ideal conditions for uncontainable wildfires. Ocean winds can even push fires uphill, hindering natural fire suppression.
One positive note is the clear skies during the current Santa Ana events, eliminating lightning as an ignition source. However, the extensive damage and loss of life highlight the immense challenges of wildfire suppression in such conditions. The current crisis will eventually subside, though the timeline remains uncertain.
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