A NOAA computer model showing the Santa Ana winds on Dec. 1, 2011.
Whipping winds returned to Southern California yesterday (Dec. 5), but with much less force than last week's winds that knocked out power to thousands across the region. Still, firefighters are on wildfire alert because of the low humidity and drier conditions.
In Southern California, these freakish winds are called Santa Ana winds, strong down-slope winds that blow through the Santa Ana Mountain passes at speeds of 40 mph (64 kph). Last week's windstorm brought gusts of up to 80 mph (129 kph) in Southern California. The storm was one of the strongest in decades, meteorologists said. Today's winds were expected to hit 40 mph with gusts to 60 mph (97 kph) possible, according to the National Weather Service.
The strong winds throughout the West Coast are caused by an extreme pressure change between the Northwest and Southwest regions. A sprawling high-pressure system following a cold front has created a difference in pressure that sends wind surging southward. In the Northwest, a strong high-pressure system built along with a clockwise flow of winds. In the Southwest, a low-pressure system developed along with a counter-clockwise flow. The tight so-called pressure gradient that formed between these systems is driving winds over mountains and through the California canyons.
The winds can be of tropical storm strength (winds less than 74 mph, or 119 kph) at lower elevations and hurricane-strength (74 mph or greater) above 1,000 feet (305 meters). These winds are even more frequent and intense during the La Niña conditions due to the dry air that sits over the Southwest.
The above image uses the U.S. National Oceanic and Atmospheric Administration's North America Model to visualize wind speed and direction on Dec. 1. The lightest blue-to-white colors represent the strongest winds — up to 80 mph. The arrows show the direction the winds are blowing. The highest winds can be seen blowing in between the San Gabriel Mountains. Winds this strong here can easily down trees and power lines and spark fires.
Today's strong winds are a wildfire threat because of the relatively low humidity in the area, Los Angeles County Fire Department Capt. Mark Savage told the Los Angeles Times. In Southern California, dry, brush-covered hills are a big wildfire risk. If a fire here gets out of hand, the strong winds will prevent firefighters from using aircraft to douse the flames.