Dry and windy conditions will continue through the week, with the most extreme Santa Ana wind conditions peaking for Orange, Los Angeles and Ventura counties on Thursday, Dec. 7. Northeast winds of 20-35 mph can be expected with gusts exceeding 60 mph over the higher terrain and in the more wind prone areas. Winds will diminish in the afternoon. Humidity will be in the 5-15% range.
The image below is from the USDA Forest Service and Predictive Services website called the Santa Ana Wildfire Threat Index that categorizes the threat levels in our region.
The site sawti.fs.fed.us is a result of a recent collaborative effort between San Diego Gas and Electric, UCLA, and the Forest Service. The site allows first responders and the public to zoom in on their area and get updated information regarding fires, wind and surface fuel conditions.
Related: Multiple wildfires burning in Southern California
Tom Rolinski, USDA Forest Service Senior Meteorolgist, compiled the chart below. It shows the average number of days with Santa Ana conditions.
Rolinski said, “The term “moderate to strong” can be defined differently, but here it generally refers to sustained winds of roughly 20 mph or greater. Keep in mind these are average numbers as some years can vary widely.”
Southern California Average Santa Ana Winds per month
Avg. number of days with Santa Ana conditions (dry air and wind gusts):
Avg. number of days with “moderate to extreme” winds and high fire risk:
Above: Image from NASA of smoke blowing offshore from fires on Dec. 5.
How the Santa Ana winds strengthen
1. A high pressure builds over Nevada, Utah and adjoining states, sending winds in a clockwise rotation across the Mojave Desert toward the sea.
The image below is a National Weather Service image from Tuesday afternoon showing high pressure systems that are expected to continue through the week.
2. Air molecules compress as they move through Southern California canyons and passes. Compression makes the air warmer and reduces its relative humidity. Winds gain more speed as they funnel through the many canyons in the Santa Ana mountains.
3. Winds roar out of local mountains. Wind speeds often top 60 m.p.h.
Wind and fuel for the fires
The map below indicates the type and rank of surface fuels for fires in the region. A high amount of surface fuels combined with intense Santa Ana Winds has led to the largest fires in state history. The state created a strategic fire plan in 2010 to try and identify the most hazardous areas and limit destruction.
Source: USDA Forest Service, Orange County Fire Authority
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Source: Oc Register