The Perfect Firestorm

Wildfires are not uncommon in California, but the recent fires that began on October 21 have been exceptional. A combination of severe drought and unusually strong winds fueled the blazes, which resulted in at least seven fatalities, destroyed over 1500 homes, burned more than 500,000 acres of land, and caused billions of dollars of damage. Even with nearly 9000 firefighters on the front lines, authorities struggled for several days to contain the raging fires. The reason: a combination of weather and climate conditions that provided a recipe for the perfect firestorm.

Two major factors contributed to the severity of the October 2007 wildfires: a severe and ongoing drought, and unusually intense Santa Ana winds. As of October 24, Los Angeles had received only 3.21 inches (8.2 cm) of rain during the 2006-2007 rainfall season; this is approximately 25% of average. In much of southern California, 2007 has been the driest year on record. This drought laid the foundation for the wildfires, but even under drought conditions wildfires can be contained with relative ease given favorable weather conditions - this brings us to the second half of the meteorological one-two punch: the Santa Ana winds.

The Santa Ana winds are a regional phenomenon that is most common during the fall and early spring in southern California. Santa Ana winds can occur when a high pressure center affects the Great Basin (located at relatively high altitude between the Sierra Nevada and Rocky Mountains). Winds spiral clockwise around the anticyclone of the high pressure system, forcing easterly winds to blow down the mountains into the coastal lowlands of southern California. To a close approximation, air obeys the ideal gas law, which states that as air is compressed (due to increasing pressure as the winds descend down the mountain slope), it must warm at a rate of approximately 9.8 degrees Celsius per kilometer of altitude it drops. This hot, dry air is then channeled through mountain passes on its way to the coast, resulting in winds that can approach hurricane force. Temperatures above 90 F and relative humidity below 20% are not uncommon during Santa Ana wind episodes. When Santa Ana winds combine with an ongoing drought, conditions are ripe for wildfires. The Santa Ana winds that fueled the recent wildfires reached speeds upwards of 60 mph. Strong winds help wildfires to grow not only by increasing the rate at which the fire spreads, but also by providing strong ventilation, bringing in more oxygen to fuel the flames.

Locally, we have had little to worry about from wildfires for much of the year. Unlike several recent years when dry conditions helped fuel wildfires in Oklahoma and Texas, 2007 has been an exceptionally wet year, providing abundant soil moisture to keep brush and grass from drying out. Ironically, this could increase the fire danger in coming weeks and months as the abundant vegetation that grew during the summer dies off over the fall and winter seasons, particularly if the relatively dry conditions we've seen over the past month persist. You can stay up to date on the local fire danger by checking the website of your local National Weather Service office - in the event of high fire danger, they will issue a Red Flag Warning and provide information about local hazards.

For more information about the California Wildfires, we recommend the following article: http://en.wikipedia.org/wiki/California_wildfires_of_October_2007.

For information about local fire weather conditions, please visit the Oklahoma Mesonet's Fire weather page, or the website of your local National Weather Service office.




Story is © Nate Snook, 2007