Some politicians have implied recently last weeks fires show we are at the beginning of a long fire season. That may certainly be true in the Sierras and northern California, but southern California has a different fire season. Let me explain…
Across much of the west, summer means melting snows in the mountains give way to warmer and drier conditions. Meadows that were green in the spring are beginning to dry. Other fuels are drying for the summer season. And fire threat is increasing.
But for southern California (outside the mountains) it is normal to be without rain for the summer. Most summer fires are not fanned by hot, dry offshore (Santa Ana) winds. They are fanned by the sea breeze as it pushes into the valleys. Besides having higher humidities, these breezes are diurnal, meaning they tend to cease late in the day after the peak heating. Thus they generally can be more easily attacked by fire fighters. To be sure, there have been exceptions…but you will find the history of large fires in southern California shows a peak season from mid September to early or mid May.
In the early fall the peak threat for the Southland begins in September when cold outbreaks in the Plains and Great Basin push southwest and turn into our offshore, or Santa Ana winds. The threat ends when fuels are wet for the winter, usually after widespread rainfall of several inches.
The threat often returns briefly around May 1. I used to call this “mini-season” at the National Weather Service in San Diego. Cool season rain has pretty much ended, it’s warming up. The fuels are drying out again, and there are a few Santa Anas left. So we generally see a short-lived increase in large fires for a while in May. But this years “mini-season” was not so mini. Two very strong Santa Anas in May, combined with fuels dry from a winter with precipitation less than 50% of normal in many places, set the stage for some catastrophic fires, especially in San Diego County.
When the Santa Anas go away, and the deep marine layer returns (like today) the threat of large fires goes with it. That is…until September!!
Jim Purpura, CCM
Director of Weather Forecasting