Two weather patterns to blame for California wildfires

Northern California and the Bay Area have been tormented with wildfires and dense smoke episodes since mid-August.

We still have over a month to go in fire season, and a record number of acres have already burned in our state.

There are two main weather patterns that trigger wildfires in our area: lightning storms and periods of strong offshore winds. As could only happen in 2020, both patterns contributed to this extreme fire season.

Summer and fall lightning events usually result when bands of tropical moisture push northward into California. In mid-August, several fire complexes (multiple lightning caused fires that merge together) were triggered when moisture from tropical storm Fausto surged up the California coast on the back of a strong subtropical jet stream flow. More than 10,000 lightning strikes, nearly 10 percent of the state’s annual average, were detected as the moist remains of Fausto moved through the state.

Two lightning caused fires, the SCU Complex (North Bay) and LNU Complex (South Bay), were not fully contained until early October. They rank as the third and fourth largest wildfires in California history, burning an area almost as large as the state of Rhode Island.
Astonishingly, the SCU and LNU fires pale in comparison to the August Complex conflagration, which is still burning after seven weeks. This fire complex has torched more than a million acres in a seven-county area between the Bay Area and Oregon border, making it the first “gigafire” in state history.

Diablo Winds

The other fall season weather pattern that is conducive to wildfire generation results when hot and dry Diablo winds develop. In this case, high pressure develops north and east of California and a surface low builds off the SoCal coast.

Strong pressure gradients develop between the Nevada deserts and the coast, forcing air down the slopes of the Sierra and then downward again as air flows across the Coastal Mountains in the area of Mt. Diablo.

All this downward push heats the air by compression and dries the air to extremely low humidity levels. To complicate things, at least as far as fire generation is concerned, the airflow accelerates as it crosses the hills. The warming, drying and accelerating winds become a weather trifecta for wildfire ignition.

We’ve had several Diablo wind events so far this fall, with each generating a new batch of wildfires. The latest round sparked several new and devastating fires that add to the loss and tragedy experienced in Northern California since mid-August.

October showers?

I hope the remainder of October will see some soothing rainstorms (without lightning) that could put a damper on the existing fires and give firefighters and their support staffs a well-deserved break.

Absent some rain events, our fire season could continue well into November. As we get later into fall, climatology would suggest that Diablo wind conditions are far more likely to develop than surges of lightning producing subtropical moisture.

However, late season tropical storms continue to develop in the eastern Pacific. The potential for a moisture surge related lightning event still exists. It is 2020, after all.

Woody Whitlatch is a meteorologist retired from PG&E. Email your questions or comments to clayton_909@yahoo.com