Alexander Gershnov

Santa Ana winds are a type of downslope wind. They accelerate down the slopes of the coastal topography here. And basically, they originate in an elevated basin at about 1,200 meters behind the Sierra Nevada mountains and goes all the way to the Salt Lake. And you have... cool air masses settling into the Great Basin.

And when you have the pressure gradients, basically high pressure around the four corners area, you get this air mass to come over and through the passes and through the gaps in the surrounding topography, basically getting pushed towards the ocean and it basically accelerates downhill under the force of gravity. And you get these dry, gusty, downslope winds that we call Santa Ana's here.

Downslope winds happen everywhere there are mountains. But these winds are very special because of the geography of the southwestern United States and specifically the fact that we have this elevated Great Basin where these winds are really rooted. They're basically the breast of the desert.

The one unusual thing about this particular event is that there's a lot of what meteorologists call upper air support. And basically, it's the jet stream that's blowing in the high troposphere that happens to be meandering in such a way that it just intensifies that flow from the northeast coming right into Southern California.

All the ignitions are human-caused, and there are plenty of sources of ignition out there because population is expanding into that wildland-urban interface, that sloping topography where the Santa Ana winds blow. And a lot of the sources of wildfire ignitions are going down, things like cigarette butts and campfires.

But ignitions due to arson and electrical power infrastructure are actually going up. So the more the population spreads into those fire-prone areas, the more sources of ignition increase. There are.

Well, these are very dry, gusty winds, and they can be strong enough to impact infrastructure. Like, for example, cause power lines to sway and contact each other when... You know, sparks fly, they can break trees or branches of trees that then land on top of power lines, for example, and that causes shorts and sparks to fly. These are very common causes of ignition.

Basically, the moisture and rain gets squeezed out of these systems right on those same slopes where the Santa Ana winds blow the hardest.

And, you know, our power utilities are operating a lot more safely lately. They have what they call public safety power shutoffs when the weather conditions are predicted to be very fire prone, specifically Santa Ana winds in this region. And when the vegetation is very dry and the National Weather Service calls a red flag warning, then that triggers a public safety power shutoff.

You know, we've seen this before, but with climate change, we do expect the wet season to start later. Basically we expect the wet season to contract into the winter and start later and finish earlier. That together with warmer conditions basically spells a longer and more intense fire season. And I say more intense because when these fires occur in Santa Ana winds,

In December and January, they're occurring during the peak season for Santa Ana winds. So then wildfires that start far enough away from the coast and have room to burn, they can become huge. This is what happened in 2017-18 with the Thomas Fire that is the biggest wildfire on record in Southern California. I think of the Thomas Fire as the poster fire for future fires in a warmer climate.

You know, here we have a repeat of what happened seven years ago.

Well, in most places, wildfire season is in the summer, like you say, Emily. But here in coastal California, and especially in Southern California, the season for the biggest wildfires is October, because that's when the fuels are dry after the dry, hot summer. That's when you have the confluence of the winds that are capable of spreading huge wildfires and the dry fuels.

But later on in November, December, we typically get enough rain so that the fuels are not flammable or that flammable anymore. But this is an exceptional year.

My pleasure, Emily. Thank you for reaching out.

Specifically in that region where the wildfires are right now, you have a lot of vegetation dry and ready to burn. And that's exactly where you get the strongest Santa Ana wind.