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Alexis Madrigal: Welcome to Forum. I’m Alexis Madrigal. When I first started writing about the climate almost twenty years ago, there were all these different scenarios for what might happen. The climate was going to get weird. The Arctic would melt. The tundra would thaw out. And it wouldn’t just be hot—things would happen that no one alive could remember happening, things that just didn’t exist in our modern records. And it wouldn’t happen just once, but with increasing frequency. A stable climate system would swing into a new and unstable zone.
A heat dome is setting up over basically the entire western United States. Temperatures we might reach a couple of times this summer—maybe in August—are going to happen this week. One atmospheric scientist, Ross Schumacher, posted: “By how much and for how long will the upcoming weather pattern be breaking March records in the West? A lot and a long time.” It sure feels like part of a new climate reality.
And here to discuss it, we have climate scientist Daniel Swain. He’s with the California Institute for Water Resources at the University of California. Thanks so much for joining us, Daniel.
Daniel Swain: Thanks for having me.
Alexis Madrigal: Let me just put it bluntly to start. Is what is happening just a sign of global warming? Incontrovertibly?
Daniel Swain: The short answer is yes. We know that in a warming world we see both more frequent and more extreme heat events. In particular, that’s the most slam-dunk type of event when it comes to thinking about extremes and climate change. And this is going to be exactly that type of event.
It will be, in a climatological and statistical sense, record-shattering. I’m using that language intentionally because we’re not just breaking records—we’re breaking long-standing records by enormous margins. Essentially to a point where it would be almost impossible to have heat waves of this kind of magnitude if it weren’t for the warming that’s already occurred.
Alexis Madrigal: When we started thinking about doing this show, part of the reason was that meteorologists—weather forecasters and people looking at atmospheric models, temperatures, wind patterns—were kind of gasping with horror at the unprecedented nature of this event, at least in our modern records. What was the thing you saw that made you think, “Oh, this is different from a normal heat wave coming in March?”
Daniel Swain: Well, first of all, in most of the West there is no such thing as a normal heat wave in March. The main thing that really jumps out is the seasonality of it. This is still, in most of the West, effectively the last month of winter—the time when snowpack is usually still accumulating and often reaching its peak.
Historically we measure peak snowpack around April first. But that is not going to be the case this year. Our snowpack was already in bad shape, and as people talk about in this conversation, it’s going to get worse—much worse—during this heat wave.
For me, the remarkable thing is that this looks like a legitimately summer-like heat wave in the middle of March. That’s an incredible anomaly—just as much as if we had record-breaking heat in the summer. In some ways it’s even more conspicuously weird because it’s going to be eighty, even ninety degrees in places that would typically be seeing snow this time of year.
Alexis Madrigal: How is this happening meteorologically? People are talking about the strongest ridge ever observed—I think that’s your quote—for the November-to-March period. What does that mean?
Daniel Swain: When we talk about ridges, it’s a bit of meteorological inside baseball. But fundamentally it means there’s high pressure in the atmosphere—not just at the surface, like you might see on a TV weather map with the H’s and L’s—but high pressure throughout the whole lower half of the atmosphere.
So not just near ground level, but all the way up tens of thousands of feet.
When we say the magnitude of the upcoming ridge over the West is the greatest we’ve ever experienced—not just in March, but in any cool-season month in the historical record—it means that the entire lower half of the atmosphere is going to be as warm or warmer, on average, than it has ever been in recorded history in this region.
That’s actually a more significant statement than just saying the surface is record warm. It means the whole lower half of the atmosphere—well above the tops of the highest mountains in the West—is going to be record warm. Not just some thin layer near the surface, but the whole cake, as it were—all the layers.
Alexis Madrigal: Why is that important? Does it extend the length of the event because there’s more warm air sitting there?
Daniel Swain: In a sense, yes. When anomalous warmth extends throughout the entire lower half of the atmosphere, that’s what forms the ridge—or what’s sometimes colloquially called a heat dome.
“Heat dome” isn’t an official meteorological term, but it gives the right sense of what’s happening. It’s a dome-like structure in the atmosphere. The air within it is prevented from rising and escaping. Instead, the air is descending—from higher altitudes down toward the surface—in the middle of this high-pressure system.
As that air descends, it warms and dries. And through various meteorological processes, the bigger and warmer and more persistent the ridge becomes, the more it reinforces itself. That’s what we call positive feedback. The stronger it gets, the more likely it is to remain strong and persistent.
Right now, parts of the West have already been record warm for the past week. This week is going to blow last week out of the water, and next week is looking pretty darn hot for March as well. So this does look like a prolonged event.
Alexis Madrigal: I saw it described online as an anticyclone. Is it almost like the strengthening we see in hurricanes—where a feedback loop keeps intensifying the system—but for heat?
Daniel Swain: An anticyclone is essentially just another word for high pressure. A cyclone is low pressure, and an anticyclone is high pressure.
You can have a weak, low-impact anticyclone that just brings a few days of mild weather and clear skies—that’s not unusual. But this anticyclone is part of a broader ridge system that does have some self-reinforcing feedbacks.
That said, it’s actually very different from how hurricanes or other low-pressure systems strengthen. Low-pressure systems can intensify extremely rapidly. High-pressure systems generally cannot.
Alexis Madrigal: So there’s no “bomb heat dome”?
Daniel Swain: Exactly. You can get explosively deepening low-pressure systems—what we call bomb cyclones—but you can’t really do that with high pressure.
That’s part of what makes this event remarkable. To reach this level of intensity, the system has to sit in the same place and gradually strengthen day after day after day. It has to be persistent and positioned just right for that to happen, and this one appears to be doing exactly that.
Alexis Madrigal: How are ocean temperatures playing into this? When I look at the maps, the center of this system seems to be over the interior West, though we’re feeling it here in the Bay Area too. Are ocean temperatures helping drive it?
Daniel Swain: Yes—but not in the way you might think. Not directly.
The ocean near California is unusually warm right now, and that will probably make coastal areas—like downtown San Francisco, Half Moon Bay, and the North Coast—warmer than they might otherwise be during this event. Normally the cool ocean acts like natural air conditioning, but that effect will be weaker.
However, the ocean’s bigger role in driving this heat wave is actually thousands of miles away in the central Pacific, near Hawaii.
Over the past week, there’s been record-breaking rainfall and catastrophic flooding there from what’s called a Kona low—a cyclone near Hawaii. That storm is pumping huge amounts of moisture and energy into the atmosphere.
When clouds form and rain falls, condensation releases heat—what meteorologists call latent heat. In a warm, moist atmosphere, a lot of energy is stored in that latent form. When moisture condenses and falls as rain, that energy is released as sensible heat—meaning heat you can actually feel.
So that moist air near Hawaii releases energy, becomes warmer and drier, and gets caught up in atmospheric flow along the edge of the ridge. That energy then travels across the North Pacific toward British Columbia, where an atmospheric river extracts even more latent heat. From there, the energy wraps around and feeds into the giant ridge over the western United States.
It sounds complicated—and a bit like a Rube Goldberg machine—but believe it or not, the warmer-than-average ocean temperatures near Hawaii are connected through this chain of processes to the heat wave we’re experiencing in the West.
Alexis Madrigal: We’re talking about the heat wave that’s starting today with climate scientist Daniel Swain. We want to hear from you. What questions do you have about this heat dome or our changing climate? Email us at forum@kqed.org. I’m Alexis Madrigal.
