Why did we have a slow Atlantic hurricane season in September?

// This is a machine generated transcript. Please report any transcription errors to will-help@illinois.edu.

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Speaker 0: The weather and climate often cause concern and curiosity, but understanding the facts behind the phenomena can be a challenge. We're here to help. From the University of Illinois Urbana-Champaign and Illinois Public Media, welcome to Weather Realness, a weekly podcast about the skies above Illinois and the world.

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Speaker 1: A note about our next segment of Weather Realness. It is about the unusually quiet Atlantic hurricane season. Our segment was taped weeks before Hurricane [Isaias] was formed. As of October 8th, it is churning in the Gulf of Mexico. There are hurricane warnings from New Orleans to Tallahassee, Florida. Stay up to date on the storm by going to weather.gov.

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Eddie Wolf: I'm Eddie Wolf, graduate research assistant in the Department of Climate, Meteorology and Atmospheric Sciences at the University of Illinois. September is typically the most active time for hurricanes in the Atlantic, but so far this year, there have been no hurricanes in the entire basin. Experts are saying this has smashed the record for the longest wait for a hurricane in the satellite era. So why are we having such a slow hurricane season? And does that mean we won't be seeing any hurricanes at all in the Atlantic this year? Joining us today is Dr. Kim Wood, associate professor in hydrology and atmospheric sciences at the University of Arizona. Thank you for joining us, Dr.

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Dr. Kim Wood: Wood. Thank you for having me. I'm happy to be here.

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Eddie Wolf: So I guess to start off, you know, like we said, there's been no Atlantic hurricanes so far this season, but there have been quite a few tropical depressions and tropical storms. For listeners who maybe aren't as familiar with tropical meteorology, can you explain to folks what sort of the difference in these terms is?

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Dr. Kim Wood: Yeah, the broad term that we use for all these types of storms is tropical cyclone, a cyclone that spins up in a generally tropical or subtropical area. Tropical depression, tropical storm, and hurricane are the categories by which we then further, uh, delineate between them, and that's based off of their maximum sustained winds near the surface. So, and that's over the ocean, so a storm that's say is 75 miles an hour, that would be a hurricane, but over land they may not actually measure those winds because the estimate is over the ocean.

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Eddie Wolf: Interesting, yeah, so I'll talk a little bit more about sort of how we measure these things in a little bit because I think that's really interesting too. Um, but as we already mentioned, you know, September is generally the peak for hurricane season in the Atlantic. How long does the season normally last? What do we consider to be hurricane season?

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Dr. Kim Wood: Officially, the Atlantic hurricane season begins June 1st and goes through November 30th. But in a typical year, and I do mean typical, not 2026, the most activity occurs in a two-month period between mid-August and mid-October. So that's partly why it's so surprising that September has come and gone, and we still don't officially have our first hurricane in the Atlantic. The Easter North Pacific hurricane season is about the same starting May 15th instead of June 1st. But those are the official dates during which the National Hurricane Center monitors and issues forecast outlooks for those regions. But technically, a storm could form outside those dates. It's just very, very rare.

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Eddie Wolf: So before we get into why this season is such an outlier, I think it'd be really interesting to talk a bit about how hurricanes actually form. So, when we talk about storms in the Midwest, our listeners, you might remember that we talked about the ingredients we need like shear and instability and moisture, but how are tropical storms different from that? What kind of ingredients do they need to form?

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Dr. Kim Wood: It's funny that you mentioned those three because when we think about hurricanes, the ingredients aren't super different. The big thing is what do they benefit from and what tends to get in their way. So shear is a bad thing for hurricanes. Warm water is a good thing for hurricanes, moisture is a good thing for hurricanes, instability is a good thing for hurricanes. So this year, what we've seen out in the area called the main development region of the Atlantic, where many storms tend to form, those conditions are working against storm development. So, we've got less instability, we have more shear, and the moisture isn't really there either.

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Eddie Wolf: Interesting, yeah, so can you talk a little bit about why wind shear is harmful to these storms, whereas it's helpful here in the Midwest for something like a thunderstorm.

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Dr. Kim Wood: It's hard to like think about it because when you think about Midwest storms, you're looking for that shear and they're convective, and then you look out to the tropics, and those are convective storms too, but the shear is bad. That has to do with how they're organized and what helps them persist and potentially intensify. So the complicated term is that they're warm core systems, uh, but what that has to do with is latent heat release. So the energy from the ocean is going through the center of the storm, the area we typically call the eye wall once it's strong enough, and in that area latent heat gets released. So it's warming things up in that area and helping the storm to maintain or even intensify itself. If you bring shear into that equation, you're tilting the storm, you're stretching the area over which that same amount of energy is able to get into. And so with less available energy right there and, you know, fighting against the structure the storm wants to have, shear tends to be bad for those storms.

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Eddie Wolf: Yeah, and for our listeners who have perhaps missed some of our episodes with Jeff Frame, uh, talking about things like wind shear, that's just a change in wind direction and speed with height. I should have mentioned that right off the bat there. And so we mentioned already multiple times now, no hurricanes in the Atlantic this year. You said the ingredients just aren't lining up. Sort of on a broader scale, what's causing that to happen? Why aren't these ingredients lining up right this year in particular?

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Dr. Kim Wood: You might be familiar with the idea that what goes up must come down. So we've got this enhanced rising motion out in the Pacific associated with the developing strong El Niño, and all of that energy and rising air, all that mass — once it goes up and moves away, it needs to sink somewhere. And a lot of the time during an El Niño event, that sinking is over the Atlantic where these storms tend to form. The other shift that happens is because where the rising air is geographically located, that changes the overall circulation of the atmosphere in a way that tends to increase shear over the Atlantic.

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Eddie Wolf: So another thing we mentioned earlier was this is the longest wait we've had for a hurricane in the satellite era. So I think that's maybe a term folks haven't heard necessarily. So why do we define things in terms of the satellite era, which is like 1970s to present?

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Dr. Kim Wood: We use that term because that's the time frame during which we had more reliable geostationary satellite coverage. So, a geostationary satellite is orbiting the Earth far enough away that the time it takes to orbit the Earth once is the same time it takes for the Earth to rotate once. So it's as if it's fixed in a point in space over a particular longitude over the equator. And since almost all tropical cyclones develop far enough from the coast that our measurements over land can't capture them, the eye in the sky — or really eye in space — that satellites give us, offer that perspective to be able to consistently monitor for the disturbances that could become storms and then the storms that develop from them.

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Eddie Wolf: So before that era, before we had satellites, how did we sort of measure hurricane seasons? How do we determine whether or not these storms were occurring?

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Dr. Kim Wood: That's such a great question, and it's kind of scary to think about being on a ship out in the Atlantic and like, oh hey, there's a storm here. I'm gonna write it down and hope we survive long enough to report it to somebody, because being on the water in a hurricane is a bad spot to be in. I think it's bad over land, which is obviously not good for a landfalling hurricane. You also have your ocean, you know, pitching and yawing and giant waves coming in. So, definitely not a fun experience to be one of those unlucky sailors who is thankfully telling us about these storms.

There's also, uh, back in like the early 1900s, a lot of barometer readings on land helped us kind of pinpoint where they were, such as the Galveston hurricane in 1900 — a lot of information about it came from Cuban observers of pressure readings.

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Eddie Wolf: Interesting, yeah, so it was basically ships and these land measurements that we had to rely on to kind of triangulate where maybe there were these low pressure centers out over the ocean, um, which I'm sure you can talk more about — this has definitely impacted our record of hurricanes going back into that era where we don't have a good count perhaps of how many were occurring.

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Dr. Kim Wood: Yeah, we use all of the available information, uh, and sometimes more of that will emerge over time. So there's a reanalysis project that's been going on for years and years to make sure that our earlier records are as robust as possible. But that's why a lot of work focuses on statistics relative to the satellite era, because we do have that lower confidence in knowing exactly how many storms, where were they, and how strong did they get before those observations came in.

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Eddie Wolf: Yeah, and you mentioned strength there. Again earlier we talked about these different levels of tropical cyclones from depression up to hurricane. So, what kind of tools do we use to actually determine how intense these storms are when they're still out over the ocean?

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Dr. Kim Wood: Ideally, we would have research and hurricane hunter aircraft flying every single one, but that is absolutely not viable both from a personnel and aircraft and a cost perspective. Uh, crews can only fly for so long, so if it's out way over the middle of the ocean, most of their flight time will be getting to and from the storm. So the region over which in the Atlantic we fly these tropical cyclones is not the entire basin for the most part.

So, satellite estimates come into play. There's a technique called the Dvorak technique that looks at cloud patterns of storms and relates that to how strong the storm likely is, and, uh, advances in that particular technique have given us a lot of uh, confidence in those estimates, both from a forecaster perspective and then from an automated perspective. But we also have uh, views from what are called low Earth orbiting satellites. So they don't have that constant view like a geostationary satellite would. But when they pass over a storm, they can tell us more about it, either through looking at the microwave part of the spectrum, which can quote unquote see through the tops of clouds, give us information about the structure, or if it's an active instrument sending its own signal down, when it bounces back, we can infer what the winds are, both speed and direction. And those insights, when we have them, give us more to work with when estimating how strong the storm is.

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Eddie Wolf: If you're just joining us, this is Weather Realness, the podcast about weather and climate from Illinois Public Media and the University of Illinois Urbana-Champaign. I'm Eddie Wolf, graduate research assistant in the Department of Climate, Meteorology and Atmospheric Sciences at the University of Illinois. We're talking about why the Atlantic has had its slowest hurricane season with Dr. Kim Wood, associate professor in hydrology and atmospheric sciences at the University of Arizona.

So listeners have probably been hearing on the news — and we already mentioned this earlier as well — that this year's El Niño is going to be incredibly intense and affect a lot of the world's weather. So we already mentioned earlier, you know, the impact El Niño is having on this season. What do we know about past El Niños and how those have affected other September seasons? Why is this year still such an outlier?

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Dr. Kim Wood: That's a great question, because no El Niño is exactly the same. The timing at which those uh, waters start warming up along the equatorial Pacific, some years it starts a little earlier, some years it starts a bit later, and the atmospheric response differs depending on when that ramp up occurs. Also, the rate at which it changes can have a different impact. You know, you think about gently dropping a rock into a pond versus slamming a rock into the pond, but ripples are gonna change in response to that forcing. Not that that's what El Niño is doing, but there is a magnitude difference in the response of the atmosphere based on how a given El Niño starts shaping up.

And in the North Atlantic, it is typical to see a decrease in activity in September with an El Niño, but not to this degree, not to the point that it feels like it's been shut down. Yes, there have been tropical cyclones in the North Atlantic, but they're typically not forming in the typical place they should. There was [Gonzalo] and everything else was higher latitude, including [Idaey] that disappeared and came back because at higher latitudes, the conditions were more favorable than in the main development region. That's weird. But having that below average activity in September, which is typically a more active month for the Atlantic, that is relatively common in an El Niño year.

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Eddie Wolf: And speaking of things that are, you know, a little bit out of the ordinary in the Atlantic, we also just had a nor'easter, uh, go up the east coast, which also not necessarily expected this time of year. Can you talk a little bit about that and how that kind of differs from these tropical cyclones we've been talking about?

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Dr. Kim Wood: Yeah, a tropical system is much more likely in the peak of the hurricane season, which is September, and a more mid-latitude system is more in the October, November going into our Northern Hemisphere winter time frame. So seeing one in September was a bit of a head scratcher. Now, having warm water off of the coast of uh, North America does help, you know, that warm water does support a nor'easter, even though it's not a tropical cyclone — that's energy available to the storm for it to develop and intensify. But one of the things that El Niño does is it affects the entire global circulation of the atmosphere, and so the flow of, say, the jet stream will be impacted. And those downstream effects can, at least as we saw this year, result in a relatively early season nor'easter compared to [little Ti].

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Eddie Wolf: And of course we've been talking a lot here about the Atlantic, but we're also seeing ramifications from El Niño over in the Pacific, where it's actually been one of the more active seasons on record. So we can talk maybe about how El Niño is affecting those storms differently and why that's the case.

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Dr. Kim Wood: Typically speaking, the circulation shifts that result in higher shear and more sinking air, and even drier air over the Atlantic — that impact is a more rising air, warmer water, more moisture, less shear over the eastern Pacific. So we tend to see this kind of seesaw effect in activity, such that the eastern Pacific is more active and the Atlantic less active in El Niño, and the reverse being true if it were a La Niña.

Uh, that said, it's more than just El Niño. The other factor is there are pretty warm waters up at higher latitudes along the Mexican and even up to the southern California coast. Not that tropical cyclones are gonna reach Southern California because the waters are warmer than usual, but still too cold to support tropical cyclones. But having that greater extent of warm water, not just at the equator affecting the circulation, but also in the areas where the storms themselves can form, contributes to the activity you can see.

Another element of the seesaw effect — a lot of the quote unquote seeds, the disturbances that could become tropical cyclones, form over Africa in something called easterly waves, and then they march across the Atlantic, and if they don't form there, but are able to kind of stick together in some coherent manner, they can then serve as seeds in the eastern Pacific once those disturbances are reaching those ingredients of warmer water, less shear, more moisture, and more rising air.

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Eddie Wolf: Yeah, it's really interesting how, you know, we think of these as separate basins, but they're so interconnected in so many ways where what's happening in the Atlantic is then feeding back in the Pacific and vice versa. It's really interesting. I think you mentioned there warmer waters than normal. I think when people hear that things are warmer than normal, the obvious question that comes to mind is climate change. So is there any signal that we're seeing here from climate change in terms of how the season is playing out?

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Dr. Kim Wood: So one of the challenges of the satellite era being as short as it is, is we don't have the period of record that lets us really easily say, yeah, this is climate change, or no, this isn't climate change. That said, everything is warm right now. And that does have an impact, especially when you look at how deep is the warm water. So it's not just is the sea surface warm, but also are the waters warm below that, because a tropical cyclone, as we talked about, was rated by its wind speed. You strengthen those winds, it's gonna churn up more water. And the more water that is churned at the surface, the more water from below gets brought up in a process called upwelling, and that — if that upwelling brings cooler water up — it creates something called a cold wake.

We've actually seen that with a couple of storms so far this year. I'm not gonna say the name right — [Odali] was one of the ones that created a cold wake in the form of not necessarily chilling the water, but the SSTs decreased because the storm had used up the energy to be what it was. Uh, but a storm like Hurricane Polo — one of the reasons that it was able to intensify so quickly is because the water there was not just warm at the surface, but it was warm really deep. And even after Polo passed, the waters were still above normal. They had cooled, but they were still — they were so far above normal to start that the water that came up from below dropped it, you know, 4 degrees Fahrenheit to a point that was still pretty conducive to hurricanes, which is why Hurricane Rachel was encountering a cold wake like uh [TD9E] was.

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Eddie Wolf: If you're just joining us, this is Weather Realness, the podcast about weather and climate from Illinois Public Media and the University of Illinois Urbana-Champaign. I'm Eddie Wolf, graduate research assistant in the Department of Climate, Meteorology and Atmospheric Sciences at the University of Illinois. We're talking about why the Atlantic has had its slowest hurricane season with Dr. Kim Wood, associate professor in hydrology and atmospheric sciences at the University of Arizona.

Yeah, I'm really glad you brought up Hurricane Polo there and how it rapidly intensified. I think, you know, in the last couple of years we've seen a lot of storms that have been undergoing this rapid intensification and it's something that our forecasts still kind of struggle with. Can you talk maybe a bit about when we're forecasting these storms, why do we struggle with these rapid intensification types of systems?

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Dr. Kim Wood: It's a combination of factors that includes the degree to which they're rapidly intensifying. So, the definition that we established long ago for what we even mean by rapid intensification is an increase of 30 knots or 35 miles an hour in a 24-hour period, which is a pretty rapid change. But the degree to which rapid intensification is occurring above that threshold is increasing, in part because these really warm waters alongside, uh, you know, humidity and rising air and low shear, there's more for a hurricane to work with to get stronger. And we're seeing more instances of double that threshold. So I said it was 30 knots in 24 hours. Well, we're seeing more instances of 50, 60 knots or more in 24 hours.

So, forecasters are getting really in tune with signals of this is gonna rapidly intensify, but it's harder to say it's gonna rapidly intensify by double the threshold that we actually look for. Uh, and so ongoing research gives us insight into which conditions, and then it's not just the atmosphere, it's also what is the storm doing? What shape does it have? How big is it, that sort of thing. And so, when we have enough information and our models are capturing these processes, we're better able to forecast the extreme end of rapid intensification.

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Eddie Wolf: I mean we've been talking a lot here about rapid intensification, how we struggle with that, but I think it's also important to note that in general our forecasts have gotten really good, both in terms of tracking and intensity over the last few decades. Can you talk a bit about how they've improved, sort of what kind of changes we've seen?

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Dr. Kim Wood: Some of the big investments came in the wake of the extremely active and destructive 2004 and 2005 hurricane seasons. So, among that being Katrina and getting a lot of investment across federal science to support figuring out what it was we needed to know, and then implementing what we learned from pursuing answers to those questions, and then navigating the questions that come up because we learned more about hurricanes. That really helped drive a lot of our progress. So there were programs such as the Hurricane Forecast Improvement Program that helped invest in the whole thing from observation through analysis and modeling to forecast. So it's not just knowing more about storms, but are we able to tease out signals that can be used within the limitations of how long a forecaster has to figure out what a storm's currently doing and what it'll do next?

Because forecasters are issuing updates every 6 hours, and they are taking the whole 6 hours ahead of that to make their forecasts. It's half that if they're lucky. And so there's only so much time they have to delve through all the information. So it's important to have tools that can be readily interacted with to be like, all right, there's a signal for [RI] here, there's over here this other signal for [RI], oh look, four different models are calling for [RI] and I have confidence in each of them to some degree. OK, then we get that consensus that helps the forecasters have confidence in calling for a more, uh, a higher end forecast, shall we say, and having experience both in the forecaster seat and in analyzing what they see directly, looking at satellite data, what's coming from a plane that might be flying through the storm, as well as those tools such as models, both statistical and a computer simulating the atmosphere.

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Eddie Wolf: All right, well, thank you so much for joining us, Dr. Kim Wood. We really appreciate having you here and hearing all of your insights on this.

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Dr. Kim Wood: Thank you again for having me. It was a lovely conversation.

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Eddie Wolf: We've been talking to Dr. Kim Wood, associate professor in hydrology and atmospheric sciences at the University of Arizona. I'm Eddie Wolf, graduate research assistant in the Department of Climate, Meteorology and Atmospheric Sciences at the University of Illinois. Thank you for joining us on Weather Realness. Do you have a weather or climate question you want us to answer? Leave us a voicemail at 217-333-2141 or email weatherrealness@illinois.edu.

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Speaker 1: One more note about this hurricane season, we taped this conversation before Hurricane [Isaia] formed in the Gulf of Mexico. As of October 8th, there are hurricane warnings along the Gulf Coast from Louisiana to Florida. You can track the storm at weather.gov.

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Joseph Trujillo Falcon: Weather Realness is produced by Trent Ford, Jeff Frame, Deanna [Hen], Steven Nesbitt, Stephanie Orellana, Eddie Wolf, and me, Joseph Trujillo Falcon. Production is partially funded by the [Backland] Charitable Trust. This podcast is a partnership between the Department of Climate, Meteorology and Atmospheric Sciences and Illinois Public Media, a service of the University of Illinois Urbana-Champaign. Thank you for listening.