Hurricane season 2021 could be intense, forecasters say

2021 Atlantic Hurricane Season ly To Be Another Active Season, Says First Outlook From Colorado State

Hurricane season 2021 could be intense, forecasters say

  • Forecasters at CSU say another busier than average hurricane season is ahead.
  • Warmer water in the Pacific may lead to a quieter Atlantic.
  • Slower winds in the Atlantic may allow for more storms.

The first outlook for the 2021 Atlantic hurricane season says that we might be in for another active year.

The outlook, which is championed by Dr. Phil Klotzbach of Colorado State University, says that there is about a 6 in 10 shot of another active hurricane season ahead.

While it is much too early in preparation season to forecast the number of tropical storms and hurricanes, climatologists are able to look at some of the slow-motion drivers that may assist or hinder hurricane activity next summer and autumn.

The two such drivers that Dr. Klotzbach says are key to figuring out how busy next season will be are both related to water – the status of El Niño Southern Oscillation (or ENSO) and how water temperatures in the northern Atlantic change in the next year.

Here's how each may play a part:

1. ENSO: Will La Niña stick around?

Water temperatures have cooled in the Pacific since this past summer in a phenomenon called La Niña. This co-operative oceanic/atmospheric occurrence is one of the reasons that this past hurricane season was record-breakingly busy.

La Niña typically allows for more favorable atmospheric conditions in the Atlantic while its counterpart pattern, El Niño, typically allows fewer tropical storms and hurricanes to form.

So the question is, where will water temperatures be during the upcoming hurricane season?

Most modeling suggests that the Pacific will gradually warm through this preparation season and into the first half of hurricane season. By the time hurricane season heats up, in July, August and September (JAS, located at the right of the below diagram), water temperatures should be warm enough to be considered at least neutral.

ENSO model prediction plume from mid-November for the next several months. Figure courtesy of the International Research Institute for Climate and Society. (Annotated by Colorado State University)

The team at Colorado State University is currently hinging on this idea and that El Niño will not develop next hurricane season, which would require faster warming of the Pacific than currently forecast.

This could mean that one of the favorable factors that led to the crazy busy hurricane season this year may not be in play in the upcoming hurricane season, potentially lowering the number of storms.

But ENSO is only one factor that determines how hurricane season will go. El Niño is “one of the reasons why we don't put out a formal forecast with numbers in December is because there's a tremendous amount that can change,” Klotzbach said in an interview with “El Niño forecasts for next summer and fall have enormous error bars.”

Another factor that Klotzbach thinks is fundamental to the busyness of the upcoming hurricane season is how warm waters are in the Atlantic.

2. AMO and the Bermuda High

For the last quarter-century, water temperatures have been running naturally above average at the height of a cycle called the Atlantic Multidecadal Oscillation (or AMO).

“The typical period of the AMO is about 60 years, with the period length varying between as short as 40-50 years and as long as 70-80 years. This means that we typically have 25-35 years of above-average Atlantic basin major TC activity and similar length periods with considerably reduced amounts of major TC activity,” according to CSU.

These bursts in activity are also “characterized by more hurricanes and more major hurricanes and overall more hurricane hit(ting) the United States.”

Given the busyness of the last five hurricane seasons, the CSU says that the enhanced AMO is ly to show up in 2021.

But there's one catch. Even during these long cycles, a year or two can “turn off” and that appears to be the case right now as signaled by the colder water in the North Atlantic (see the black box below).

One key question that remains is whether or not the cycle will flip back on by next hurricane season.

But why does a patch of cold water off the coast of Greenland and Ireland matter to hurricanes, which form between Africa and the United States?

Klotzbach says that the enhanced AMO phase slows down the Bermuda High.

The Bermuda High, which is typically located between Bermuda and the Azores during hurricane season is greatly responsible for steering (or shoving) tropical waves, tropical storms and hurricanes westward from Africa toward the Caribbean and United States.

“If you have a lot of pressure that is higher than normal in the subtropics, that tends to favor stronger winds blowing across the tropical Atlantic that then causes cooling relative to average,” says Klotzbach. Cooler water temperatures in this area are less favorable for the development of tropical waves. This phenomenon is something that the CSU team begins to figure out in February and March.

Also, the slower the Bermuda High rotates, the slower tropical systems move westward and the more ly they are to develop and intensify.

Weather vs. Climate

Of course these two elements – ENSO and the AMO – are only two of the mechanisms that can change how many storms we will see.

Both of these are climate-related. Typically, in a given hurricane season we will see around 13 named tropical storms and 7 hurricanes.

These two features either elevate or decrease the favorability of conditions that may lead to more or fewer named storms and hurricanes.

That's where weather comes in.

It won't be until next April that we can start to talk about the weather conditions that may lead to more or fewer storms. This is why CSU does not issue a forecast with numbers, i.e.

12-15 tropical storms, 5-8 hurricanes, just yet. We just don't know what the weather will be during hurricane season.

As this report says, we're still figuring out the slower-moving climate signals for next year.

Klotzbach also says that the record-breaking 2020 season has little bearing on how 2021 will go. He says that an El Niño event can knock down storm activity even after a busy season.

Stay tuned right here to The Weather Channel for the latest on hurricane season in the months to come. Remember, it is never too early to prepare for what weather may come our way next.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.


What a Busy 2020 Hurricane Season Could Mean for 2021

Hurricane season 2021 could be intense, forecasters say

We get it.

With a record-shattering, nonstop onslaught of an Atlantic hurricane season in 2020, the last thing you probably want to think about is more hurricanes.

So with that in mind, there's some good news and some not-so-good news about what 2020 could tell us about 2021.

First and foremost: It’s too far from the start of the 2021 season to really get a great sense for what might come.

“You really have to get a look at the patterns around May 1 to know what June through November might hold,” said Spectrum News Chief Meteorologist Mike Clay, based in Tampa. “There is no telling.”

Long-range tropical and seasonal forecasting is still very much in its infancy, and more than six months from the start of the next hurricane season, a lot can and will change. We’re still months away from accurately assessing or even dipping our toes into most key long-range meteorological metrics.

But even with that in mind, there's perhaps at least a little that we can look at with more than six months to go until June 2021, including the history of previous big hurricane seasons and what followed immediately afterward.

Historical Precedents

The main meteorological indicators about what the 2021 season could hold are too far into the future to accurately assess now. More on that in a minute.

The one definitive metric that we do have, though, is history. This is where, at least on the surface, there's at least a smidge of good news.

The 2020 season recently topped the 2005 season in terms of total named storms, so perhaps the 2006 season might be the first place to look for clues about what 2021 may hold.

The good news is that the 2006 hurricane season fell below average, producing only 10 named storms and five hurricanes, both below the long-term full-year averages of 12 named storms and six hurricanes in the Atlantic.

The relative bad news, though, is that the primary culprits for those low-end totals in 2006 were a developing El Niño event and a persistent layer of Saharan dust in the Atlantic Ocean. Again, it’s just way too far out to say if those things will be in play for next summer and fall.

And with a sample size of one, there’s simply no way we can attribute the low-end 2006 season to what might happen in 2021 with any kind of accuracy.

So with that 2006 micro-sample size in mind, let’s expand our view a bit to include some of the biggest hurricane seasons over the last 100-plus years.

If you average the top-10 seasons since 1900 in terms of Accumulated Cyclone Energy (ACE), the most widely used metric for assessing a hurricane season’s overall intensity, you’d get an average ACE of 219.5. An exactly average Atlantic season features an ACE of 100, so 219.5 signals a hurricane season with more than double the average of tropical systems.

If you average the nine years that followed a top-nine season in terms of ACE since 1900 (2020 is number 10), you’d get an average ACE of 122.38, a notable drop-off but an indicator that, on average, an above-average-but-not-off-the-charts hurricane season could be in the cards.

Then again, the record-smashing 2005 season followed an almost-as-big season in 2004. Simply put, history offers a scattered, at best, view of what the year after a huge hurricane season might look .

But, at the very least, there is some historical precedent for at least a slight reprieve after a big season 2020.

The ENSO Cycle

It's arguably the single-most-important determining factor in a long-range forecast, or for at least a top-level view about how a tropical season could play out. 

The ENSO cycle is the measure of sea-surface temperatures in the central Pacific Ocean. THe cycle states that warmer-than-average temperatures in the central Pacific is an El Niño, while cooler-than-average readings are a La Niña.

Generally speaking (though with notable exceptions), El Niño events can lead to below-average seasons, and La Niñas produce above-average seasons.

In short, an El Niño creates a meteorological domino effect of global weather that often ends up boosting wind shear across the Caribbean and the Atlantic, tearing apart storms before they fully develop.

A La Niña, meanwhile, reduces wind shear and can create a more favorable environment for the formation of tropical systems in the Atlantic.

A moderate-to-strong La Niña is in place, and it’s widely cited as one of the key puzzle pieces for the record-breaking 2020 season.

Currently, the Climate Prediction Center (CPC) confidently predicts the current La Niña to persist into the spring of 2021. Beyond that, though, uncertainty grows about what the ENSO cycle could look in the summer and fall of 2021.

“La Niña is ly to continue through the northern hemisphere winter 2020-21 (~95% chance during January-March) and into spring 2021 (~65% chance during March-May),” the CPC wrote in its late November memo.

Watch this closely, though: Some computer models are hinting at moderating sea-surface temperatures in the central Pacific, perhaps an indicator that the current La Niña may start to relax next year.

Other Factors

If you’re a weather buff, you probably know that sea-surface temperatures are also one of the main indicators of what the upcoming tropical season could look .

Warmer sea-surface temperatures act as fuel for tropical cyclones. The warmer it is in the Caribbean Sea, the Gulf of Mexico, and the Atlantic Ocean, typically, the more favorable the environment there is for tropical cyclone formation.

Dust from the Saharan Desert can also limit tropical development in the Atlantic, especially earlier on in the season.

​These two metrics are where it’s basically impossible to get any sort of pulse. At this point, we simply don’t know what sea-surface temperatures, the Saharan dust layer (SAL), and other critical and shorter-range factors the Madden-Julian Oscillation, Atlantic high pressure and Rossby waves will look across the Atlantic in six months.

We certainly don’t want to leave you with a cliffhanger, but we also don’t want to mislead you.

While there are a few hints about 2021, the reality is there’s just not much we can gather from the current data about what the 2021 hurricane season might look . Hopefully, though, we won’t have to deal with 30 named storms in a single season again anytime soon.


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