22 June 2023ArticleILS

Hurricane outlook: the only certainty is uncertainty

First, the good news: before the 2023 Atlantic hurricane season began on June 1, forecasters reached a consensus on the outlook.

The bad news? The consensus was that no-one can say with confidence what is going to happen.

While most agree that the season is likely to be slightly below average, there was a caveat. No-one can be sure which of two climate phenomena will dictate the frequency and severity of storms.

The El Niño/Southern Oscillation (ENSO) historically dampens hurricane activity in the Atlantic and most forecasters had worked that into their models early on.

But sea temperatures, which are on track to reach 30-year highs, have the opposite effect, causing more storms of greater intensity, and no forecaster dares say with certainty whether the two factors will cancel each other out, or whether one will dominate.

Munich Re’s hurricane forecast describes it this way: “Hurricane activity is essentially influenced by two factors: the water temperature in the tropical North Atlantic and the natural climate oscillation ENSO in the Pacific.

“Current predictions for this year anticipate sea surface temperatures of up to 1°C above average during the main months of hurricane activity from August to October. Even now, temperatures in the tropical North Atlantic are showing a significant increase.

“A warmer than usual tropical North Atlantic is known to create generally more favourable conditions for the development and intensification of hurricanes. It also ensures lower air pressure and weaker trade winds, which likewise contribute to an environment favouring hurricanes.

“The second factor is the ENSO. Even though this is actually a temperature swing in the Pacific, it exerts a strong influence on extreme weather in very distant regions. After three years with La Niña conditions, which favour hurricanes, a swing to what may become a strong El Niño phase is expected in the late summer.

“El Niño years are accompanied by strong winds at high altitude over the North Atlantic. This is known as vertical wind shear, which inhibits cyclones because it literally tears storm systems apart. According to current forecasts, vertical wind shear is likely to increase to a high level during the main period of the hurricane season.

“Although the models are predicting a strong El Niño phase for the late summer with relative certainty, the level of hurricane activity will also depend heavily on the current high ocean temperatures. These conflicting signals make it difficult to provide a reliable forecast for this year’s hurricane season.”

A dozen storms?

Munich Re is forecasting some 12 named tropical storms in the tropical North Atlantic in the 2023 season. It says, based on an analysis of independent forecasters, that around six could develop into hurricanes, and two into major hurricanes with wind speeds in excess of 110 mph.

Colorado State University (CSU), which has been forecasting hurricanes for almost 40 years, increased its forecast for accumulated cyclone energy for the season by about a quarter, to a reading of 123 to match the 1991–2020 average.

CSU raised its forecast for named storms by one to 14, hurricanes from six to seven and major hurricanes from two to three.

It said: “While we anticipate a robust El Niño for the peak of the Atlantic hurricane season, the tropical and subtropical Atlantic have continued to anomalously warm to near-record levels.

“El Niño increases vertical wind shear in the Caribbean and tropical Atlantic, but the anomalous warmth in the tropical and subtropical Atlantic may counteract some of the typical El Niño-driven increase in vertical wind shear. The probability of US major hurricane landfall is estimated to be near the long-period average.”

Almost all late spring forecasts featured upgrades on the number of storms and their severity as the high sea temperatures became clear.

If predicting the overall nature of a hurricane season is difficult, it’s even harder to predict whether and where hurricanes will make landfall. Nonetheless, CSU is forecasting that landfall probabilities are at the historic average.

CSU sees a 43 percent chance of major hurricane landfall on the US coastline, a 21 percent chance of an east coast landfall and a 27 percent chance of a Gulf coast landfall, all matching the 140-year averages.

All forecasters agree on one thing, however. Regardless of how many hurricanes are predicted or take place, it takes only one to turn a below-average season into a catastrophic one.

“Hurricane Ian last year, with insured losses of $60 billion, illustrated that a single severe storm can push up the loss potential—irrespective of whether a large number of storms is predicted,” says Thomas Blunck of Munich Re.

“It is important to remember that predicting tropical storms is a very complex process, one that depends on a range of factors. Insurers should therefore not make decisions based solely on the expected El Niño phase.”

With the first hurricane of the season, Arlene, already on the books, no-one is letting their guard down.

But since the short-lived hurricane in the Mexican Gulf moved south before dissipating, it confirms that the most predictable thing about the 2023 hurricane season is its very unpredictability.