A stormy season ahead

Hurricane forecasters expect an above-average hurricane season as high sea temperatures offset the effects of El Niño.

Hopes that the 2023 hurricane season would be calmer than in previous years are fading as the storm season warms up.

As Bermuda:Re+ILS was going to press, no fewer than three tropical storms were lined up across the Atlantic, and there were three more tropical disturbances being watched.

At the same time, in the Pacific, Hurricane Hilary, downgraded to a tropical storm, had come ashore in Baja California, killing at least one person, before sweeping north past San Diego and Los Angeles. Palm Springs had received half a year’s rainfall—admittedly just two inches—in just six hours.

The sudden upsurge of storms comes after a relatively quiet first two months of the hurricane season, when there were four named storms, none of them causing any significant damage.

Nonetheless, hurricane modellers were already revising their forecasts from early in the year when a near normal—whatever that is these days—season was forecast.

The reasons for optimism, both for residents of peak hurricane regions and catastrophe reinsurers was grounded in the fact that 2023 is an El Niño year, when the periodic weather phenomenon was expected to weaken windshear in tropical systems, meaning that they could be less severe. Those forecasts were always tempered by concern that sea surface temperatures, which have been steadily rising in the last 20 years, would continue to heat up, cancelling out the effect of El Niño .

As a result, the most recent predictions have been for an above-average season.

Colorado State University (CSU), which has been making hurricane predictions since the 1980s, revised its forecast on August 3, saying the probability of a US major hurricane landfall is now above average.

“While a robust El Niño has developed and is likely to persist for the peak of the Atlantic hurricane season, most of the tropical and subtropical Atlantic has record warm sea surface temperatures for this time of year,” the revision says. “El Niño increases vertical wind shear in the Caribbean and tropical Atlantic, but the extreme anomalous warmth in the tropical and subtropical Atlantic is anticipated to counteract some of the typical El Niño-driven increase in vertical wind shear.

“The probability of US major hurricane landfall is estimated to be above the long-period average. As is the case with all hurricane seasons, coastal residents are reminded that it only takes one hurricane making landfall to make it an active season for them. They should prepare the same for every season, regardless of how much activity is predicted.”

The CSU Tropical Meteorology Project team is now predicting 18 named storms in 2023, including the five named storms that have already formed (January subtropical storm, Arlene, Bret, Cindy and Don), since which time Emily, Gert and Franklin had formed.

“Of those, researchers expect nine (including Don) to become hurricanes and four to reach major hurricane strength (Saffir/Simpson category 3-4-5) with sustained winds of 111 miles per hour (mph) or greater,” says the CSU report.

“The most recent predictions have been for an above-average season.”

Lots of uncertainty

So far, the 2023 hurricane season is exhibiting characteristics similar to 1951, 1969, 1987, 2004, 2005, 2006 and 2012, the report says.

“Our analogue seasons exhibited a wide range of outcomes, from below-normal seasons to hyperactive seasons,” says Phil Klotzbach, research scientist in the Department of Atmospheric Science and lead author of the report. “This highlights the large uncertainty that exists with this outlook.”

The team predicts that 2023 hurricane activity will be about 130 percent of the average season. By comparison, 2022’s hurricane activity was about 75 percent of the average season, although it noted that it was notable for two major hurricanes—Fiona, which brought devastating flooding to Puerto Rico before causing significant surge, wind and rain impacts in the Atlantic Provinces of Canada as a post-tropical cyclone.

“Ian made landfall as a Category 4 hurricane in southwest Florida, causing over 150 fatalities and $113 billion dollars in damage,” the report says.

The report also includes the probability of major hurricanes making landfall for the remainder of the season:

• 48 percent for the entire US coastline (full season average from 1880 to 2020 is 43 percent)

• 25 percent for the US East Coast including the Florida peninsula (full season average from 1880 to 2020 is 21 percent)

• 31 percent for the Gulf Coast from the Florida Panhandle westward to Brownsville (full season average from 1880 to 2020 is 27 percent)

• 53 percent for tracking through the Caribbean (full season average from 1880 to 2020 is 47 percent)

The report is broadly in line with other forecasts, including the revised prediction from the US National Oceanographic and Atmospheric Administration’s (NOAA) hurricane centre. The NOAA increased the likelihood of an above-normal Atlantic hurricane season to 60 percent (increased from the outlook issued in May, which predicted a 30 percent chance). The likelihood of near-normal activity has decreased to 25 percent, down from the 40 percent chances outlined in May’s outlook. This new update gives the Atlantic a 15 percent chance of seeing a below-normal season.

NOAA’s update to the 2023 outlook—which covers the entire six-month hurricane season that ends on November 30—calls for 14 to 21 named storms (winds of 39 mph or greater), of which six to 11 could become hurricanes (winds of 74 mph or greater).

“Of those, two to five could become major hurricanes (winds of 111 mph or greater),” the report says. “NOAA provides these ranges with a 70 percent confidence.”

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