The grip of a double-dip La Niña is a key clue in a leading hurricane forecast’s early prediction for an above-normal storm season come June 1.
Colorado State University’s first review of the global climate patterns that could influence the 2018 hurricane season found a 60 percent probability that a more active season will unfold, giving a below-average season only a 20 percent probability of occurring.
The December outlook does not predict the number of storms but forecasts the amount of accumulated cyclone energy, or ACE, for the season. ACE is a measure of the strength and longevity of a tropical cyclone.
Phil Klotzbach, lead author of the early outlook and protege of storied hurricane expert William Gray, is quick to point out that while CSU’s forecasts are based on 35 years of experience, any prediction this far out is tricky.
“There is always considerable uncertainty as to how much activity an Atlantic hurricane season is going to generate at such a long lead time,” the report notes. “No one can completely understand the full complexity of the atmosphere-ocean system.”
Even springtime forecasts can be a bust. CSU predicted a slightly below-average 2017 hurricane season in its April forecast, calling for only 11 named storms, four hurricanes and two major hurricanes. The forecast for ACE was 75.
Instead, the record-setting season ended with 17 named storms. That included 10 hurricanes, six of which became major hurricanes of Category 3 or higher. The season ended with an ACE of 226.
CSU based its April prediction on the belief a cyclone-battering El Niño would rise up. When it didn’t, the university’s subsequent predictions were more accurate. By August, the forecast was up to 16 named storms.
That’s why many hurricane experts caution against giving too much credence to predictions made before the seasonal transition from winter to summer, which can shake up the atmosphere and lead to significantly different outcomes than forecasts made six months before.
“The things that will end up affecting the season overall are really hard to predict very far in advance,” said James Franklin, the National Hurricane Center’s former chief of the hurricane specialist unit. “There are frequently going to be big errors in these forecasts, even when they come out in April.”
Franklin said long-range hurricane forecasts may be of use to the insurance and reinsurance industry, but are not helpful to people planning for the next storm season.
“For coastal residents, and even those not on the coast, the only thing to take away from a seasonal forecast is a reminder that one needs to be ready for hurricanes every year,” Franklin said.
The Climate Prediction Center believes a strengthening La Niña will last through early spring, with a shift to a neutral phase before summer. In 2017, La Niña lasted into January before transitioning to a neutral phase. Some scientists believe a La Niña hangover lingered in the atmosphere, bolstering the 2017 season.
While El Niño acts to tear hurricanes apart with strong west winds, La Niña is more accommodating to tropical cyclones with less storm-deterring wind shear.
The Nevada-based Climate Forecast Applications Network also issued a forecast this month calling for an above-normal 2018 hurricane season, but not based on El Niño or La Niña.
President Judith Curry said her company, which is hired by private firms looking for specialized forecasts, has identified specific sea surface and atmospheric circulation patterns that it feels are a better forecast tool. Using those patterns, the Climate Forecast Applications Network predicts an 80 percent probability of an above-normal 2018 season.
But Curry also cautioned against relying on a December forecast to be 100 percent accurate.
“It may have some interest to the general public, but at this point, it’s not useful to emergency managers,” Curry said. “It helps the financial sector hedge their bets when they have to make decisions by Jan. 1.”
What’s the ACE?
- It means “accumulated cyclone energy”
- It is a measure of the strength and longevity of a tropical cyclone
Colorado State University’s December prediction for the 2018 season gave five forecast possibilities, with the two that were above average having the combined probability of 60 percent.
The ACE for the above average seasons ranged between 130 to 170. The historic average mean ACE is 93. The median is 87.
For perspective, the 2017 record-setting storm season, with 17 named storms (10 hurricanes, six of which became major hurricanes of Category 3 or higher), ended with an ACE of 226.