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With Two Category 4 Eastern Pacific Hurricanes In Four Days, Should We Be Worried About the 2018 Atlantic Hurricane Season?
Published: June 13, 2018
The short answer: It's unclear, according to our analysis of past similar quick eastern Pacific season starts.
Other Two-Hurricane East Pacific Starts
In reliable records dating to 1971, NOAA's historical hurricane database has 10 other years in which the first two named storms in the eastern Pacific basin were hurricanes: 2015, 2011, 1999, 1996, 1995, 1990, 1983, 1976, 1973 and 1971.
(Lt Adam Abitbol, P-3 Pilot, NOAA/AOC)
Admittedly, this is by no means a large sample size for any statistical study.
In those 10 years, the average number of Atlantic named storms (12.3), hurricanes (6.6), and major hurricanes (2.8 Category 3 or stronger) almost exactly matched the 30-year average from 1981-2010 of 13, 7 and 3.
Of course, averages mask extremes. There was quite a range in the number of named storms, from a mere 4 in 1983 to 19 in both 1995 and 2011, and in hurricanes from 3 in 1983 to 11 in 1995.
So, this isn't terribly helpful from a seasonal forecast perspective.
Two Category 4 Hurricane East Pacific Season Starts
In the two years that started with a pair of Category 4 eastern Pacific hurricanes like this year – 2015 and 1995 – the Atlantic hurricane seasons couldn't have been more different.
In 2015 there were 11 named storms, but only 4 hurricanes and 2 major hurricanes. However, in 1995, there were 19 named storms, 11 hurricanes and 5 major hurricanes.
Bottom line: We need to dig deeper.
Another driver of the overall activity of both the eastern Pacific and Atlantic hurricane seasons is whether the central and eastern equatorial Pacific Ocean is in a state of El Niño (warmer-than-average water temperatures), La Niña (cooler-than-average), or neither.
In general, stronger El Niños tend to suppress numbers of Atlantic named storms and hurricanes by producing increased wind shear, the change in wind direction with height that tends to rip apart tropical cyclones.
(MORE: Early Signs of El Niño)
In the eastern Pacific basin, the effect is the opposite, with warmer water and lower wind shear contributing to a more active season during stronger El Niños.
That's exactly what happened in 2015. An intensifying El Niño kept a relative lid on the Atlantic season, but helped to spawn a frenetic eastern Pacific season with 26 named storms, 16 hurricanes and 11 major hurricanes.
In 1995, the equatorial Pacific Ocean trended toward La Niña during hurricane season, but not nearly as strong as the 2015 El Niño.
In La Niña, the impacts are reversed. Less wind shear in the Atlantic basin contributes to a more active season, there, while the eastern Pacific season is suppressed, to some extent.
Two-Hurricane Eastern Pacific Starts With Similar El Niño Evolution
Finally, we looked for seasons starting with two eastern Pacific hurricanes that also had a similar evolution of El Niño as the current forecast for summer/fall 2018.
The preponderance of outlooks currently call for a neutral (neither El Niño nor La Niña) scenario to evolve to a weak El Niño later in the 2018 hurricane season.
Of the 10 hurricane seasons that started with two Eastern Pacific hurricanes, we found two such years with a similar "neutral to weak El Niño" evolution: 1990 and 1976.
Both of these years occurred during the multi-decadal period of lower activity from the 1960s through the first half of the 1990s. So, that was another factor in play.
The number of named storms in each of those two years ranged from 10 to 14, with 6 to 8 of those becoming hurricanes and 1 to 2 becoming Category 3 or stronger hurricanes.
In other words, roughly average years by those metrics.
The Takeaway: Be Prepared Every Season
"I think the conclusion from two Category 4s to start the eastern Pacific season is that there is no predictability in Atlantic hurricane activity," said Dr. Michael Ventrice, atmospheric scientist at The Weather Company, an IBM Business.
Ventrice stressed that there are other factors in play when forecasting Atlantic season activity.
Sea-surface temperatures in the tropical Atlantic and the state of ENSO (El Niño, neutral, or La Niña) possess the highest correlation with Atlantic activity, he said.
Ventrice noted an index developed at The Weather Company more reflective of the atmosphere's reponse to El Niño or La Niña is forecast to trend toward El Niño later this summer.
Sea-surface temperatures are also starting out cooler than average in the zone between Africa and the Lesser Antilles, according to Dr. Phil Klotzbach, Colorado State University tropical scientist.
Both factors may be suppressing influences on the hurricane season.
Unfortunately, this entire discussion so far has left out a crucial point: There is no strong correlation between the number of storms or hurricanes and U.S. landfalls in any given season.
All it takes is one intense, landfalling hurricane to make many forget about El Niño, or cooler Atlantic Ocean water temperatures earlier in the season.
The poster child of this was 1983, which had only 4 named storms, the least number of named storms in any season in modern times. However, one of those four was destructive Hurricane Alicia in the Houston metro area.
Residents near the coast should prepare each year, no matter what seasonal outlooks say, and no matter how the season starts in another basin.
Jonathan Erdman is a senior meteorologist at weather.com and has been an incurable weather geek since a tornado narrowly missed his childhood home in Wisconsin at age 7. Follow him on Facebook and Twitter and subscribe to The Weather Channel podcast on Apple, Google Play, Stitcher, or wherever you get your podcasts.
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.