An expected delay in the onset of El Niño may increase the number of tropical storms and hurricanes to form in the Atlantic during 2017.
El Niño is part of a routine fluctuation in sea surface temperatures over the tropical Pacific. When these waters enter a warm phase of a few degrees Fahrenheit or greater compared to average for several consecutive months, the pattern is designated as El Niño.
When El Niño occurs, it typically creates strong winds from the west at mid-levels of the atmosphere. These winds tend to prevent formation, limit strengthening and can lead to an early demise of tropical storms.
This spring and early summer, sea surface temperatures over the tropical Pacific have been hovering above average but only very slightly so.
"We now believe that the tropical Pacific waters will not warm enough to officially have an El Niño through the middle of the autumn and effectively the bulk of the 2017 Atlantic hurricane season," according to AccuWeather Lead Long-Range Meteorologist Paul Pastelok.
"Until this point, we felt a weak El Niño was possible as by the end of the summer or early in the autumn," Pastelok said.
Meanwhile, waters over much of the Atlantic basin have risen above normal in many areas. The temperature of most of these waters is now above average. Warm water, especially when this warmth extends to the depths, helps to fuel tropical storms.
Another consideration is the formation of two tropical systems, Bret and Cindy, within a few days of each other during the middle of June, on top of the system, Arlene, from early in the spring.
A lack of El Niño conditions well into the fall may also allow tropical storms to form right up to the end of hurricane season, which is Nov. 30.
"We are increasing our forecast numbers in all categories, compared to this spring," according to AccuWeather Hurricane Expert Kottlowski.
Including the three systems that have occurred already, from 11 to 15 tropical storms are predicted, along with six to nine hurricanes and three to four major hurricanes.
"We expect disruptive winds at mid-levels of the atmosphere to ease by late July and early August, which will create conditions more favorable for tropical storm formation in the main development region," Kottlowski said.
A higher number of tropical systems does not necessarily correlate with an increased impact to land, unless the weather pattern supports that.
"Back in May, we expressed concerns about direct impacts on the southeastern United States, especially the area along the eastern Gulf coast," Kottlowski said. "We remain concerned about this area."
Multiple developed systems are likely to reach the southwestern part of the Atlantic basin.
"Deep, warmer-than-average water over the Gulf of Mexico and the Caribbean Sea could support at least one high-impact hurricane for the U.S., perhaps as strong as Joaquin in 2015 and Matthew in 2016," Kottlowski said.
The last hurricane to make landfall in the U.S. as a Category 3 was Wilma on Oct. 24, 2005. Wilma struck the Florida Peninsula as a major hurricane.
The clockwise flow around a strong area of high pressure is likely to direct tropical storms and hurricanes farther west than average. Hence the concern for the Gulf of Mexico in addition to the Caribbean.
As the orientation of this high pressure area changes toward the middle and latter part of hurricane season or during autumn, the risk of tropical storms and/or hurricanes having close encounters with the southern Atlantic Seaboard of the U.S. is likely to increase.
Correspondingly, this risk will then lower over the western Gulf of Mexico later in the season.
Depending on the weather pattern at the time of tropical systems approaching Florida or the Carolinas, there may be a risk for significant direct or indirect impacts farther north in the eastern U.S.
"So not only may the 2017 Atlantic hurricane season be more active than 2016, the number of storms that make landfall in the U.S. may be similar to last year," Kottlowski said.
"As experienced last year, all it takes is one hurricane to have high impact on a coastal area."
Including Cindy, which made landfall along the Louisiana coast in late June, three to four tropical storms and/or hurricanes are projected to make landfall in the U.S. for the 2017 season.