By Jon Erdman and Linda Lam and Chris Dolce From weather.com

Story Highlights
Colorado State University has slightly increased its forecast to 16 named storms, eight hurricanes and three major hurricanes this season.
Warm water temperatures and vanishing El Niño odds are reasons for the increased numbers.
Five named storms have already formed this year in the Atlantic Basin; all but one was relatively short-lived.

The number of named storms forecast to develop in the 2017 Atlantic hurricane season has increased slightly in the final forecast issued Friday by the Colorado State University Tropical Meteorology Project.

A total of 16 named storms and eight hurricanes are expected in the Atlantic Basin, according to the CSU forecast, led by Dr. Phil Klotzbach. Three of the eight hurricanes are forecast to be Category 3 or stronger on the Saffir-Simpson Hurricane Wind Scale.

Tropical Storms Arlene, Bret, Cindy, Don and Emily – all of which have already occurred – are included in the seasonal forecast numbers in the outlook.

The updated forecast is above the Atlantic Basin’s 30-year historical average (1981-2010) of 12 named storms, six hurricanes and three major hurricanes.

CSU has slightly increased the forecast for named storms compared to the last update from July 5, which called for 15 named storms. It’s significantly higher than the initial April forecast of 11 named storms and four hurricanes.

Warm water temperatures in the tropical and subtropical Atlantic Ocean along with the dwindling chance of El Niño’s development later this summer are reasons why the forecast has steadily nudged upward.

“A warmer-than-normal tropical Atlantic is generally associated with lower surface pressures, increased mid-level moisture and weaker trade winds, creating a more conducive dynamic and thermodynamic environment for hurricane formation and intensification,” said the CSU team.

Widespread and hostile upper-level winds typically associated with El Niño will also be a no-show during the heart of the hurricane season ahead, but unfavorable winds aloft still occur at times each season in the Atlantic Basin whether El Niño is present or not.

The CSU team does caution that considerable uncertainty remains. This is in part due to conditions present that would hinder tropical development, such as the cooler than normal water temperatures in the far North Atlantic and unusually strong wind shear over the past few weeks in the Caribbean. However, hurricane-enhancing conditions are likely to dominate, leading to the expectation of an active season.

CSU found five years since 1950 that most closely resemble what’s expected in the atmosphere above and waters of the Atlantic Basin from August through October. Those years included 1953, 1969, 1979, 2001 and 2004. The average of those six seasons is slightly lower than the CSU forecast for 2017.

The Weather Company, an IBM Business, updated its seasonal forecast in July and also expects a total of 15 named storms, eight hurricanes and three major hurricanes this season. This is an increase from its earlier forecasts as well, mainly due to the same factors stated in the CSU outlook.

The official Atlantic hurricane season begins June 1 and runs through Nov. 30. Occasionally, storms can form outside those months, as Tropical Storm Arlene did this year. It also occurred last season with January’s Hurricane Alex and late May’s Tropical Storm Bonnie.

While five named storms had already formed through early August, all were relatively short-lived.

Using the ACE (Accumulated Cyclone Energy) index, a measure of tropical cyclone activity calculated by adding each tropical storm or hurricane’s wind speed throughout its life cycle, Klotzbach found the 2017 Atlantic season is off to its slowest start in eight years.

However, Klotzbach also cautioned that by the same ACE index measure, about 90 percent of a typical Atlantic hurricane season still remained.

What Does This Mean For the United States?

There is no strong correlation between the number of storms or hurricanes and U.S. landfalls in any given season. One or more of the additional named storms forecast to develop this season could hit the U.S., or none at all. Therefore, residents of the coastal U.S. should prepare each year, no matter the forecast.

A couple of classic examples that show the need to prepare each year occurred in 1992 and 1983.

The 1992 season produced only six named storms and one subtropical storm. However, one of those named storms was Hurricane Andrew, which devastated South Florida as a Category 5 hurricane.

In 1983, there were only four named storms, but one of them was Alicia. The Category 3 hurricane hit the Houston-Galveston area and caused almost as many direct fatalities along the Texas coast as Andrew did in South Florida.

In contrast, the 2010 season was active. There were 19 named storms and 12 hurricanes that formed in the Atlantic Basin. Despite the large number of storms that year, no hurricanes and only one tropical storm made landfall in the U.S.

In other words, a season can deliver many storms but have little impact, or deliver few storms with one or more causing major impacts to the U.S. coast.

The U.S. averages one to two hurricane landfalls each season, according to NOAA’s Hurricane Research Division statistics.

In 2016, five named storms impacted the Southeast coast, most notably the powerful hit from Hurricane Matthew and its subsequent inland rainfall flooding.

Prior to that, the number of U.S. landfalls had been well below average over the previous 10 years.

The 10-year running total of U.S. hurricane landfalls from 2006 through 2015 was seven, according to Alex Lamers, a meteorologist with the National Weather Service. This was a record low for any 10-year period dating to 1850, and considerably lower than the average of 17 per 10-year period dating to 1850, Lamers added.

Of course, the record-breaking 2005 hurricane season was outside that current 10-year running total. It was also the last season we saw a Category 3 or stronger hurricane (Wilma) hit the U.S., the longest such streak dating to the mid-19th century.

The bottom line: it’s impossible to know for certain if a U.S. hurricane strike, or multiple strikes, will occur this season. Keep in mind, however, that even a weak tropical storm hitting the U.S. can cause major impacts, particularly if it moves slowly and triggers flooding rainfall.

IMAGES:
The named storms that affected the U.S. in 2016 were clustered in the Southeast.
2017 Atlantic hurricane season names.
Numbers of Atlantic Basin named storms, those that attain at least tropical storm strength, hurricanes, and hurricanes of Cat. 3 intensity forecast by NOAA, The Weather Company, an IBM business and Colorado State University compared to 30-year average.

For more on this story go to: https://weather.com/storms/hurricane/news/2017-hurricane-season-forecast-atlantic-colorado-state-august-update