Friday, May 26, 2017

NOAA Predicts Above-Average 2017 Atlantic Hurricane Season

Jon Erdman
Published: May 25,2017

The 2017 Atlantic hurricane season is forecast to be more active than historical averages with regards to the number of named storms and hurricanes, according to a forecast released Thursday by the National Oceanic Atmospheric Administration.
(MORE: Hurricane Central)
NOAA's forecast calls for:
  • Eleven to 17 named storms – including April's Tropical Storm Arlene.
  • Five to 9 of which would become hurricanes.
  • Two to 4 of which would become major hurricanes.
The 30-year historical average (1981-2010) for the Atlantic Basin is 12 named storms, six hurricanes and three major hurricanes. A major hurricane is one that is Category 3 or stronger on the Saffir-Simpson Hurricane Wind Scale.
(MORE: 5 Changes Coming to Hurricane Season Forecasts)
Numbers of Atlantic Basin named storms, those that attain at least tropical storm strength, hurricanes, and hurricanes of Cat. 3 intensity forecast by The Weather Company, an IBM business, NOAA, and Colorado State University compared to 30-year average.

According to NOAA, "The outlook reflects our expectation of a weak or non-existent El Niño, near- or above-average sea-surface temperatures across the tropical Atlantic Ocean and Caribbean Sea and average or weaker-than-average vertical wind shear in that same region."
Strong El Niños typically lead to increased wind shear in parts of the Atlantic Basin, suppressing the development or intensification of tropical cyclones in the Atlantic, so the prediction for weak conditions increased the chance for more activity this season.
"The climate models are showing considerable uncertainty, which is reflected in the comparable probabilities for an above-normal and near-normal season," NOAA added.
The Weather Company updated its seasonal forecast earlier in May and expects a total of 14 named storms – seven hurricanes and three major hurricanes – this season. This is an increase from its forecast compared to April due to a couple of factors.
One of the reasons is that warmer sea-surface temperatures have been observed in the North Atlantic, which have correlated with more active seasons in the past. In addition, there are indications that further warming is likely.
Another factor the outlook cited is that there is a reduced potential for the development and strength of El Niño later this summer.
Given the current trends, there is the potential for another increase with the next update in June. "The historically strong North Atlantic blocking event in early May also suggests the possibility of continued increases in North Atlantic sea-surface temperature anomalies, so it would be no surprise if we increased our forecast numbers again," said Dr. Todd Crawford, chief meteorologist with The Weather Company.
(MORE: The Latest on El Niño's Possible Development)
The Colorado State University (CSU) Tropical Meteorology Project outlook headed by Dr. Phil Klotzbach calls for a lower number of named storms, with 11 expected. CSU forecasts fewer hurricanes this year compared to average, with four expected in the Atlantic Basin.
The official Atlantic hurricane season begins June 1 and runs through Nov. 30. Occasionally, storms can form outside those months as happened this year with Tropical Storm Arlene, which formed in April. This also occurred last season with January's Hurricane Alex and late May's Tropical Storm Bonnie.
2017 Atlantic hurricane season names.
An important note is that Arlene is included in the seasonal forecast numbers in the outlooks.
(MORE: 10 Things We Remembered Most About the 2016 Season)

What Does This Mean For the U.S.? 

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 11-14 named storms forecast to develop this season could hit the U.S., or none at all. Therefore, residents of the coastal United States should prepare each year no matter the forecast.
A couple of classic examples of why you need to be prepared 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 there 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, not a single hurricane and only one tropical storm made landfall in the United States.
In other words, a season can deliver many storms, but have little impact, or deliver few storms and have one or more hitting the U.S. coast with major impact.
The named storms that affected the U.S. in 2016 were clustered in the Southeast.

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 U.S. coast, most notably the powerful scraping of the coast from Hurricane Matthew, and its subsequent inland rainfall flooding.
(MORE: Hermine Ended Florida's Record Hurricane Drought)
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, 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.
(MORE: 10 Reasons the U.S. Major Hurricane Drought is Misleading)
The bottom line is that 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, resulting in flooding rainfall.

Will El Niño play a role?

As mentioned earlier, El Niño could return at some point during the 2017 hurricane season, but there remains plenty of uncertainty regarding that.
This periodic warming of the central and eastern equatorial waters of the Pacific Ocean tends to produce areas of stronger wind shear (the change in wind speed with height) and sinking air in parts of the Atlantic Basin that is hostile to either the development or maintenance of tropical cyclones.
The effects of El Niño in the eastern Pacific, Caribbean and western Atlantic Ocean.
NOAA put the odds of El Niño's development at slightly lower than 50 percent during the summer to fall period, according to their latest update.
Crawford said in The Weather Company hurricane season forecast that the latter portion of the season could be less active if El Niño conditions develop. But it's unclear how much and how soon any type of atmospheric response there would be if El Niño did materialize.
In the CSU outlook, Klotzbach said the potential development of El Niño is different than anything seen since 1980, complicating the forecast.
"Current SST (sea-surface temperature) anomalies in the Nino 1+2 region are some of the warmest ever observed," wrote Klotzbach.
"These warm SST anomalies off the west coast of South America may be a harbinger of a developing El Niño event." Klotzbach also cautions there is considerable uncertainty regarding the eventual strength of El Niño, assuming it even occurs.
The most recent El Niño strengthened quickly during the 2015 season, which featured 11 named storms, 4 hurricanes and 2 major hurricanes. Hurricane Joaquin's prolonged pummeling of the Bahamas was the most notable hurricane that season.
Strong wind shear near the Caribbean Sea and other parts of the Atlantic Basin contributed to the eventual demise of five named storms during the heart of the 2015 season.
Klotzbach found that June through October 2015 Caribbean wind shear was the highest on record dating to 1979. Klotzbach also said the magnitude of dry air over the Caribbean Sea in the peak season month of August and September also set a record.

Any Other Factors in Play?

Dry air and wind shear can be detrimental to tropical storm or hurricane development no matter whether El Niño is present or not.
The 2013 and 2014 seasons featured prohibitive dry air and/or wind shear during a significant part of the season, but El Niño was nowhere to be found.
Named storm tracks in the 2016 Atlantic hurricane season.
Sea-surface temperature anomalies may continue to trend upwards as we head into hurricane season due to a change in the weather pattern near the north Atlantic Ocean, according to The Weather Company outlook. This could result in atmospheric conditions becoming more favorable for the development and strengthening of Atlantic hurricanes.
"The historically-strong negative NAO (North Atlantic Oscillation) event in May would suggest further warming of the North Atlantic sea surface temperaturs, which is a bullish factor that may cause future forecast numbers to increase a bit," Crawford said.
MORE: Hurricanes From Space - Satellite Imagery

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.

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