Below-Average 2015 Atlantic Hurricane Season Likely, Forecasters Say

NOAA, which released its final forecast update of the season Thursday, calls for a 90 percent likelihood of:

- 6-10 named storms (including three tropical storms that formed earlier in the season: Ana, Bill, and Claudette)- 1-4 of which would become hurricanes- 0-1 of which would become major hurricanes – those of at least Category 3 strength on the Saffir-Simpson Hurricane Wind Scale.

This is below the 30-year average of 12 named storms, six hurricanes and three major hurricanes.

NOAA noted the modern-era record low number of Atlantic hurricanes in any season is two in 2013 and 1982. 

The NOAA forecast is consistent with The Weather Channel forecast, updated in mid-July, which also predicted below-average activity, of:

- 10 named storms- 4 hurricanes- 1 major hurricane

(MORE: Florida Hurricane Apathy) 

Colorado State University's (CSU) forecast update issued Aug. 4 calls for a season total of eight named storms, including two hurricanes, one of which is predicted to attain major hurricane status.

The CSU outlook is headed by Dr. Phil Klotzbach in consultation with long-time hurricane expert Dr. William Gray and is based on a combination of 29 years of statistical predictors, combined with analog seasons exhibiting similar features of sea-level pressure and sea-surface temperatures in the Atlantic and eastern Pacific Oceans.

“We remain rather bearish on the season as a whole," said meteorologist Dr. Todd Crawford of The Weather Channel Professional Division in a mid-July outlook. "The behavior of the Atlantic hurricane season thus far is going generally as planned." 

Here are four questions about the various hurricane season outlooks and what it means for you.

Q: Does this mean a less destructive season?

There is no strong correlation between the number of storms or hurricanes and U.S. landfalls in any given season.

"It is important to note that our  – The Weather Channel – forecasts are for the total number of storms that may occur anywhere within the Atlantic Ocean, and do not attempt to predict the number of storms that will make landfall in the U.S.," said Dr. Peter Neilley, vice president of Global Forecasting Services at WSI.

The 2014 season featured the fewest number of named storms in 17 years (eight storms), but also featured the strongest landfalling hurricane in the mainland U.S. in six years (Hurricane Arthur on the Outer Banks), and featured two back-to-back hurricane hits on the tiny archipelago of Bermuda (Fay, then Gonzalo).

Furthermore, six of those eight storms became hurricanes, and Gonzalo was the strongest Atlantic hurricane since Igor in 2010.

(RECAP: 2014 Hurricane Season)

In 1983, there were only four named storms, but one of them was Alicia, a Category 3 hurricane which clobbered the Houston-Galveston area.

The 2010 season featured 12 hurricanes and 19 named storms, which tied 1995 for the third most named storms in any Atlantic season, at the time. But not a single hurricane, and only one tropical storm, made landfall in the U.S during that active season.

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.

Therefore, it's important to be prepared for hurricanes and tropical storms every year, regardless of seasonal forecasts.

Q: Is El Nino playing a role?

El Nino was first officially declared by NOAA as winter wound down. As of mid-July, El Nino, a periodic warming of the equatorial Pacific waters, has been given an 80 percent chance of persisting into spring 2016, and is likely to become strong, according to NOAA's Climate Prediction Center.

(MORE: Strong El Nino Probable)

"The atmosphere is now responding to El Nino at an amplitude that has not been observed for developing El Nino events in the early half of summer, according to our Atmospheric ENSO index (AEI)," Crawford said. "We often see enhanced low-level easterly flow and upper-level westerly flow across the Atlantic (in El Nino events), which increases vertical wind shear."

Vertical wind shear weakens existing tropical cyclones and tends to inhibit new cyclones from forming by displacing thunderstorms away from the low-level circulation center.

CSU's Klotzbach says the magnitude of wind shear over the Caribbean Sea in July was at record high levels.

This warm, dry, sinking air aloft suppresses thunderstorms needed for tropical cyclones to organize.

Klotzbach and Gray of CSU found five other hurricane seasons with comparable sea-surface and atmospheric conditions to June-July 2015, during strong El Ninos and during which the atmosphere was deemed generally unfavorable for development: 1965, 1972, 1982, 1987, and 1997. Those years averaged 6-7 named storms, three hurricanes, and one major hurricane.

Despite the low numbers in those years, there were two historic hurricanes during those seasons:

- Hurricane Betsy (1965): This was the benchmark for New Orleans before Katrina. Betsy drove a 10-foot storm surge up the Mississippi River Gulf Outlet and into Lake Pontchartrain, submerging some homes up to their rooftops. Betsy was the first billion-dollar hurricane in U.S. history, with total damage estimated at $1.4 billion. Officially, 75 deaths were attributed to Betsy in Florida and Louisiana.

- Hurricane Agnes (1972): More noted for its inland flooding in the East than its Florida landfall, Agnes and its remnant flooding claimed 122 lives and was the costliest weather disaster in U.S. history, at the time ($2.1 billion). A dike breach sent a wall of water through Wilkes-Barre, Pennsylvania.

(MORE: El Niño Facts Behind the Impacts)

Q: Any other factors in play?

Sea-surface temperature anomalies in the MDR are at levels comparable to what was experienced in both 2002 and 2014, which were both relatively quiet Atlantic hurricane seasons, Klotzbach said.

Looking at the Atlantic Basin as a whole in July 2015, warmer sea-surface temperatures (hereafter, SSTs) were in place in the western Atlantic Ocean, northern and central Gulf of Mexico and northwest Caribbean Sea, but generally cooler-than-average SSTs dominated from just west of the Cape Verde Islands off the western African coast to the Windward Islands, eastern and southern Caribbean Sea.

All other factors – such as the amount of wind shear and dry air aloft – being equal, warmer waters offer more heat to fuel the tropical cyclone.

"Note that all of the developing storms this year (so far), have been over warmer-than-average SSTs," said Crawford referring to the three named Atlantic storms through the end of July, all near the U.S. East Coast or Gulf Coast.

It is important to note, however, that a large majority of the destructive hurricanes during the record-setting 2005 hurricane season developed in the western Atlantic Basin. 

(MORE: The Heart of the Hurricane Season)

NOAA cited another possible reason for lower numbers of named storms this season.

Q: Why were the last two seasons relatively quiet?

We mentioned the somewhat paradoxical 2014 Atlantic hurricane season earlier. Fewest named storms since 1997, but back-to-back strikes on Bermuda, as well as Hurricane Arthur ruining the July 4th holiday on the Outer Banks of North Carolina. 

In 2014, Klotzbach and Gray noted July sea-surface temperatures in the main development region between the Lesser Antilles and Africa were the coolest since July 2002. Interestingly, sea-surface temperatures were actually warmer than average in a broad swath of the western Atlantic Ocean, western Caribbean Sea, and Gulf of Mexico.

Vertical wind shear, namely the change in wind direction and/or speed with height, was found to be near the strongest on record in July 2014 over the Caribbean Sea, according to the CSU study. Wind shear disrupts tropical cyclones or inhibits them from developing by displacing thunderstorms from the center of circulation.

After Arthur, five remaining named storms forming in the Atlantic Ocean all took north, then northeast turns away from the U.S. mainland, thanks to the orientation of winds aloft and the orientation of the Bermuda high. Tropical Storms Dolly and Hanna buried themselves in eastern Mexico and Central America, respectively.

In the 2013 season, for the first time since 1994, no hurricanes stronger than Category 2 developed. Since the satellite era began in 1960, only four other seasons failed to produce a single Category 3 or stronger hurricane (1994, 1986, 1972, 1968).

"By most measures, 2013 was one of the strangest years in the tropical Atlantic in many decades," Crawford said. "The 'usual suspects' of pre-season indicators suggested a reasonably active season as relative warm Atlantic SSTs and an expected lack of El Niño resulted in fairly bullish seasonal forecasts."

While the number of storms predicted (14) in 2013 was above the long-term average, the dominance of dry air and wind shear limited the intensity of existing storms or squelched the development of others.

The NOAA outlook mentioned the debate which has surfaced as to whether we are still in this high-activity era of the 25-40 year Atlantic multidecadal oscillation, or AMO, which had been in place since 1995. 

The relatively quiet 2013 and 2014 seasons and the cooler-than-average sea-surface temperatures in the eastern Atlantic Ocean just before the 2015 season have fueled speculation we may be transitioning to the less active cycle of the AMO which was last in place from 1971-1994.

During that less-active period, there was an average of only 8-9 named storms, 5 hurricanes, and 1-2 major hurricanes each season. 

"The current suppressive conditions are expected to make 2013-2015 the only 3-year period since 1992-94 in which three consecutive Atlantic hurricane seasons were not above normal," the NOAA hurricane season forecast team said in its latest update.

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