What a hurricane season! It started very early with Arlene in April but the real action held off until the last week of August, when Hurricane Harvey flooded Texas and Louisiana. Harvey was the first hurricane to make landfall in Texas since Ike in 2008 and the first Category 4 hurricane in Texas since Carla in 1961.

Irma, the 11th strongest Atlantic storm on record (using central pressure, the most reliable measure), had major impacts on islands like Barbuda and St. Martin, the Virgin Islands, the Turks and Caicos and southern Bahamas. Then crossing northern Cuba it curled back into Florida. It was the first landfalling hurricane and major hurricane in Florida since Wilma in 2005.

Jose too became a major hurricane but never made landfall, though it created large swells along the eastern seaboard and pounded southeastern New England, Cape Cod and the islands with tropical storm winds and coastal flooding as it stalled for days.

Maria, the third major hurricane of the season and 10th strongest Atlantic storm on record, crossed the northern Leeward Islands and plowed through Puerto Rico, doing catastrophic damage to the island. It then moved north into the Atlantic, close enough to pound the Atlantic Coast with large swells from Florida to New Jersey.

And then Hurricane Nate avoided another “Katrina moment” for New Orleans but produced storm surge damage to southeast Louisiana, Mississippi, Alabama and the Florida Panhandle.

Before the landfall of two major storms on the U.S. we had gone just short of 12 years without a major hurricane landfall, the longest such lull since the 1860s.

The quiet period came after three big years. Isabel made landfall on the Mid Atlantic in 2003. Charley, Frances, Ivan and Jeanne in 2004 and Dennis, Katrina, Rita and Wilma in 2005 all made landfall on the mainland. Emily in 2005 was another major hurricane but turned west into Mexico. 2005 holds the record for five Category 4 or greater and four Category 5 impact storms. Some speculated this was the new norm for the Atlantic before nature gave us that 12-year break.

So what causes long quiet spells and then big years like 2004 and 2005 and now 2017?

Nothing is new in weather. Great Colonial hurricanes in the Northeast with storm surges up to 20 feet occurred in 1635 and 1675. A Katrina-like storm made landfall in Louisiana in 1722 with major flooding and damage in Louisiana. The Great Chesapeake storm in 1769, like Isabel in 2003, brought major flooding to North Carolina and Virginia. In the Caribbean, the Great Hurricane of 1780 killed an estimated 27,500 people while ravaging the islands of the eastern Caribbean with winds estimated to top 200 m.p.h. It was one of three hurricanes that year with death tolls greater than 1,000.

1893 had at least 10 hurricanes. Of those, five became major hurricanes. Two of the hurricanes caused over two thousand (2,000) deaths in the United States. At the time, that season was the deadliest in U.S. history.

1886 came close with at least 10 hurricanes, seven making landfall. Four of the hurricanes were major hurricanes.

The Galveston Hurricane in 1900 killed at least 8,000 people with some estimates as high as 12,000, making it the deadliest natural disaster in U.S. history.

Okay, major hurricanes have occurred even during cold periods, but is there a trend in the modern record?

ACCUMULATED CYCLONE ENERGY INDEX A MEASURE OF SEASONAL TROPICAL ACTIVITY

The Accumulated Cyclone Energy index takes into account the number, duration and strength of all tropical storms in a season. The ACE index is a wind energy index, defined as the sum of the squares of the maximum sustained surface wind speed (knots) measured every six hours for all named storms while they are at least tropical storm strength.

The ACE index for the Atlantic shows a cyclical behavior with no long-term trend but with spikes in 1893, 1926, 1933 and 1950 then again in 1995, 2004 and 2005. 2017 ranks 8th now with still weeks to go this season.

So what causes long breaks and then big years like 2004 and 2005 and now 2017?

OCEAN TEMPERATURE AND PRESSURE PATTERNS


The North Atlantic, like the Pacific, undergoes multi-decadal changes in ocean temperature and pressure patterns. It has long been known that when the Atlantic is in what is called its warm mode, there are more storms. Since 1995, when the current warm Atlantic mode began, we have average 14.6 named storms per year, more than five greater than the long-term 1851-2017 average.

An important factor that affects whether hurricanes affect the United States is El Niño and La Niña. When El Niños develop, more storms develop in the eastern and central Pacific, threatening Mexico, Hawaii and sometimes in weakened forms Arizona and California.

These storms enhance high-level winds that cross into the Atlantic. These winds produce shear that disrupts developing storms, causing them to weaken or dissipate and/or turn harmlessly north into the North Atlantic. Storms can still develop near the coast where the water is warm like in the Gulf and near the Gulf Stream off the southeast coast.

When La Ninas develop there are usually fewer storms in the eastern Pacific and less shear to disrupt the Atlantic storms.

In warm Atlantic years, that means trouble as the storms can track the entire basin with more time to turn into major hurricanes. Even the East Coast is more vulnerable to a landfalling hurricane. We had eight high-impact East Coast hurricanes from 1938 to 1960 and nine from 1988 to 2012.

The last important La Niña stretch was in 2010/11 to 2011/12. We avoided a major hurricane hit, though major hurricanes at sea made final landfall in the NYC metro — Irene (as a tropical storm) in 2011 and Sandy in 2012 (as a post-tropical cyclone). They caused massive flooding (from rains with Irene in upstate NY and Vermont and from a storm surge with Sandy in New York City and New Jersey).

We are still in the latest Atlantic warm period. This year, a spring attempt at an El Niño failed and La Niña-like conditions developed. Had El Niño succeeded we may have had Harvey, which developed near the Texas coast, and Nate, which came out of the bath water in the western Caribbean, but maybe Irma and Maria would have been weakened or deflected. But with La Niña conditions developing, no shear and warm Atlantic water we saw a return to big storms just as we saw in 2004 and 2005.

It may not be over as in 2005 we had Wilma come out of the Caribbean in late October.

At WeatherBELL, Joe Bastardi led the team in the tropical outlook and correctly called for a big season. When the water in the Main Development Regions warmed further, Joe upped the ACE forecast and the team began alerting that come mid August, big things would happen.

So when we get a year like 2017 or back-to-back bad years like 2004 and 2005, we have to accept that is how the weather works. Permadroughts ended with record wet years for Texas and California this decade. The record nearly 12-year major hurricane “drought” ended with 2017.

Joe D'Aleo is currently a senior co-chief meteorologist with WeatherBELL Analytics. Joe is a CCM, fellow of the AMS and former chair of the AMS Committee on Weather Analysis and Forecasting. He was a college professor of meteorology/climatology, the co-founder and first director of meteorology at The Weather Channel and chief meteorologist with three companies. He is the executive director of Icecap.us since 2007.