Why the 2018 Hurricane Season Was More Active Than We Predicted

In our end-of-season recap, we spoke to a few of the experts who issue seasonal forecasts to learn why the year was more active than expected.

Finally, we'll look back at some of the notable hurricanes and numbers we'll remember for years to come.

(PODCAST: Dr. Phil Klotzbach Explains Why This Season Overachieved)

Why It Was More Active Than Expected

Before the season began, forecasts called for generally above-average activity. In August, the final forecast updates had wrongly trended downward, calling for near-average activity.

What appeared to be better news for the season was based on a couple of factors:

- The expectation of cooler Atlantic temperatures, which would lessen activity in the heart of the tropics- The expectation that the suppressive nature of El Niño would lower numbers

But these factors just didn't pan out.

"It turns out the tropical Atlantic ocean temperatures warmed up to near average as we went through August," said Gerry Bell, Ph.D., lead seasonal hurricane forecaster at NOAA’s Climate Prediction Center. "Also, El Niño did not form as the models had predicted, which means El Niño would not be a suppressing factor on the season."

(MORE: NOAA's Review of the 2018 Season)

It's worth noting that forecasts in May and June were closer to reality than forecasts released later in the season. These forecasts were released when an El Niño and cooler Atlantic temperatures were anticipated to have less of an effect on the season.

Two other curveballs increased the seasonal numbers even more.

First, far more subtropical storms formed in the basin than usual. Subtropical storms draw their energy from both the ocean, like typical tropical storms and hurricanes, and from the jet stream overhead, like low-pressure systems with cold and warm fronts.

"The conditions over the central North Atlantic, where we had a lot of these subtropical systems, were extremely conducive," according to Dr. Bell. "We had record-warm ocean temperatures and also record-weak vertical wind shear. ... That was certainly a player in the overall strength of the hurricane season, as far as numbers go."

The season produced seven subtropical systems, a record for any season, in a few spots that are typically more hostile for development.

Most tropical activity occurs in the southern Atlantic, Caribbean and Gulf of Mexico, but this year, as indicated by the numerous storm symbols in the graphic below, several systems formed in the northern Atlantic.

(MORE: What Is a Subtropical System?)

Those subtropical systems were likely unaffected by the slightly cooler waters of the Atlantic and the warm-neutral El Niño conditions.

"Storms that form in the subtropics typically don't get as strong and they don’t necessarily respond to large-scale factors like El Niño in the same way," according to Dr. Phil Klotzbach, lead author of Colorado State Univerisity's seasonal outlooks.

Subtropical storms were not officially recognized until the beginning of the satellite era, and they weren't named until 2002.

(MORE: Why This Cold November Increases Odds of a Cold Winter in the East)

The second surprise occurred when the Atlantic roared to life in early September.

A tropical cyclone outbreak – defined by at least three tropical storms or hurricanes forming in relatively close positions and timing – occurred in early September.

Five named storms formed in the first 12 days of September, which, according to Klotzbach, tied the record for tropical storm formation in this period. What was then Tropical Storm Florence was the first of the month to form, but was later joined by hurricanes Helene and Isaac on Sept. 10. This was the 11th year on record with three simultaneous hurricanes in the Atlantic.

Even during this spell of heightened activity, the Caribbean stayed mostly quiet, which might have been a function of the El Niño-like conditions.

The Caribbean wind shear was extremely strong even though we didn’t meet the threshold that would designate an official El Niño, said Klotzbach. This was notable because no hurricanes developed or tracked into the Caribbean this year – what we would expect in an El Niño season.

Notable Hurricanes

Hurricane Florence (Recap)

Florence slowed on approach to the Carolinas in mid-September and then meandered for days, inundating New Bern and the Outer Banks amid record-setting rainfall throughout the Carolinas. Florence finally came ashore after two weeks of unusual movements across the Atlantic.

In its cross-Atlantic voyage, Florence became a Category 4 storm twice: once on Sept. 5, then again on Sept. 10, with a stint as a tropical storm in between. Winds rapidly intensified from 75 mph to 130 mph in just 25 hours, ending at noon EDT Sept. 10.

Florence made landfall near Wrightsville Beach, North Carolina, at 7:15 a.m. EDT Sept. 14 with maximum sustained winds around 90 mph.

Florence's water was a major story days before it slowed down and made landfall. Surf came up on the shores of North Carolina as Florence passed south of Bermuda, then flooded the town of New Bern in the eastern part of the Tar Heel State with more than 10 feet of storm surge.

Then rainfall became a big story as Florence drifted inland.

Two state tropical cyclone rainfall records were broken: North Carolina, with 35.93 inches near Elizabethtown (old record: 24.06 inches – Floyd) and the other in South Carolina, with 23.63 inches near Loris (old record: 17.45 inches – Beryl).

The incredible rainfall and its resulting flooding cut off Wilmington, North Carolina, from surrounding areas.

At least five river gauges observed record flooding in North Carolina, topping what was seen in hurricanes Matthew (2016) and Floyd (1999). Also, the Waccamaw River near Conway, South Carolina, had record flooding, topping Matthew by more than 3 feet.

Hurricane Michael (Recap)

Michael slammed the Florida Panhandle in early October at its peak intensity, Category 5 strength. Michael was the strongest hurricane to hit the panhandle in records going back to the 1800s.

Michael quickly intensified in the Gulf of Mexico in the days before landfall. The system grew from a tropical depression with 35 mph winds to a Category 1 hurricane with 75 mph winds in the 24 hours ending at 11 a.m. EDT on Oct. 8.

A little more than 48 hours later, Michael made landfall near Mexico Beach, Florida, with maximum sustained winds of 160 mph.

At the coast, Michael brought a peak storm surge of 9 to 14 feet from Mexico Beach to Apalachee Bay. The inundation was worst from Mexico Beach to Port St. Joe, where several cuts were made through the barrier islands. Combined with high tide, Michael pushed water levels more than 20 feet high at some spots.

The area between Mexico Beach and Panama City, including Tyndall Air Force Base, suffered catastrophic wind damage that damaged or destroyed hundreds of homes.

Winds higher than 115 mph knocked out power and ravaged crops from the central Florida panhandle into southwestern Georgia.

By the Numbers

One: Category 5 hurricane to make landfall in the Florida Panhandle in history (Hurricane Michael).

Three: Category 4 or 5 hurricanes to make landfall in the continental U.S. in two years – Michael, Harvey and Irma – a record.

Fourth-strongest: Landfalling sustained winds in the mainland U.S. (160 mph) for Hurricane Michael. This trailed only behind the Labor Day Hurricane of 1935, Camille (1969) and Andrew (1992), which were all Category 5 hurricanes.

12.5 weeks into hurricane season: Until early September, wind shear and ocean temperatures were near average.

16.5 days: Leslie was a tropical storm longer than any other named storm in the Northern Hemisphere in 2018.

919 millibars: Michael had the third-lowest pressure for a U.S. landfalling hurricane on record, trailing only the Labor Day Hurricane of 1935 and Hurricane Camille.

Some of these statistics were provided by Dr. Phil Klotzbach.

Closing Remarks

This season, especially by midseason, was forecast to be near average. As we know, this isn't what happened.

We often say, "It only takes one," but this year, two such historic storms struck the American coastline.

"This season really highlights the importance of being prepared," said Dr. Bell. "Being prepared ahead of time made such a huge difference in terms of, not only for people with individual situations, but being able to evacuate more people. That was certainly critical with Michael that destroyed everything in its path."

The key message is to be prepared every hurricane season, regardless of seasonal forecasts.