Why Hurricane Florence Is Such a Dangerous Storm

The category four hurricane is expected to hit with intense winds and cause major flooding.Wednesday, 12 September 2018

By Sarah Gibbens
In this image taken by the International Space Station on 10th September, 2018, Hurricane Florence gains strength in the Atlantic Ocean as it moves west.

Later this week, Hurricane Florence is expected to hit the southern portion of the U.S. East Coast. It harbours the potential for life-threatening conditions and has already forced mandatory evacuations.

As Florence spins toward the Carolinas with 130 mile-per-hour winds, the northern Atlantic ocean is spinning two other named hurricanes—Isaac and Helene. In the Pacific, tropical storm Olivia is on track to hit Hawaii, dumping large amounts of rain on the islands.

It's no coincidence that the ocean has suddenly begun churning out intense storms all at once. According to NOAA, late August to October is the peak of hurricane season when conditions are primed to create the perfect storm, though deadly hurricanes can form through the whole season (June through November).

Storms that hit the U.S. East Coast tend to be strongest when they originate off the coast of Africa, says NOAA hurricane support meteorologist Joel Cline.

“Any time it comes off Africa, they have two weeks worth of warm water to build up,” says Cline. Surface temperature, he notes, “gets things going,” but windy conditions have to stay out of a hurricane's way if it's going to build up to a category four or five.

How Hurricanes Form

The surface of typical low-pressure storms tilt toward cold air, but Cline notes hurricanes are built differently.

“In a hurricane, there's no real cold air. It's built more like a chimney,” he says.

As air flows in through the bottom, it comes out through the top. If wind shear is high, it tilts the hurricane, making it more difficult to funnel warm air through the hurricane and weakening the storm. When wind shear is low, the hurricane stays centred and cycles warm air through more efficiently.

Cline says Hurricane Florence was hit by high shear but was still able to maintain its circulation centre.

“Dangerous Storm”

Along its path, Florence also stayed strong because there were few land masses in its way to slow the storm's progress. That means when Florence finally hits land (expected on Thursday), it could hit with full force.

(Read hurricane safety tips here.)

“Slight weakening can happen, but it cannot be overstated that Florence is still a dangerous storm. If you are told to evacuate, it is crucial that you listen. Plan to be out of affected areas for an extended period of time as Florence will have extensive impacts,” FEMA administrator Brock Long said in a tweet.

Heavy Rain

Once Florence has dispersed on land, it's expected to dump large amounts of rainfall.

Because Florence is driven by warm air, its capable of carrying more water—because warm air generally holds more water than cold air. Cline notes that air currents coming off the Appalachian mountain range could force more rain to fall when it mixes with storm conditions.

“Florence has the potential to bring rainfall well beyond the coast,” says Cline.

He notes that in inland regions, mudslides could become a major threat.

Season Outlook

Hurricane season in the Atlantic will last until 30th November, and NOAA has predicted that 2018 will be average or slightly above average. While this time of year is often the most active, a number of weather conditions like El Niños and La Niñas can influence storm outcomes.

(Why a quiet hurricane season isn't necessarily a good thing.)

Anywhere from one to four major hurricanes are expected, but so far, Florence is the first.

This story was originally published on NationalGeographic.com