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How a thunderstorm with hurricane force can capsize a ship

Sunday, April 18th 2021, 10:27 am - An unrelated ship in the general vicinity of Port Fourchon, La. recorded a wind gust of 188 km/h, equivalent to a Category 3 hurricane.

While ships capsizing during rough waters at sea is not unheard of, this particular storm that sank a vessel on Tuesday had some extraordinary powers.

A commercial ship encountered a strong thunderstorm while churning off the Louisiana coast, capsizing amid fierce winds and heavy rain. Nineteen people were on board, with several bodies recovered and about half of the crew still missing.

The 129-foot Seacor Power liftboat came across a line of potent thunderstorms shortly after launching from Port Fourchon Tuesday afternoon. The ship was around 13 kilometres south of the port when it issued a distress call just before 4:30 p.m.

US Coast Guard District 8/capsized ship A U.S. Coast Guard boat heads toward a capsized 175-foot, commercial liftboat April 13, searching for people in the water about 13 kilometres south of Grand Isle, La. Photo: Glenn Harris/U.S. Coast Guard District 8.

HOW STRONG WAS THE THUNDERSTORM?

The thunderstorm development resulted from a stationary front positioned across Louisiana and other Gulf Coast states. It was considered to be a mesoscale convective system -- a long-lasting, large-scale cluster of storms that are typical of producing very heavy rain, powerful winds, flooding and tornadoes.

A special marine warning was in place for the potential for steep waves for the coastal waters in and near Port Fourchon, La.

The system trekked across fairly rapidly, possibly faster than the ship's crew could handle in a reactionary manner. The storm also strengthened as it moved offshore.

So, how powerful was it?

High waves and winds of 64-96 km/h were reported at surrounding land stations, persisting as long as 12 hours after the boat capsized. An unrelated ship in the general vicinity of Port Fourchon recorded a wind gust of 188 km/h, equivalent to a Category 3 hurricane.

US Coast Guard District 8/capsized ship The crew of the Coast Guard Cutter Glenn Harris pulls a person from the water April 13 after a 175-foot commercial liftboat capsized close to 13 kilometres south of Grand Isle, La. Photo: Glenn Harris/U.S. Coast Guard District 8.

To make matters worse, a phenomena known as gravity waves was believed to have been a factor in the ship capsizing.

The U.S. National Weather Service stated that gravity waves had was contained below a strong temperature inversion -- an increase of the air temperature with height -- slightly above the surface, approximately the lowest 1,500 feet of the atmosphere. That reversal resulted in "waves of stronger winds mixing down in gusts over land, and sustained gales with storm-force gusts over larger water bodies."

"The key with the gravity waves/temperature inversion is that it allowed for stronger winds to reach the surface, so not technically making it have hurricane strength, but allowing for some of the stronger upper winds to mix down," said Jessie Uppal, a meteorologist at The Weather Network.

Thumbnail courtesy of Glenn Harris/U.S. Coast Guard District 8.

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