Severe, unforecasted winds caused a containership to breakaway from a pier and damaged equipment at the Napoleon Avenue Container Terminal in New Orleans, Lousiana, on August 2, 2020, according to the National Transportation Safety Board's (NTSB) Marine Accident Brief 21/18 issued late last week.
As longshoremen loaded and unloaded cargo from the Malta-flagged containership CMA CGM Bianca, a sudden, localised thunderstorm passed through the area. Ten of the vessel's 16 mooring lines parted in the high winds, and the ship moved away from the pier.
Containers lifted by shoreside gantry cranes struck the ship. One damaged container dropped in the water spilling a cargo of plastic pellets, known as nurdles.
A crane operator suffered a minor injury while damages totaled approximately US$15.1 million.
In its report, the NTSB said the crane operators and CMA CGM Bianca's crew reported extreme high winds that came on "in seconds" during heavy rains. Rains were heavy enough to completely obscure the visibility of security cameras at the terminal (pictured).
Although the closest official weather station recorded winds peaking at 31 mph (26 knots), a vessel located very close to the accident reported a wind gust at 73 mph (63 knots).
CMA CGM Bianca's master said that the storm was "in the form of a tornado."
According to the report, the evidence suggests that CMA CGM Bianca was struck by outflow winds from a downburst.
The National Weather Service classifies downbursts as "powerful winds that descend from a thunderstorm and spread out quickly once they hit the ground. These winds can easily cause damage similar to that of a EF0 (65–85 mph ~ 56–73 knot winds) or even EF1 (86–110 mph ~ 74–95 knot winds) tornado and are sometimes misinterpreted as tornadoes."
Investigators determined the probable cause of the accident to be the sudden onset of unforecasted severe winds likely originating from the outflow of a thunderstorm-generated downburst.