Winds locally topped 100 mph, abruptly whipping up hazardous seas. The ship was south of Grand Isle when the 129-foot vessel issued a mayday call about 4:30 p.m.
The Coast Guard was continuing its search Wednesday, working against intermittent thunderstorms that have been producing flash flooding ashore in Louisiana.
More active weather as search operations continue
The active weather is the result of a stalled stationary front draped across the area. Low pressure over East Texas has helped pool moisture along the front, with upper-level energy supporting vigorous bands of downpours and thunderstorms.
At least 1.84 inches of rain had come down in Morgan City, La., with more than 2 inches falling near Grand Isle. Some places could wind up with up to half a foot of rain by Thursday or Friday.
Conditions will remain unsettled intermittently into Friday, with several opportunities for more storminess.
The National Weather Service Storm Prediction Center had issued a level 2 out of 5 “slight risk” for severe weather that encompassed much of Louisiana from the Mississippi River Delta westward Wednesday; the risk area was trimmed toward the coast in the afternoon. The Weather Service had also hoisted a flash flood watch for the Interstate 10 corridor from the Texas-Louisiana border east to the Florida Panhandle.
Severe freshwater flooding, rivaling that one might see from a hurricane, is continuing to affect southern Louisiana. 8.75 inches of rain have already fallen this month in New Orleans, 6.79 inches more than average and the second most on record month-to-date, according to Weather.com’s Jonathan Erdman.
Marine warnings issued once again
While the bulk of strong to severe thunderstorms had hovered over land before noontime Wednesday, the instigating front was sagging southward, allowing thunderstorms to blossom over the northern Gulf of Mexico near the Louisiana coastline. The Weather Service issued several special marine warnings, advising boaters to be cautious for waterspouts and gusts of at least 39 mph.
How Tuesday’s weather spawned a disaster
Special marine warnings were also issued Tuesday before a line of powerful thunderstorms surged into the gulf, but it appears a combination of atmospheric factors made storms even more potent than originally forecast.
First, a band of severe thunderstorms was sloshing southward toward the coast during the afternoon hours, having produced a few instances of quarter-sized hail and winds of 50 to 60 mph that resulted in minor structural damage west of New Orleans. The storms had largely become outflow-dominant by the time they reached the coast — meaning they were exhaling more air than they were ingesting. That’s often the precursor to a line of gusty winds, but also signals a storm’s demise.
Along most of the line, winds of 40 to 55 mph were common — significant, but typically not enough to capsize such a large vessel. South of Grand Isle, however, the storms were more potent. In fact, there was a “mesovortex,” or area of mid-level spin, in the atmosphere.
Sometimes when a line of storms develops, strong winds cause the middle to bow outward, the tail ends lagging behind and curling back on themselves. The left, northern end of the line takes on counterclockwise rotation, becoming what meteorologists call a “northern bookend vortex.” These vortices, which are usually 10 or more miles across and a mile or two above the ground, can intensify thunderstorms and linger even after storms have died.
In the case of Tuesday’s storms, the northern bookend vortex slipped south of Grand Isle, and helped anchor the most powerful thunderstorm cell within the line. Storm tops towered to 51,000 feet directly overhead the vortex. The storm, which was capable of producing egg-sized hail, was also rotating. That means that, in addition to 80 mph or greater straight-line winds on the open water, microbursts or tornadic waterspouts were possible.
It’s unclear what exactly produced the 117 mph gust reportedly measured by a ship south of Port Fourchon, La.
Wake lows are areas of low pressure that form as a result of air sinking and warming on the backside of a thunderstorm complex. They’re difficult to detect in real-time unless weather stations are close enough together. Strong winds often accompany the wake low due both to air aloft drying and accelerating downward and air being drawn into the low from a region of high pressure. This prolonged the period of strong winds, and even prompted the issuance of several additional severe thunderstorm warnings and even high wind warnings in coastal Mississippi and Alabama.
A final ingredient that contributed to the disaster was the presence of “gravity waves,” according to the Weather Service. As the wake low arrived, so too did an inversion — or an increase in air temperature with height — about a half-mile to a mile above the surface. That narrow layer of warm, dry air extended behind the storms, and acted as a blanket beneath which individual atmospheric wavelets induced by the storms rippled. Those gravity waves helped mix down momentum from aloft, and were able to more efficiently carry down strong winds over the water.
Weather station data from southern Lafourche Parish in Louisiana indicates a few air pressure fluctuations probably commensurate with both a wake low and one or more gravity waves.
The Coast Guard was continuing their rescue mission Wednesday, but optimism was waning as the potential increased for it to become a recovery mission.
Jason Samenow contributed to this report.