Hurricane Francine’s unusual structure led to heavy rain, high winds in New Orleans area

Throughout its short life, Hurricane Francine was a storm that seemed barely able to hold itself together.

It developed in the western Gulf of Mexico, taking its time to congeal into a tropical storm but then spinning up into a Category 2 hurricane in just over 50 hours. As it hurtled toward the Louisiana coast, dry air and high-altitude winds prevented it from ever fully developing the strong center characteristic of monster storms.

Then, almost immediately after it came ashore southwest of Houma, its eyewall — the swirling innermost bands of a hurricane that typically pack the hardest punch — began to fall apart.

In that deteriorated condition, looking less like a hurricane and more like a line of brutal thunderstorms surging in from the Gulf, Francine arrived in the most populated parts of New Orleans and Jefferson Parish. And while that put a damper on its wind speeds, it resulted in intense rainfall, concentrated along its northern arc, that deluged communities across the metro area.

“As it made landfall, you saw the southern part of the eyewall degrade, and it ended up kind of shifting a lot of the thunderstorm activity to the north and northwest,” said Dan Brown, a senior hurricane specialist overseeing forecasters at the National Hurricane Center. 

Forecasters with the National Hurricane Center and the Slidell office of the National Weather Service said that unusual structure meant the storm had a broad eye and area of impact. And for all the strengths displayed by forecasters in their modeling and forecasting of its track, intensity and likely effects, it made Francine’s actions immediately after landfall an uncertain affair for the residents in its path.

“It was a very large eye, almost 50 miles in diameter when it was offshore. And because of that, this storm was trying to establish its true center throughout that period,” NWS New Orleans lead forecaster Phil Grigsby said.

Disrupting the hurricane was a combination of southwesterly wind shear, resulting from a frontal system in Texas, and the influence of dry air around the storm’s southwestern edge, Brown said.

A key result of that unusual storm formation was that its intense thunderstorms that delivered the most rainfall set up over a broad area. The heaviest rain started falling even before the center of the storm came ashore, and continued as that intense semi-circle of thunderstorms, what remained of the northern eyewall, moved north.

As it moved inland, forecasters contended with a difficult task of determining where its center was located. Measurements within its 50-mile-wide eye found competing points of lowest air pressure three different times. The point of lowest air pressure is used to mark the storm’s movement. So the competing readings resulted in a track that looked like a bunch of zig-zags across southeastern Louisiana even though the eye itself didn’t really change in size or direction.

“We could see it on radar happening, and that’s why it has that weird zig-zag, just because it was such a broad system as it turned less tropical and moved north of the lake,” Grigsby said.

Eventually, the pretense of a center began to disappear entirely.

“Once it moved on land, it became more open and was just kind of a broad line of thunderstorms,” he said. “There wasn’t a true center of circulation.”

At the same time, the storm began to slow down, increasing the amount of time the downpours lingered over the New Orleans area, State Climatologist Jay Grymes said.

Though the intensity of Francine’s rainfall may have caught some by surprise, the NHC’s forecast was largely in line with the storm’s eventual path and impacts.

The storm never left the cone of uncertainty released by forecasters a day before landfall, and when Francine’s center passed by New Orleans it was only about 45 miles from the center of that track.

Forecasts that said 4 inches to 8 inches of rain would fall across the New Orleans area, and that some parts of the region could see even more, also proved prescient.

“The rainfall totals ranged between 6 and 8 inches, and some places got closer to 9,” Grigsby said. “About 5 to 7 of those inches fell within an hour and a half, so the rainfall rates were easily 2 to 3 inches per hour during that time-frame and maybe even higher in some spots.”