June 21, 2021

Claudette Strengthened Into A Tropical Storm Over Land—Twice

Claudette regained its tropical storm status over North Carolina on Monday as the system picked up speed and touched ocean water for the first time as a named storm. This is the second time that Claudette reached the all-important (yet arbitrary) threshold of becoming a tropical storm, and both times it reached that milestone while the center of circulation was over land. That's not unheard of, but it is a little odd. Here's how it happened.

Claudette's Journey

This storm spent its first couple of days of existence as a "Potential Tropical Cyclone," a bureaucratic title that allows the National Hurricane Center to issue watches and warnings for a disturbance that's expected to develop into a tropical storm and hit land within the next 48 hours.

Before this special designation, forecasters had to wait until a system was officially a tropical cyclone to issue watches and warnings. There have been instances where this didn't happen until just a few hours before landfall, in which case the warnings were all but useless.

The disturbance existed as a broad low-pressure system as it traversed the northern Gulf of Mexico during the day on Friday. Upper-level wind shear kept its thunderstorms far removed from the center of the system. This asymmetry is why Alabama and Florida received the bulk of the flooding rains and tornadoes on Friday and Saturday.

Word came in Saturday's 5:00 AM advisory that the disturbance had met the criteria necessary to become Tropical Storm Claudette. Its center of circulation was just over southern Louisiana by that point, sitting over swampy land southwest of New Orleans. 

Claudette quickly weakened into a tropical depression as it traversed the southeastern United States, restrengthening into a tropical storm as its center passed near Fayetteville, North Carolina. The system picked up speed as finally entered open waters, and it's expected to lose its tropical characteristics as it heads toward Atlantic Canada.

Lows Don't Fit Your Silly Labels

This was a weird storm. But you know what? Some storms are just weird.

Lows, or cyclones, exist on a spectrum with extratropical cyclones on one end and tropical cyclones occupying the opposite end.
A large extratropical cyclone swirls over the northern Atlantic in March 2020. (Source: NOAA)
An extratropical cyclone is the "everyday" type of low-pressure system featuring fronts that can herald big swings in temperature and humidity as they pass through. These systems are driven by diverging winds in the upper levels of the atmosphere, which draw air up and away from the surface to create the center of low pressure.
Hurricane Joaquin in 2015 (Source: NOAA)
A tropical cyclone is a low-pressure system that's warm and humid throughout the storm (in other words, no fronts). Tropical cyclones are powered by thunderstorms around the center of circulation, which derive their energy from warm ocean waters. As the thunderstorms grow more powerful, their updrafts pull more air away from the surface, deepening the pressure at the center of the low, which intensifies the storm and continues the cycle.

Lows don't always fit into the "extratropical" or "tropical" boxes. There's a whole array of hybrid cyclones that can exist somewhere in the murky middle between the two types of cyclones.

Some of these hybrid systems are called subtropical cyclones, which receive names and full forecast packages from the National Hurricane Center because they're similar in structure and impact to tropical cyclones.

Others are...debatable. Hurricane Sandy is an infamous example of a system that straddled the line between tropical and extratropical as it approached landfall in New Jersey. Sandy officially lost its title as a hurricane at the moment of landfall, which turned out to have huge implications for insurance companies.

Claudette the Land-Dweller

Claudette is another fine case study in nebulous low-pressure systems that are tough to categorize.  The system's structure, surface wind circulation, and thunderstorm placement precluded the experts from calling it a tropical depression or tropical storm for most of its journey through the northern Gulf of Mexico. Forecasters observing the storm say it didn't meet that threshold until its center had just crossed the coastline. 

Technically, this tropical storm existed entirely over land for 48 hours, and only touched warm ocean waters for the first time on Monday afternoon when it scooted over North Carolina's Outer Banks into the western Atlantic Ocean.

It's possible for tropical cyclones to form and strengthen while their center is over land. Tropical cyclones are more than just the pinpoint at the very center of the system. Claudette spanned more than a hundred miles across over the northern Gulf Coast, consisting of thunderstorms that were both near and far from the center of the storm.

The disturbance became a tropical storm when its center was just over the Louisiana coastline. This area is swamp. It's barely land to begin with, really, and it's not a stretch for a storm to take advantage of swamps to maintain strength or intensify.

We've seen storms form or strengthen over the Florida Everglades, for instance, and one storm (Tropical Storm Julia in 2016) was declared a tropical storm while its center was over land in northeastern Florida. 

There are also plenty of documented cases of tropical cyclones maintaining composition and strength after they make landfall. The "brown ocean effect" describes the phenomenon when warm and moist soils can act like warm ocean waters that help sustain a tropical cyclone while it's over land.

What made Claudette a tropical storm over Louisiana? Well, the National Hurricane Center's forecast discussion early Saturday morning was...vague:
The system that we have been tracking for a few days finally has enough of a well-defined center and organized convection to be considered a tropical storm. While the organization is not classical by any means, and there are some hybrid characteristics, the cyclone most resembles a sheared tropical storm, so the system is now Tropical Storm Claudette.
I'm willing to bet they were short on technical details on purpose due to the subjectivity of a call like that. It would've been great to have a seat on the operations floor to listen to the discussion surrounding that call.

Forecasters issued a beefier discussion for Claudette's second upgrade to tropical storm status while its center was over eastern North Carolina on Monday morning:
The elongated low-level center of Claudette is located over eastern North Carolina this morning, while a curved band of convection continues pushing eastward across the adjacent coastal and offshore waters. Surface pressures have fallen slightly overnight near the estimated center position, and Frying Pan Shoals buoy 41013 off the coast of southeastern North Carolina reported a brief period of sustained tropical-storm-force winds shortly after 06 UTC. Therefore, Claudette's initial intensity is raised to 35 kt with this advisory, making it a tropical storm once again.
This probably isn't going to be the last oddball storm we see this year. There are always systems that walk a fine line between a sad sack of clouds and a formidable cyclone.

It's all about the impacts, anyway. Claudette was physically unimpressive but its heavy rains wreaked havoc on parts of the northern Gulf Coast. Names and classifications are largely arbitrary, especially when there's a foot of rain or a storm surge on the way.

[Top Image: NOAA]

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I graduated from the University of South Alabama in 2014 with a degree in political science and a minor in meteorology. I contribute to The Weather Network as a digital writer, and I've written for Forbes, the Washington Post's Capital Weather Gang, Popular Science, Mental Floss, and Gawker's The Vane. My latest book, The Skies Above, is now available. My first book, The Extreme Weather Survival Manual, arrived in October 2015.