• Stephen Strum

Hurricane Ida

Hurricane Ida made landfall along the southeastern coast of Louisiana at midday on August 29, 2021, as a strong Category 4 hurricane with 150mph winds. Here is an IR satellite image of Ida as it was moving inland. The dark reds indicate the highest and coldest cloud tops within the storm.

Ida was an incredibly well-forecasted storm as the plot below shows. Every NHC forecast for track and intensity (denoted by line color) is shown in the plot below. The initial forecast as the storm was just developing was a bit too far west, but all the forecasts after that initial one were very close to the eventual landfall location, and the forecasts 24 hours out were nearly perfect. In any event, all of southeastern Louisiana saw hurricane conditions with Ida. Hurricanes are not a point and hurricane-force winds extended out about 50 miles from the center.

So, why was Ida so strong, and why did it undergo rapid strengthening in the Gulf of Mexico? The answers in this case are pretty straightforward. Water temperatures were extremely warm along the track of Ida, and wind shear was very low. That is, winds were generally uniform with height, with no significant changes in wind direction or speed in the atmosphere. Wind shear can prevent storms from organizing, and limit their ability to strengthen by potentially disrupting the circulation or by restricting the flow of air out from the center of the storm, which in turn keeps the air pressure from decreasing as quickly in the eye of the storm, and thus keeps wind speeds lower. The plot below shows water temperatures across the Atlantica basin on August 29th.

While surface water temperatures are important, what really fuels hurricanes is the total heat content of the ocean under the storm. In many areas, surface waters can be very warm, over 85F, yet that warmth is often shallow. As a storm grows stronger, and wind speeds increase, so do wave heights. As wave heights increase, water is mixed to increasingly deeper depths by the churning motion of the wave activity. That can result in water temperatures decreasing under the storm, limiting the overall intensity. But, when extremely warm water exists to great depths, wave action does little to lower water temperatures. The plot below shows the heat content of the ocean. Note that the pool of greatest heat content in the entire Atlantic basin was located in the central Gulf of Mexico, right along the track of Ida. It was that deep pool of warm water that led to the rapid deepening of Ida. Had Ida tracked farther east over the cooler water of the eastern Gulf, the storm would likely not have reached the same intensity.

I've put together a 30-hour long radar loop of Ida as it made landfall, as well as a couple of satellite loops showing the evolution of the storm. Both can be found below.

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