What has changed 20 years on from Hurricane Katrina?

Devastation caused by Hurricane Katrina in New Orleans, August 2005

Huge improvements in hurricane forecasting have been made since Katrina hit the city of New Orleans this day 20 years ago.

From starting out as a cluster of storm clouds over the Bahamas, Hurricane Katrina went on to become the deadliest (since 1928) and costliest natural disaster recorded in the United States.

As part of an Atlantic hurricane season that was the most active in documented history, Katrina took nearly 2,000 lives, destroyed or made uninhabitable an estimated 300,000 homes, and caused more than $100 billion in property damage.

Although the US National Hurricane Center (NHC) provided an accurate forecast of the storms track three days in advance, the resulting devastation led to a concerted effort to significantly improve hurricane modelling, prediction, and warning capabilities.

In addition to planes and drones, more data is collected from the sea using gliders, floats and drifters

Technological advances have been the biggest driver of improved forecast accuracy.

A combination of satellites, aircraft observations, numerical forecasting models, as well as historical data are used to predict what will happen.

Back in 2005 forecasters had access to satellite images every 30 minutes. Today, satellites typically capture imagery every 10 minutes, and during severe weather this can be increased to every 30 seconds.

Now more data from satellites and radar can be fed more quickly into more sophisticated models to produce a more detailed forecast.

In 2005 data from aircraft flown into storms was not being used as part of the modelling. Today unmanned drones are used as part of research to improve hurricane science.

Twenty years on scientists have access to more data from the sea. Collected by gliders, floats and drifters, these instruments provide valuable information about the most turbulent region of the hurricane environment, called the boundary layer, where the air meets the ocean.

Computer models now also take into account forecast errors for the previous five years.

All these, plus advances in computing power and a better understanding of the physics of a hurricane through more data and research, have led to the significant improvements. In 2005 the average tracking error in a 48-hour forecast was 110 nautical miles (200km). Since then, according to the US National Oceanic and Atmospheric Administration (NOAA), the error has reduced by about 50 per cent.

Forecast track of Hurricane Katrina issued two days before landfall

Forecasting a storms intensity however remains more challenging. Small changes in atmospheric conditions can have a large impact on whether a storm strengthens or weakens.

A 2024 study, external suggested the two main drivers to this uncertainty are changes in wind speed and direction through the atmosphere, known as vertical wind shear, and atmospheric moisture.

Vertical wind shear tends to rip storms apart, preventing intensification or even causing a storm to weaken. Moisture in the atmosphere provides the energy to build clouds to great heights and enables storms to "spin up".

The most unpredictable storms occur when there is a moderate amount of wind shear and moisture in the atmosphere.

Improvements in computing power and new technologies to gather data hope to make these "high uncertainty" storms easier to forecast.

Whilst a weather event can now be a forecasting success it is still possible for it to be a communications failure, as demonstrated by Hurricane Sandy in 2012, which hit the east coast of the US.

A review found the forecasts issued, including surge forecasts, were remarkably accurate, but were not communicated in ways that made it easy for officials and the public to understand.

Sandy killed 159 people, including 44 in New York City, many from coastal flooding, never having expected such a storm could reach so far north.

Dr Leanne Archer of the University of Bristol believes that forecasting is only one part of the disaster management cycle.

She says, "It is vital that efforts continue to improve early warning systems, risk mapping, evacuation plans, scientific research and governance structures to ensure that places are prepared to act when the next hurricane season arrives.

"There is a still a considerable challenge where the forecast science is clear and yet the warnings do not meet the right people at the right time."

An aerial view showed the damage caused by Hurricane Sandy to the New Jersey coast on 30 October 2012

Part of the communication issues may be down to how hurricanes are classified, using the Saffir-Simpson hurricane wind scale. The scale was introduced in the 1970s and revised in 2010 after Katrina. It categorises hurricanes based on sustained wind speeds, ranging from Category 1 to Category 5.

Strong winds will often cause the most damage to buildings and structures, and they can pick up debris which creates further impact.

However, powerful winds are not the only deadly force during a hurricane. According to NOAA, the greatest threat to life actually comes from the water – in the form of a storm surge.

Storm surge is water from the ocean that is pushed toward the shore by the force of the winds swirling around the hurricane. Storm surge combined with waves and high tides can lead to flooding over a large area and cause extensive damage.

In the case of Katrina, a storm surge over 8 metres was pushed onto the vulnerable Gulf Coast. A levee system constructed to divert storm waters away from New Orleans burst in multiple places, sending floodwaters pouring into residential areas, the cause of most of the lives lost. At the height of the disaster, about 80 percent of New Orleans was underwater, more than 4 metres deep in places.

For over a decade Wireless Emergency Alerts, which include hurricane warnings, have been sent directly by government agencies to mobile devices. But it was not until 2017 that the NHC began issuing a storm surge warning graphic.

Warmer waters and more atmospheric moisture due to climate change will impact hurricanes

A recent study by Climate Central has found that climate change intensified Hurricane Katrina, external.

Its analysis found that the warm waters over which Katrina strengthened were made up to 18 times more likely by climate change, increasing Katrinas maximum sustained wind speed by 5mph (8km/h).

Every 10th of a degree of ocean warming increases the risks of stronger storms and higher sea levels. And because of climate change, water temperatures in 2025 are higher than in 2005.

Dr Daniel Gilford, Meteorologist and Climate Scientist at Climate Central, said: "If Katrina had formed in todays climate, it likely would have been even more powerful."

As our atmosphere continues to warm and sea surface temperatures continue to rise, rainfall from hurricanes and the risk of coastal inundation due to storm surge could be greater.

Although there are not expected to be more hurricanes in total in the future, a greater number of them are forecast to be more powerful.

Perhaps the Saffir-Simpson Scale will need to be updated again to add a Category 6 storm?