Stalling hurricanes worry insurers

In August 2017 Hurricane Harvey came ashore at San Jose Island off the coast of southern Texas. Weather forecasters predicted that it would pass inland quite quickly, before curving off north and then going east, dissipating as it went.

However, much to everyone’s surprise, the storm didn’t follow the predicted script. Instead it stalled in place, unloading a huge amount of water in the form of rain over a small geographical footprint, leading to extensive flooding—and extensive losses.

At the time it was seen as a bit of a freak event caused by abnormal conditions. But in 2018 Hurricane Florence did something similar, stalling in place. Although the flooding was not quite as bad, it was another sign that stalling hurricanes could be something to worry about.

As TigerRisk’s new white paper, “Stalling Hurricanes Wreak Havoc”, points out, this kind of hurricane behaviour is not new or unusual.

According to Anna Neely, research and development analyst at TigerRisk, what causes a hurricane to ‘stall’ after making landfall is still something of mystery.

“Our understanding of hurricane motion is constantly developing in the name of forecasting,” Neely says. “In general, a hurricane stalls because of local atmospheric conditions, but research on this topic is ongoing.”

High pressure

The TigerRisk white paper reveals that the path and forward speed of a tropical cyclone depend on a large number of factors such as trade winds, high and low pressure systems, jet streams and vertical wind shear. In the Atlantic basin for example, trade winds steer hurricanes from an area off the coast of Africa where they tend to develop, westwards towards the Caribbean Sea and North American coasts.

Their path can be further influenced by the location of the high-pressure system known as the Bermuda High, the Azores High, or the subtropical ridge. According to TigerRisk, hurricanes typically ride the strongest winds of the Bermuda High along the periphery. However, if the high is positioned to the east, then such storm systems are more likely to move north-eastward out into the open Atlantic. Tropical cyclones are more likely to track towards Florida, Cuba and the Gulf of Mexico when the high is positioned to the west and extends south, effectively blocking any north-eastern movement.

TigerRisk points out that a persistent high-pressure system (ridge) over the eastern US and the western Atlantic during the 2004 hurricane season, helped steer storms along a more westerly track; in the 2005 season, the ridge was further south and west than in 2004 which helped steer the storms in the Gulf of Mexico. The primary driver of the ridging along the East Coast in 2005 is believed to be from warming sea surface temperatures in the Pacific causing heat to spread downstream across North America and into the Atlantic Ocean.

Stalling storms

TigerRisk’s white paper adds that its writers had found that some of the stalling cyclones were impeded by neighbouring high-pressure systems. Upon Harvey’s Texas landfall, the cyclone became embedded in light steering currents that were sandwiched between two high-pressure systems located to the east and west, preventing Harvey from advancing further inland or back out to sea.

Florence’s westward movement in 2018 was blocked by a high-pressure system over eastern North America. Tropical storm Claudette in 1979 was forecast to keep moving as it approached Texas, however it was blocked to the north by a ridge causing it to slowly turn westward describing a tight loop where 42 inches of rainfall fell over a 24-hour period in Alvin, Texas.

These stalls can also happen to smaller storms, as when tropical storm Allison stalled over Texas in 2001, inundating many of the same areas that Harvey flooded, such as the Port of Houston which received about 37 inches. Allison’s track was like Harvey’s in that it moved inland via a northward track due to a subtropical ridge off the south east. That ridge later weakened and moved further south east while another ridge developed over New Mexico causing Allison to turn southward back out to sea.

Moreover, Allison experienced two distinct stalls separated by six hours when the cyclone’s forward speed reached 6.35 mph; together these two stall events lasted about 72 hours, with average maximum sustained winds around 23 mph.

Stall rates

“Based on historical rates, we expect one landfalling storm to stall every five years or so on average. Observing storms that stall in back-to-back years is certainly above the long-term average, but no significant trend exists in the historical data,” Neely says.

She also points out that in a long duration wind and rain event, the distribution of modes of damage differ from that of a typical storm. This results in a greater contribution of damage from wind-driven rain.

“The insurance industry should learn from these recent events to understand how loss was affected from this storm,” Neely says.

Modelling vendors will also ingest the outcomes of these events, and she expects that they will be incorporated into the models over time.

The TigerRisk white paper concludes that this is an area that needs to be studied in more detail.

Neely explains that further study is needed because the modes of damage are different and are not currently captured in modelled damage from storms.

“If we understood the reason for a stall we could better differentiate a hurricane, based on damages being primarily driven by water versus wind,” she says.

“Over the past several years, with the events that have occurred, we are reminded that each storm is different. Improving our understanding of storm behaviours and patterns helps us to be more proactive in our ability to prepare and protect insurers.”