Hurricane Steering

MYTH #1: "They always turn away at the last minute."

This myth is based on an idea that what will happen during a particular hurricane event will be similar to what happened during prior events. However, as in the financial markets, past performance offers no guarantee of future outcomes.

Satellite image of Hurricane Katrina superimposed on a map showing the Katrina's eye almost right on top of New Orleans

This particular myth was very popular in New Orleans before Hurricane Katrina and might have contributed to giving area residents a false sense of security. It may have been reinforced by the track followed by Hurricane Ivan just a year before, as that system threatened but ultimately turned away from the New Orleans area, striking Pensacola, Florida, instead.

What the last hurricane has done with regard to a particular city has no bearing whatsoever on what the next one will do, any more than one flip of a coin influences the next. Hurricanes that turn away are near-misses, not false alarms, and a series of near-misses at a location most certainly does not constitute a pattern of false alarms. Whether a storm will turn in one direction or another at a particular point in its trajectory is a matter of probabilities with regard to future states of the atmosphere, and in particular, the atmospheric currents that steer hurricanes.

Katrina embedded in environmental steering currents

Hurricanes normally move together with the winds in which they are embedded, as shown in the adjacent graphic showing the winds responsible for steering Katrina to its ultimate destination.

Less intense hurricanes are steered mostly by winds in the lower to middle layers of the atmosphere; higher-level wind currents also factor into the steering of the more intense storms. This is one way in which track and intensity may be understood as mutually causative: a storm could become more intense for whatever reason, and its track could then change from what it might have been, as a result of having come under the influence of the higher-level steering currents.

The motion of hurricanes depends on the atmospheric steering currents present at that time and on the internal dynamics of the individual hurricane. The dominant steering influence for Atlantic hurricanes during most of their journey through the tropical latitudes is the trade winds, which flow from east to west and set developing hurricanes on their westward course across the Atlantic, toward North and Central American coastlines.

Large atmospheric features, such as high and low pressure systems, produce winds capable of changing or maintaining the course of hurricanes that enter their area and come under their influence. The Bermuda High Pressure System or Subtropical Ridge, a semi-permanent high pressure system that occupies various positions in the Atlantic Ocean, contributes to the motion of most Atlantic hurricanes, whether they be in the Atlantic, the Caribbean, or the Gulf of Mexico.

Hurricanes can no more cross through high pressure than a bouncing ball can pass through a brick wall; they have to go around it. In the northern hemisphere, the winds around high pressure flow in a clockwise direction. Those clockwise winds around the Bermuda High are capable of capturing a passing hurricane that might have been heading in a westward direction and "turning" it back toward the north or northeast.

Many Atlantic hurricanes undergo such recurvature around the Bermuda High, especially the Cape Verde hurricanes, so named because they originate just off the West Coast of Africa, near the Cape Verde islands. Cape Verde hurricanes are peak season storms - most typical in August and September - and have earned a reputation for producing historic levels of destructiveness. Katrina was one such hurricane.

The appearance of a second high pressure system to the west of the Bermuda High is capable of blocking this process of recurvature, bouncing the northward-moving storm back on its original westward or northwestward course for a while longer, or preventing recurvature from ever even getting started. This is what happened to Hurricane Dean of 2007. With high pressure constantly to its north, Dean had nowhere to go but west, missing the southern United States altogether.

Frontal system passing across the United States

Conversely, passing low pressure systems in the midlatitudes, with their associated cold fronts, rather than performing a blocking function, have the ability to absorb hurricanes into their own circulations. These frontal systems move across the North American continent from west to east, themselves steered by the prevailing westerlies, or midlatitude winds that flow from west to east. As these frontal systems pass through, they and the prevailing westerlies in which they are embedded may dip down into the subtropical latitudes, where they could encounter a passing hurricane.

Should a hurricane come under the influence of those winds, it is likely to be pulled off its westward or northwestward track and into the passing low, then carried off along with it in a northeasterly direction, again leading the storm into recurvature. This is what happened with Hurricane Wilma in 2005: it encountered a passing frontal system in the Gulf of Mexico, which pulled it off course into a northeasterly direction, bringing it into direct contact with the west coast of Florida. Still under the influence of the passing low, Wilma crossed the state on its way back out to the Atlantic Ocean.

The "turning" of hurricanes, then, is highly dependent on changes in surrounding atmospheric features. High and low pressure systems, such as the Bermuda High positioned correctly or a passing frontal system timed just right, have the ability to change the course of a hurricane and steer it into or away from a particular coastline.

Like the hurricane itself, these features are in constant motion. They are similarly subject to changes in size, and what the spatial extent of a nearby high or low pressure system is at any given time will affect whether a hurricane positioned at a given distance from its center will come under the influence of its winds and change its course accordingly.

Because the motion of the hurricane depends in part on such features, predictions about their sizes and motions also have to be made; those predictions are themselves part of what goes into predicting the track of the hurricane. A Bermuda high positioned in the Central Atlantic, as opposed to its being positioned just off the eastern seaboard, for instance, could easily be (and often has been) the difference between a hurricane striking Florida and a hurricane striking Texas, and this is only one example.

Uncertainties in the future positions and sizes of nearby high and low pressure systems thus contribute to the uncertainty in the tracks (as well as in the intensities) of hurricanes. As the surrounding atmosphere, and the configuration of specific steering currents, are dynamic and ever-changing, predicting the track of a hurricane has to be much like chasing a moving target embedded within another moving target.

The state or development of high and low pressure systems capable of affecting hurricanes, like the state or development of the hurricanes themselves, are highly sensitive to initial conditions, such that small errors in their prediction tend to become magnified over time. Regardless of technological advancements, it may never be possible to predict the steering currents that drive the tracks of tropical cyclones much beyond current forecast time scales.

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