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'TROPICS IN DEPTH' PART 4: Steering Currents

In this fourth part of Tropics In Depth, First Coast Storm Experts explain how a hurricane navigates the globe to help answer the question, "where is it going?"

JACKSONVILLE, Fla. — "Will it hit my house?"

It's a question many meteorologists and forecasters answer during hurricane season. Over the past few decades, answering that question has become easier, because forecasting improvements have enabled meteorologists to better understand where hurricanes will go once they develop. So how do hurricanes, or all tropical cyclones, travel the globe? In short, that answer is steering currents.

The atmosphere is made up of trillions and trillions of parts that are constantly in motion, and hurricanes are steered by those moving parts.

One of the first factors we look into are the winds. Not just the winds at the surface or within the hurricane, but rather at different heights of the atmosphere. A weaker system is typically steered by the wind at 10,000 feet. A stronger system will need wind up near 18,000 feet to direct it.

Pressure in the atmosphere is another factor that plays a role in steering and directing a hurricane. Think of high pressure as mountains in the atmosphere and low pressure as valleys. It's easier for a river to flow through a valley than it is a mountain. Or it's easier to build a road around a mountain rather than up it. When dealing with pressures, a hurricane's track is usually routed around high pressure. However, pressure is always changing in shape and intensity so that route is always changing.

One of the toughest parts of forecasting the track is during the development of a cyclone. A developing hurricane will thrive in areas of little shear, however, shear is oftentimes found in the main currents of air. So, a developing cyclone tends not to form in the main current of air, but rather the 'eddies' around it.

In other words, the stronger a hurricane is the more likely it is in a weak steering environment. When in this situation, the hurricane wants to go where it can thrive, where the warmer water is, where there's little wind shear. We'll also look at what part, or quadrant, of a system has the most convection (vertical movement). This can also help pull or direct a storm.

Despite all of these variables, variables that are constantly moving and changing, the track forecast continues to improve. We are probably within a year of saying that the 7-day forecast of a hurricane's track is better than the 3-day forecast was 30 years ago.

It's also good to remember that a track forecast does not include impacts. And that a storm's impacts will spread beyond its path.

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