Atmospheric Factors that Influence the Potential Path of a Hurricane

As hurricane season begins to ramp up across the Atlantic basin, meteorologists are increasingly looking for clues that signal an elevated chance of tropical activity. There are several signs that indicate that tropical weather is likely, including clues that determine if a developing feature will impact the U.S. or if it will move out into the open waters of the Atlantic. Here is a look at what forecasters will be monitoring in the weeks ahead.

Interaction of High Pressure and Wind Flow Steers Tropical Weather Direction

Tropical weather events are steered in any given direction by wind flow embedded throughout the deepest layers of the atmosphere. These steering winds behave much like a steering wheel on a vehicle, hence the name "steering." How do these steering winds influence the direction of a feature and if the U.S. needs to be put on alert?

Tropical storms and hurricanes that make a direct strike on the U.S. mainland typically come together and intensify while roaming the Caribbean Sea or the Gulf. There are also times earlier in the season when homegrown storms that form near the coastline of the Southeast are more common.

The bulk of the activity that impacts the U.S. gets its start as a tropical wave coming off of the coast of Africa. These areas of low pressure then pick up steam and strengthen as they churn over the warmer ocean waters that circulate in the Caribbean Sea.

Once the disturbances reach the central Atlantic, they are likely to interact with what is known as the Bermuda-Azores high. This zone of high pressure works as a barrier, keeping the storms from tracking to the north. As a result, developing features are more likely to move to the west for a longer period of time. This westward journey puts the U.S. in the path.

A southward dip in the jet stream is a development that forecasters monitor when predicting if a storm will head toward the U.S. coastline or drift to the north into the open seas. Without this southward dip, the feature has a harder time turning to the north in time to avoid contact with the U.S.

Looking back at a few recent storms, Hurricane Isais encountered higher wind shear and dry air when it interacted with Hispaniola in the Caribbean, limiting its impacts. However, the Bermuda-Azores high was powerful enough to keep Isias from drifting to the north. Additionally, the jet stream's dip to the south was far enough to the west to send Isais away from the U.S. The end result was a storm that pushed along the Atlantic Seaboard, dumping heavy rain and whipping up winds and storm surge from the Carolinas and up through New York.

How the Recurve Impacts the Journey of Tropical Weather Events

The majority of hurricanes that form shortly after exiting Africa do not survive the track across the Atlantic. Known as Cabo Verde hurricanes, these features are more likely to become destructive if they do happen to make it long enough to strike the U.S.

The exception is hurricanes that develop in areas farther to the north. Because of the higher starting latitude, they are more likely to track away from the U.S.

A more pronounced southward jet stream dip along the eastern coast of the U.S. also forces a tropical feature to curve to the north before moving to the northeast. This is what meteorologists call "recurving." The recurving movement spares the U.S. as the storm heads back to the northeast and into the open ocean waters.

There are times when these Cabo Verde hurricanes are able to reach Bermuda. This generally happens when the Bermuda-Azores high is more focused on the Azores as opposed to farther to the south over Bermuda. A storm that takes longer to turn to the east can also move across the Canadian Maritimes.

This was recently demonstrated by Hurricane Sam in 2021. Sam reached the status of a Category 4 storm when it roamed the central Atlantic. The jet stream pattern along the East Coast paired with a weak Bermuda-Azores high to send Sam well to the northeast and away from land. This was fortuitous, as Sam could have unleashed significant damage along the East Coast had it not made that curve to the northeast.

The noted examples detail how there are several factors that influence where a storm heads after it gets its start. We will continue to monitor these factors in the coming weeks as hurricane season reaches its peak time of activity in the Atlantic.

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