At a Glance

The large-scale weather pattern over the Atlantic Ocean and North America often determines a storm's path.A stronger, more expansive Bermuda high will often steer a storm or hurricane to the U.S.A weaker Bermuda high allows a storm to recurve out to sea, away from the U.S.

Each hurricane season, meteorologists pay close attention to weather patterns that could either steer a hurricane safely out to sea, or push it toward a strike on the U.S. mainland.

Hurricanes that strike the U.S. most often first become hurricanes in the Caribbean Sea, Gulf of Mexico or near the Southeast coast, as we discussed in a .

By simple geography,hurricanes in those areas have a better chance of at least indirectly impacting parts of the U.S.

But during the heart of hurricane season, attention also turns much farther east, to the so-called main development region between the Lesser Antilles and Africa.

About 85% of all Atlantic major hurricanes (Category 3 or stronger on the ) have origins traceable to the disturbances known as .

Storms forming soon after the easterly wave emerge off western Africa – known as Cabo Verde storms, since they develop soon after passing by the Cabo Verde Islands – have an expansive area of the Atlantic Ocean over which to strengthen, assuming other factors such as dry air and wind shear are absent.

Given that, which pattern puts the U.S. mainland at the highest risk of a hurricane landfall from a Cabo Verde storm? Which pattern helps deflect these away?

Lower U.S. Threat Scenario

First, you may ask what steers hurricanes and tropical storms in the first place.

Hurricanes are steered by wind flow through a deep layer of the atmosphere up to the level of the jet stream. These Cabo Verde storms like to track around the edge of the Bermuda-Azores high. Southward dips in the jet stream, or troughs, are also crucial to the system's later track.

In this first scenario, the Bermuda-Azores high is weaker, and confined closer to the Azores in the eastern Atlantic Ocean. Along or just off the U.S. East Coast is a sharp southward dip in the jet stream.

Upper-level steering pattern helping to deflect so-called Cabo Verde tropical cyclones away from the U.S.

A Cabo Verde tropical cycloneinitially moving around the southern side of the Bermuda-Azores high would then get caught between the high and jet stream dip, negotiating a so-called recurve to the northwest, north, then northeast away from the U.S. coast.

One classic, recent recurve was 2021's .

Despite reaching Category 4 intensity, a jet-stream trough along the East Coast and a weaker Bermuda high curled Sam to the north, then northeast far from any land concerns.

Track history of Hurricane Sam, color-coded by intensity from Sept. 22 through Oct. 5, 2021.

(Track data: NOAA/NHC)

Tropical cyclones starting off farther north in the eastern or central Atlantic have a better chance of recurving since they have more latitude to start with.

Occasionally, one of these recurve storms can impact Bermuda, and may also survive into the Canadian Maritimes (Nova Scotia or, more often, Newfoundland).

Hurricane Larry in mid-Sept. 2021 also recurved, but then , Canada, at Category 1 intensity, producing extensive tree damage and power outages.

There is one caveat in this scenario.

A tropical cyclone forming in the western Caribbean or southwestern Gulf of Mexico could get either picked up by the eastern U.S. trough and flung toward the U.S. or can miss the trough altogether and head for the far western Gulf of Mexico.

Greater U.S. Threat Scenario

For a Cabo Verde storm to threaten the U.S., we look for a much more expansive Bermuda-Azores high, serving as a northern wall, forcing the tropical cyclone to track to the west or west-northwest over a larger part of its journey.

In this scenario, the tropical cyclone stays farther south in the main development region, the region to the east of the Lesser Antilles. It may also not develop into a tropical cyclone as soon as in the first recurve scenario, remaining as an open tropical wave until it's at least halfway between Africa and the Lesser Antilles.

(MORE: The Importance Of Tropical Waves In Hurricane Season)

Upper-level steering pattern that would put the U.S. coast at a greater threat from a Cabo Verde tropical cyclone.

Lacking a dip in the jet stream aloft, or trough, anywhere near the eastern U.S., the alley for this Cabo Verde storm track implicates a large swath of the U.S. coast, not to mention the Caribbean Sea.

Sometimes the trough arrives a bit too late in the East, is not far enough east, or isn't sharp enough to recurve the storm.

In late August 2011, the Bermuda-Azores high was just strong enough, far enough west, to keep Hurricane Irene on a westward trajectory, pushing it into the northwest Bahamas on Aug. 25. That same day, a vigorous dip in the jet stream was bypassing Irene to the north and northeast. Instead of being pushed out to sea, .

So, the next time we're tracking a system in the central Atlantic, keep in mind, among other things, that it's the steering patternforecasters keep a close watch over when diagnosing a potential threat to the U.S.

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