Throughout the hurricane season, U.S. Gulf and East Coast interests obviously pay close attention to potential disturbances forming nearby in the western Caribbean, near the western Bahamas or in the Gulf of Mexico. By simple geography, tropical storms or hurricanes in those areas have a better chance of at least indirectly impacting parts of the U.S. coast.
During heart of the Atlantic hurricane season, however, attention also turns much farther east, to the so-called main development region between the Lesser Antilles and Africa.
Roughly 85 percent of all Atlantic major hurricanes (Category 3 or stronger on the Saffir-Simpson Hurricane Wind Scale) have origins traceable to the easterly waves originating from Africa.
Storms forming soon after the easterly wave emerges off Africa, or Cape Verde storms, 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 steering pattern in the upper atmosphere puts the U.S. mainland at a greater risk of a hurricane landfall from a Cape 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.
Mature hurricanes are steered by wind flow through a deep-layer of the troposphere, the layer of the atmosphere where virtually all weather occurs. These Cape 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 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.
A Cape Verde tropical cyclone initially moving around the periphery 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.
Tropical cyclones starting off farther north in the eastern or central Atlantic also 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).
This all said, in this scenario, a tropical cyclone forming in the western Caribbean or southwest 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
Conversely, for a Cape 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 portion of its journey.
In this scenario, the tropical cyclone stays farther south in the aforementioned 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.
Lacking a dip in the jet stream aloft, or trough, anywhere near the eastern U.S., the alley for this Cape 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 east enough, 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 enough of a westward trajectory, pushing it into the northwest Bahamas on Aug. 25, 2011. 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, Irene tracked up the Eastern Seaboard.
In late October 2012, a dip in the jet stream anchored too far west (over the Ohio Valley) and blocking high pressure aloft over the Canadian Maritimes and North Atlantic conspired to steer Superstorm Sandy toward the East Coast, with devastating results.
So, next time we’re tracking a system in the central Atlantic, keep in mind, among other things, that it’s the steering pattern forecasters keep a close watch over when diagnosing a potential threat to the U.S.