The forecast path of Hurricane Katia, which is expected to run nearly parallel to the U.S. East Coast before heading out to sea, is a typical one for Atlantic storms to follow.
The reason: They are steered away from land by prevailing wind patterns and surrounding environmental flow.
On Sept. 6, Hurricane Katia was a category 3 hurricane on the Saffir-Simpson scale with sustained winds of 120 miles per hour (195 km per hour). The National Hurricane Center projects that the storm will probably graze Bermuda. [Which US Cities Are Most Vulnerable to Hurricanes? ]
Atlantic hurricanes can form anywhere from just off the west coast of Africa to somewhere near the Caribbean to inside the Gulf of Mexico. Those that form out in the Atlantic tend to move westward, toward the United States, and then eventually turn north and often recurve back toward the east.
"Hurricanes are always going to try to curve northward," said Dennis Feltgen, a spokesperson for the National Hurricane Center. "A lot of them curve out to sea."
However, it is often difficult to know for certain whether a hurricane will head back out to sea, or when, or how sharp the recurvature will be, according to the National Oceanic and Atmospheric Administration (NOAA).
The direction that a tropical hurricane moves in is determined by the storm's surrounding environmental flow and the conditions throughout the depth of the troposphere, or the lower atmosphere located up to about 8 miles (12 km) in altitude.
Seasonal weather patterns, strong winds, water currents and temperature also play a large role in directing a storm's path, according to NOAA. In fact, as storms shift north, they tend to weaken because they move over cooler water. (Hurricanes are fueled by warm water as it evaporates off the sea.)
In tropical latitudes, nearer the equator, prevailing wind patterns push storms toward the west, because of a high-pressure axis called the subtropical ridge, which extends east-west of the storms. Closer to the equator, general easterly winds prevail.
The wind pattern is driven partly by the Bermuda High, a high-pressure ridge that expands and contracts across the Atlantic during the summer months and is surrounded by a clockwise circulation. Many tropical storms will skirt around the Bermuda High, enhancing the pattern of moving west, then north. The exact path of a storm can depend greatly on how far north or south the Bermuda High is at that time. And since the high is a weak system (oftentimes due to a trough in the jet stream), tropical hurricanes can recurve back toward the east, according to NOAA. [The Bermuda Triangle and Other Mysterious Places on the Seas ]
Once storms move north of Cape Hatteras, North Carolina, they tend to drastically accelerate due to the area's weather systems. A storm with a forward speed of 10 to 15 mph can accelerate to 20, 30 or even 50 mph in extreme cases.
Such extremes are rare, but a hurricane in 1938 raced from Cape Hatteras to Long Island in a matter of hours, at an average speed of 50 mph. In the morning of Sept. 21, it was off the coast of Hatteras, and by Noon it was tickling the New Jersey coast. By 5 p.m. it was in Massachusetts. Though the storm weakened, its forward speed was so great that it arrived in the Northeast before losing all its punch, creating 30-foot storm surges that swamped Providence, RI. It was in Canada by nightfall, having killed 600 people along the way.
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