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Subject: G8) Why do hurricanes hit the East coast of the U.S., but never the West coast?
Contributed by Chris Landsea
Hurricanes form both in the Atlantic basin (i.e. the Atlantic Ocean, Gulf of Mexico and Caribbean Sea) to the east of the continental U.S. and in the Northeast Pacific basin to the west of the U.S. However, the ones in the Northeast Pacific almost never hit the U.S., while the ones in the Atlantic basin strike the U.S. mainland just less than twice a year on average. There are two main reasons. The first is that hurricanes tend to move toward the west-northwest after they form in the tropical and subtropical latitudes. In the Atlantic, such a motion often brings the hurricane into the vicinity of the U.S. east coast. In the Northeast Pacific, a west-northwest track takes those hurricanes farther off-shore, well away from the U.S. west coast. In addition to the general track, a second factor is the difference in water temperatures along the U.S. east and west coasts. Along the U.S. east coast, the Gulf Stream provides a source of warm (> 80°F or 26.5°C) waters to help maintain the hurricane. However, along the U.S. west coast, the ocean temperatures rarely get above the lower 70s, even in the midst of summer. Such relatively cool temperatures are not energetic enough to sustain a hurricane's strength. So for the occasional Northeast Pacific hurricane that does track back toward the U.S. west coast, the cooler waters can quickly reduce the strength of the storm.
Recently Chenoweth and Landsea (2004), re-discovered that a hurricane struck San Diego, California on October 2, 1858. Unprecedented damage was done in the city and was described as the severest gale ever felt to that date nor has it been matched or exceeded in severity since. The hurricane force winds at San Diego are t he first and only documented instance of winds of this strength from a tropical cyclone in the recorded history of the state. While climate records are incomplete, 1858 may have been an El Niño year, which would have allowed the hurricane to maintain intensity as it moved north along warmer than usual waters. Today if a Category 1 hurricane made a direct landfall in either San Diego or Los Angeles, damage from such a storm would likely be on the order of a few to several hundred million dollars. The re-discovery of this storm is relevant to climate change issues and the insurance/emergency management communities risk assessment of rare and extreme events in the region.
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