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A look at weather extremes and the big-picture climate connections.

Atlantic Hurricane Season Heats Up

2011 Atlantic hurricane seasonal numbers to date. Credit: Russ Freedman for Climate Central.

With the typical peak of the Atlantic hurricane season approaching on September 10, it's fitting that meteorologists are watching several areas for storm development, from the Bay of Campeche near Mexico to the far eastern reaches of the Atlantic. As of the time of this writing there were three named storms — Hurricane KatiaTropical Storm Maria, and newly-formed Tropical Storm Nate (which is not yet reflected in the graphic). Katia is expected to pass between the US and Bermuda, sparing both, and then race off to the northeast toward Scotland, where it may strike land as a powerful non-tropical storm early next week. Maria, however, may pose a threat to the East Coast or Bermuda, and Nate may affect the Gulf Coast.

As Climate Central's new "hurricane scorecard" shows, so far this hurricane season there have been numerous tropical storms that never made it to hurricane strength, with sustained winds greater than 74 mph. Although water temperatures in the tropical Atlantic are running above average, other factors such as dry air in the middle and upper atmosphere as well as wind shear have taken their toll on developing storms. Even Hurricane Irene, which caused billions of dollars in damage in the eastern US and killed nearly 50 people, weakened considerably as it approached landfall in North Carolina. Although forecasters are continuing to investigate why Irene failed to maintain its strength as forecast, one of the most likely culprits was an area of dry air that got wrapped into the storm's circulation. This disrupted the storm's eyewall structure — the strongest part of the storm — and weakened Irene's winds. Instead, it was Irene's heavy rains that did the most damage, inundating states such as New Jersey, New York, and Vermont.

Tropical Storm Irene makes its final landfall near New York City on August 28, 2011. Credit: NOAA.

The 2011 Atlantic hurricane season-to-date demonstrates that, contrary to many disaster scenarios and scary media headlines during the past few years, global warming may not lead to epic numbers of major hurricanes striking the United States. Some studies have shown that in recent decades the most powerful hurricanes — Category Four and Five storms — have become more common in the Atlantic Basin. There is an emerging consensus among climate scientists and hurricane specialists that, as the planet continues to warm, hurricanes may occur less frequently but be more severe (both in terms of wind speeds and rainfall) when they do develop. The decline in frequency would come from some of the same factors that have hindered weak named storms from becoming full-blown hurricanes this year, such as atmospheric wind shear.

The increase in strength and rainfall would be caused primarily by an increase in ocean heat content, since warm waters are the main fuel source for tropical storms and hurricanes, and warming seas and air temperatures allow more water to evaporate into the atmosphere, where it can be wrung out as rainfall. Hurricane Irene alone dropped 10 to 20 inches of rain over a broad area in the Mid-Atlantic and Northeast.

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