Snow and hurricanes

As predicted, the 2010 Atlantic hurricane season has been a busy one. As no one predicted, for the United States it has been stunningly uneventful.

A total of 12 hurricanes have formed, double the long-term average. As we've stated, this is the first time in the period of record that 10 or more hurricanes developed without a U.S. landfall.

How could this happen? The paradox of a hyperactive season and the lack of a single U.S. landfall may have a connection with the record snows in the Philadelphia-Washington corridor last year. 

Last winter, the storms were particularly juicy in part because of the warming in the tropical Pacific, but the requisite cold air came courtesy of higher-than-normal pressure in the far North Atlantic.

Pressure differences fluctuate between the high and mid latitudes over the Atlantic, a pattern known as the North Atlantic Oscillation. When the pressures are higher to the north, the NAO index is said to be negative, and last winter is was historically negative.

That pattern slackened the trade winds, and that allowed the hurricane-brewing waters of the Atlantic to warm, according to Mike Halpert, a chief forecaster at the government's Climate Prediction Center. That helped supply fuel for the hurricanes.

The index remained stubbornly negative, observed Todd Crawford, forecaster for WSI Corp., in Massachusetts, and the far-north higher pressures may have taken some strength from the so-called Bermuda high. That would have affected hurricane paths.

The Bermuda high gets its name because it is typically centered near Bermuda. Winds circulate clockwise around high centers, so areas to the west of the center experience south winds. The high sometimes gets so strong that its western flank approaches the U.S. East Coast.

At a key period in the hurricane season, the high was relatively weak. Thus hurricanes caught in its circulation curved away from the U.S. mainland.

Crawford said that if it had been stronger, more storms could have approached the United States, with possibly "disastrous" results.

As it was, the season that began June 1 and ends officially Nov. 30 will become mark the fifth consecutive year without a U.S. landfalling hurricane. The only other such periods occurred from 1901-1905 and from 1910-14.

No complaints have been heard from Florida.