Subtropical Storm Sean Forms Off Atlantic

The 18th named storm of the 2011 hurricane season has formed off the Atlantic Coast, but is no threat to the U.S.

Sean, a subtropical storm, is about 445 miles southwest of Bermuda, with winds of 40 mph.

It is basically stationary now but is expected to start moving northwest in the next 12-24 hours, then eventually be pulled to the northeast and into the Atlantic.

A subtropical storm is a hybrid system. Like a hurricane, it is powered by warm sea surface waters. But it also gets energy from the interaction of cold air aloft and warm air near the surface.

Forecasters say there’s a chance Sean can transition into a tropical storm, with a more concentrated wind field and better convection, but shear is more likely to prevent that from happening.

The 2011 hurricane season has produced six hurricanes, three of them considered major, with sustained winds of at least 111 mph. An average year is 9-10 storms, 5-6 hurricanes and 2-3 major storms.

The hurricane season ends Nov. 30.

Tracking Sean

St. Petersburg Times