MIAMI (BNO NEWS) — Subtropical Storm Sean formed midway between the British overseas territory of Bermuda and the Bahamas on early Tuesday morning, forecasters said, prompting a tropical storm watch for Bermuda.

Forecasters at the U.S. National Hurricane Center (NHC) have been following the weather system since Sunday morning when a large non-tropical low pressure system emerged over the central Atlantic. It quickly became better organized on Monday.

“Satellite information and surface observations indicate the large non-tropical low pressure system that has been meandering between Bermuda and the Bahamas the past few days has acquired enough organized convection to be classified as Subtropical Storm Sean,” said NHC senior hurricane specialist Stacy Stewart.

As of 4 a.m. EST (0900 GMT) on Tuesday, the center of Sean was located about 445 miles (720 kilometers) southwest of Bermuda. Its maximum sustained winds are near 45 miles (75 kilometers) per hour, with higher gusts, forecasters said.

Stewart said the subtropical storm remained nearly stationary on Tuesday morning. “Sean has been making a small cyclonic loop during the past several hours and little overall motion is expected today as the cyclone remains trapped in weak steering flow,” he said. “By [Tuesday evening-Wednesday morning], however, Sean is expected to move slowly northwestward as the subtropical ridge to the north begins to erode and retreat to the east.”

On Thursday, a strong frontal system and associated deep-layer trough which is located across the eastern United States is expected to accelerate Sean toward the northeast, bringing it closer to Bermuda. As a result, the Bermuda Weather Service has issued a tropical storm watch.

“Sean is expected to remain over 26 degrees Celsius (78.8 degrees Fahrenheit) sea surface temperatures (SSTs) for at least the next 48 hours, which means the cyclone could possibly transition into a tropical storm as the vertical shear weakens,” Stewart said. “Overall, only a slight increase in strength is expected, assuming that an eye feature does not develop.”

Sean is the eighteenth named storm of the 2011 Atlantic hurricane season, following category two Hurricane Rina which formed over the northwestern Caribbean Sea in late October before affecting the Yucat√°n Peninsula in Mexico. It caused damage but no casualties.

According to figures released in August, NOAA’s Climate Prediction Center is expecting an above-normal hurricane season in the Atlantic this year. The outlook called for 14 to 19 named storms, with seven to ten becoming hurricanes and three to five expected to become a major hurricane (category 3 or higher).

An average Atlantic hurricane season produces eleven named storms, with six becoming hurricanes and two becoming major hurricanes. The Atlantic hurricane season runs from June 1 through November 30, with peak activity in September.