By Andrew Ryan, Globe Staff

Tropical Storm Hanna is expected churn up the Atlantic coast and lash Eastern Massachusetts this weekend with 40-mile-per-hour winds and up to 3 inches of rain.

The storm, which is gathering steam in the Bahamas, is forecast to crash into the Carolinas as a hurricane at 2 a.m. on Saturday. After cutting across the Outer Banks and back into the Atlantic, the storm is expected to pass over Eastern Long Island and take aim at Massachusetts late Saturday night or early Sunday as a tropical storm.

The current track has Hanna slashing through Bristol and Plymouth counties and cutting across Scituate as it heads into Cape Cod Bay, passing just north of Provincetown on its way out to sea.

"They are still playing with the track, but the trend has been coming right over on top of us," said Charlie Foley, a meteorologist with the National Weather Service in Taunton.

There is, however, some good news. As Hanna moves north, it is expected to morph from a tropical storm into an extratropical storm. That technical change means that the center of the swirl of clouds will cool down, causing the storm to lose some of its punch. The strongest winds will be aloft -- not on the ground -- and less rain is likely to fall than would with a warm tropical storm or hurricane.

Even as an extratropical storm, however, Hanna is still expected to cause some havoc when it hits Massachusetts. Winds are predicted to be 35 to 40 miles per hour on Cape Cod and 15 to 25 miles per hour in Boston. But that is not what has forecasters concerned.

"We're going to get a lot of heavy, soaking rain out it," Foley said. "That will be the primary focus … Too much rain in too short of a time."

That rain is expected to flood urban streets and cause some smaller rivers and streams to overflow their banks.

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