(Adds details, previous Washington)

MIAMI, July 18 (Reuters) - The third tropical depression of the 2008 Atlantic hurricane season formed just off the U.S. East Coast on Friday and appeared likely to move parallel to the shoreline in a northeasterly direction, the U.S. National Hurricane Center said.

The system was located off the coast of South Carolina about 65 miles (105 km) south-southeast of Charleston. It had top sustained winds near 30 mph (45 kph).

The Miami-based hurricane center said some strengthening in the depression was forecast and it was expected to become a tropical storm on Saturday. A tropical storm warning was issued for the coast from the South Santee River in South Carolina to Currituck Beach Light in North Carolina.

If the depression does strengthen into a tropical storm, with winds of at last 39 mph (64 kph), it would be called Cristobal.

The hurricane center said the environment "did not appear conducive to rapid development" and it was impossible to pinpoint a likely point of landfall.

Computer models indicated the weather system would continue on a northeasterly course that would graze the coast and eventually take it into cool northerly waters.

Tropical storm-force winds would not pose much of a threat to the South Carolina or North Carolina coast and there was no indication the depression would reach hurricane strength with winds of at least 74 mph (119 kph).

Storm experts have forecast a busy six-month hurricane season this year, after two quiet seasons.

The record-busting 2005 hurricane season had 28 storms, including Hurricane Katrina, which swamped New Orleans and killed about 1,500 people on the U.S. Gulf coast. (Additional reporting by Chris Wilson; Reporting by Michael Christie; Editing by Peter Cooney)

AlertNet news is provided by