(Updates with additional background, comments from BP executive) By Susan Daker and Angel Gonzalez Of DOW JONES NEWSWIRES HOUSTON (Dow Jones)--The U.S. government could decide as early as Thursday evening whether to evacuate vessels working on oil-spill recovery as the third tropical depression of the season approaches the Gulf of Mexico, the federal response commander said. If officials and BP PLC (BP) decide to send away the flotilla of ships, mobile drilling units and other vessels, the spill response could be delayed by 10-14 days, Ret. Coast Guard Adm. Thad Allen said, adding a cap on the mile-deep broken well will be left in place to prevent oil from again gushing into the Gulf. "We are prepared to leave the well unattended for this particular event," Allen said during a teleconference from Mobile, Ala. The cap was put on top of the well last week and has been keeping up to 60,000 barrels of oil a day from flowing into the gulf. A decision about the evacuation could be made about 8 p.m. local time, Allen said. The storm, currently dubbed Tropical Depression 3, would have to rise to a level of a tropical storm, meaning its winds would have to increase to 39 miles per hour, in order to provoke an evacuation, Allen said. The National Hurricane Center shows the storm tracking toward the oil spill site off the coast of Louisiana. Gale force winds could reach the site by Saturday morning, Allen said. The vessel drilling the relief well that is closest to completion would be the first to leave, Allen said. BP is drilling two relief wells and one has been on track to finish by mid-August. Its completion is heralded as the ultimate way to plug the well. BP Chief Operating Officer Doug Suttles said Thursday that the evacuations resulting from the storm could delay drilling of the relief well by as much as two weeks. On Wednesday, the company put a storm plug on the relief well 300 feet below the sea bed, he added. In addition to the evacuation of vessels near the spill site, equipment closer to shore will be moved. U.S. Coast Guard Rear Adm. Paul Zukunft, the on-scene coordinator for the oil spill response, told local officials in a letter released Thursday that there are concerns about the effect of a storm surge on the thousands of feet of oil-containment boom placed along the Gulf's shores. During two recent storm surges, tens of thousands of feet of boom were washed onto the shoreline and marshes of the area, causing damage to the environment, the letter said. The Coast Guard has set up staging areas in four states that rest on high ground to help deal with the problem as the storm approaches, the letter said. Richard Knabb, the tropical weather expert at the Weather Channel, said Thursday that the tropical depression is unlikely to form into a hurricane but would still push sea water, which could be mixed with oil and dispersant, onto land in Louisiana, Mississippi and Alabama. "Some places would get onshore flow, no matter what strength [of storm]," Knabb said. However, the government announced Thursday that it had reopened one-third of the Gulf area that had been closed to fishing as a result of the spill. -By Susan Daker and Angel Gonzalez, Dow Jones Newswires; 713 547-9208; susan.daker@dowjones.com (Russell Adams contributed to this story) (END) Dow Jones Newswires July 22, 2010 17:04 ET (21:04 GMT)