Howard County Executive James N. Robey warned yesterday that Hurricane Isabel could bring heavy rains and high winds to the county in a short time and that flooding could hit low-lying areas such as lower Main Street in Ellicott City.

Despite showing signs of diminishing, Isabel is still expected to pack a double-barreled blast of high winds and heavy rain across the region by tomorrow, Robey and other public safety officials said at a briefing for county agencies and neighborhood groups.

"What was the worse-case scenario a couple days ago is now a lot-less-worse-case scenario," Robey told the group. But he added, "I don't think we can let down our guard."

Officials said that today's weather projections would be critical in their decision on whether to step up their emergency preparations. A weather report yesterday indicated that strong winds and up to 6 inches of rain were possible, they said.

"We're going to get very substantial flooding if that does occur," James M. Irvin, director of the county Department of Public Works, said at the meeting. "It's more than likely that we're going to have a significant flood event."

Officials said they expect flooding in several of the county's low-lying areas, many of which, such as Main Street, have a history of flooding.

Officials displayed several photographs showing the effects of Tropical Storm Agnes in 1972, which flooded Main Street and other parts of the county, blocked roads with debris and caused several deaths.

"Six inches of rain in a compressed period of time would bring water into lower Main Street," Robey said. He said he is prepared to close Main Street and order a mandatory evacuation of businesses and residents if necessary.

"We're not going to leave one or two people who, quite frankly, are being unreasonable about their safety," Robey said.

Other potential flood areas include Elkridge; North Laurel; U.S. 29 between the Route 108 and Broken Land Parkway overpasses; and the Allview Estates in Columbia.

"Our main concern is going to be the water," said Fire Chief Joseph A. Herr, head of the county's Department of Fire and Rescue.

The Police Department is working on a detour plan to get motorists around various potential flood spots. Police also will ensure that looting doesn't become a problem for businesses or residents, Police Chief Wayne Livesay said.

"We may be on security posts for three, four, five days," Livesay said.

Robey and other county officials plan to make key decisions on emergency preparation plans today after an 11 a.m. update on the hurricane from the Maryland Emergency Management Agency.

John R. O'Rourke, Howard's schools superintendent, said yesterday that he would not decide on school closings until after today's briefing.

"We want to make a call about school probably early to mid-afternoon [today] so parents have plenty of information," O'Rourke said.

For updates, residents can call a county hot line, 410-313-2900, that carries a recording on Hurricane Isabel. Residents can also access the county government's Web site at www.co.ho.md.us.