A freezing rain forecast to move into the Washington region before dawn today could glaze roads and sheathe tree limbs and power lines with up to a half-inch of ice in western and northern jurisdictions.

With climatic conditions simply perfect for slip-sliding disaster on many bridges, overpasses and roadways, crews in Maryland and Virginia are braced for a long, frigid day of work to keep ice from building up. The Virginia highway department expected to have 849 salt trucks loaded and staged along interstates and major thoroughfares by 10 last night, and its Maryland counterpart asked hundreds of workers to report at midnight.

The District's public works force was also on guard and planned to deploy more than 125 trucks in early morning hours to treat city streets with a mixture of salt and liquid de-icer.

Yet conditions across the region still could become hazardous, even treacherous, especially during the morning commute. Officials already were urging many motorists to stay home.

"Imagine driving a car on an ice rink. There's no control," said National Weather Service meteorologist Michelle Margraf.

Utility companies are on high alert, too, with some meteorologists predicting that areas from the Blue Ridge Mountains to Fauquier and Loudoun counties could have up to six hours of icing.

"This is a real time of concern for us because we know if there's an ice storm, there's not much we can do," said Le-Ha Anderson of Dominion Virginia Power, which yesterday began moving up crews from eastern and central parts of the state. Some were told to "come packed and . . . ready to be sent out" for as long as five days of line repair.

The anxiety recalls the massive damage that followed a fierce ice storm in January 1999, when hundreds of thousands of residents lost electricity -- some for nearly a week. Both Dominion Virginia and Pepco now have toll-free phone lines that officials say can receive as many as 100,000 calls an hour from customers needing to report power outages or downed lines. Dominion Virginia customers needing to report power problems should call 1-888-667-3000. For Pepco customers, the number is 1-877-737-2662.

"Ice is definitely more challenging and damaging than snow," Virginia transportation spokesman Ryan Hall said.

A winter weather advisory covers much of the region, with the Weather Service predicting that the freezing rain would begin about 5 a.m. Ground-level temperatures at that hour are crucial to determining how bad things could become.

Blame a phenomenon called "cold air damming," Margraf explained.

In the stratocumulus clouds that will hang overhead today, conditions are frosty enough that the precipitation actually begins as snow. As the moisture passes through warmer air at 3,000 to 5,000 feet, that snow changes to rain. But when the rain hits the ground, which has been super-chilled by winds blowing in from the north and northeast, it will change again to ice.

Margraf warned that Fauquier and Loudoun counties, as well as Frederick County and the northern part of Montgomery County, could face a quarter-inch to half-inch of ice. And if today's high remains stuck in the upper 20s in those communities, as is likely, the ice will stick around, too -- increasing the danger of downed tree limbs and power lines.

Even in the District, the high temperature may barely reach above freezing.

Surprisingly, higher elevations in the mountains should get only rain because they're part of that warmer-air swath. "It's just going to depend on your exact location and what the temperature is. It's going to be very tricky," Margraf said.

The Associated Press contributed to this report.

For updates on today's weather and road conditions and news of closings in the area, check www.washingtonpost.com.