The watch and warning covered Washington County and west in Maryland, Fulton and Franklin counties in Pennsylvania, and West Virginia's Eastern Panhandle.
Jefferson County, W.Va., emergency officials said they planned to keep a close watch on the Opequon Creek and the Shenandoah and Potomac rivers, which were expected to crest on Friday.
The Potomac River is expected to crest at 17 to 18 feet at Harpers Ferry on Friday night, Strong said. The flood stage there is 18 feet.
The Shenandoah is expected to rise slightly out of the banks on Friday afternoon at Millville, W.Va., with the river cresting at 15 to 16 feet, Strong said.
The Tri-State area was on the border between areas of snow and rain produced by the slow-moving storm that originated in the Southeast and moved north up the coast.
To Hagerstown's west, Cumberland, Md., reported snow accumulations of almost a foot by 9 p.m. Wednesday, and snow was still falling.
In Hagerstown, 65 miles east, temperatures in the mid-30s kept the precipitation falling as rain. By Wednesday night, .77 inches of rain had fallen, weather observers said.
"The storm is all around us but there's always a line between who gets the rain and who gets the snow," Strong said.
About four to five inches of snow and sleet hit Berkeley County, W.Va., Wednesday, with another four or five inches expected to fall before this morning.
Washington County schools closed Wednesday in anticipation of the slushy mess.
Berkeley County schools closed Wednesday and a two-hour delay was issued Wednesday night for this morning.
Although main roads were clear by noon, the West Virginia Department of Highways Operation was still working on secondary roads Wednesday night.
Several minor accidents were reported, though there were no injuries, according to police.
"All our primary roads have been plowed and treated. But where we haven't been it's nasty because it's slush," said foreman Bill Faircloth.
Road crews have 500-plus miles of streets to clear, Faircloth said, and the eight trucks were likely to be out all night clearing streets of Wednesday's snow and the additional five inches or so expected by this morning.
Chief Deputy Jesse Jones of the Jefferson County Sheriff's Department said there seven weather-related accidents Wednesday, with vehicles sliding off slick roads and into ditches, fences and in one case, a tree.
Most of the primary roads were in good condition, but many of the secondary roads were snow-covered in the morning, he said.
West Virginia State Police in Martinsburg reported at least a half dozen vehicles slid off Interstate 81 due to slippery conditions Wednesday night. They said they had no reports of injuries Wednesday night.
Emergency services officials in Franklin County, Pa., reported mostly rain and sleet Wednesday night with a few icy areas.
To the west and over the mountain in Fulton County, Pa., Pennsylvania State Police in McConnellsburg, Pa., said about 5 inches of snow was on the ground and it was still snowing at 9 p.m.
Police in both counties reported several minor accidents caused by the weather.
Staff Writers Richard Belisle, Clyde Ford, and Amy Wallauer contributed to this story.