Marylanders faced the grim prospect of an ice storm today, with sleet and freezing rain predicted to fall throughout the day on top of a few inches of overnight snow.

It's the latest in a series of small but troublesome storms this season that have vexed the forecasters assigned to pinpoint when and where the precipitation will fall - and how much - in all its forms.

"These storms are fun to forecast, but also very stressful to forecast," said Marisa Ferger of the Pennsylvania State University Weather Communications Group in State College. "There's a fine line as to who's getting what, or when. And the fact the fine line is in very populated areas makes it more stressful."

The threat of a prolonged period of sleet and freezing rain today prompted the National Weather Service yesterday to post a winter storm warning from Allegany County to the Chesapeake.

Federal forecasters in Sterling, Va., expected between 1 and 3 inches of snow overnight in most Maryland locations. That would be followed by a changeover to sleet and freezing rain during the morning rush hour and continuing through the day, and maybe, somewhere, to all rain, they said.

But the times for the changeovers were far from certain, and Ferger's take differed from the weather service's.

"You guys could have an interesting freezing rain event tomorrow around lunchtime," Ferger said. Then, "hopefully, it gets warm enough to turn to rain."

Or not.

"One of the [computer] models has it turning to rain; one doesn't," she said. "One has it cold enough to stay freezing precipitation. That's what makes this so fun."

The storm threat spurred Annapolis officials yesterday to order city offices closed today except for essential personnel.

Department of Public Works Superintendent Robert Couchenour said salting operations would begin at 10 p.m. and continue through the night. He reminded residents that it's their responsibility to clear their walks of snow and ice.

The relatively warm and very wet storm is coming to us from the Texas Gulf Coast, tracking northeast and spreading snow and ice from the Southern Plains to the Ohio Valley and the mid-Atlantic states.

It was running into the very cold arctic air mass that moved into Maryland earlier this week and held temperatures to about 32 degrees yesterday. The average high for this time of year in Baltimore is 46 degrees.

That cold air at the surface is dense and heavy, and it's hunkered down east of the Appalachians. That makes it especially difficult for the warmer air pushing in from the south and west - and over the mountains - to dislodge it. Meteorologists call it "cold air damming."

"The warmer air moves in at the upper levels faster than it does at the surface, especially east of the mountains," Ferger explained. "That's why it's going to change from snow to a freezing rain and sleet mix."

The air at the surface is still cold enough so that when rain falls, it freezes on contact - with cars, wires, trees, sidewalks and steps.

What's tough for forecasters is determining whether, when and where the cold air will be eroded enough for the snow to change to freezing rain and sleet, and later to rain.

"We're dealing with not just surface temperatures, but [temperatures] throughout the entire column up into the cloud," Ferger said.

The other complicating factor today will be a secondary low that is expected to form off the Delmarva Peninsula and move up the coast. "That's why this thing will be so prolonged," Ferger said. "This is a long-duration storm."

At Baltimore-Washington International Thurgood Marshall Airport, 7.9 inches of snow had been measured this season before today's storm, with a week to go in the three-month meteorological winter.

That's more than 10 inches below the long-term average for Baltimore and the least since the winter of 2001-2002.

Nearly all of this winter's total fell in two storms: 4.7 inches on Dec. 5 and 2.4 inches on Jan. 17.

frank.roylance@baltsun.com