With cold air firmly in place, and a storm gathering to our south, Marylanders are bracing today for what could be the season's first measurable snowfall, and potentially the biggest snowstorm in nearly three years.

"Some could receive a little more, some a little less. But on average it appears likely enough the area will be receiving somewhere in the ballpark of 5 inches," said meteorologist Howard Silverman, of the National Weather Service's Sterling, Va., forecast office.

A developing storm system was expected to move across the Southeast today and reorganize off the Carolina coast, generating snow and ice from Georgia to New England today and tomorrow.

With cold Canadian air well entrenched across the state, the event seemed likely to be all snow in Central Maryland. Some mixing with sleet is possible in extreme Southern Maryland and the Eastern Shore.

Snow was expected to begin falling in the Baltimore region tonight and continue until tomorrow afternoon.

Winter storm watches were issued yesterday for all of Maryland from Allegany County east to the Chesapeake Bay. The watches mean that hazardous conditions, or accumulations of 4 inches or more, were possible beginning at 5 p.m. today and lasting until noon tomorrow.

Forecasters said accumulations would depend on the storm's final track and its speed.

With last night's low forecast to reach 15 degrees, Baltimore Health Commissioner Peter L. Beilenson declared yesterday the season's first "Code Blue" alert. That opened an emergency overflow shelter and provided bus pickups for the city's cold and homeless.

If it comes, the storm will bring the first measurable accumulations in Baltimore since a 2.3-inch storm Jan. 19. That was all the city received last winter. And only 8.7 inches fell in the winter of 2000-2001. There hasn't been a snowfall deeper than 3.7 inches at the airport since Jan. 25, 2000, when 14.9 inches fell.