Some of Baltimore's coldest December weather in a decade is forecast to arrive by Thursday morning, bringing chances for snow and ice.
A cold front with frigid Arctic air behind it is expected to pass through the region before sunrise Thursday. Some snow showers were possible overnight.
Temperatures on Thursday and Friday are not expected to rise out of the mid-20s — lower than normal for this time of year. They could fall to 15 degrees overnight Thursday into Friday — with wind chills dropping to the single digits.
"It's going to be close to 20 degrees below normal," said Brian LaSorsa, a meteorologist with the Baltimore-Washington forecast office of the National Weather Service.
Area health officials opened warming shelters and urged people to check on homeless and vulnerable people. Baltimore Health Commissioner Dr. Leana S. Wen issued a Code Blue advisory from Wednesday night through noon Friday.
"Code Blue means that we encourage residents to stay indoors in heated areas," Wen said. "This is the time to watch out for those most vulnerable among us, especially children, older adults and those with chronic medical conditions."
Baltimore County officials opened a freezing-weather shelter at the North Point Government Center in Dundalk, and also encouraged residents to warm up at county libraries and senior centers.
AAA Mid-Atlantic urged drivers to be watch for signs of dying batteries, to check tire pressure and to pack an emergency kit to reduce the risk of being stranded in the cold or snow.
Three people died of cold-weather-related causes such as hypothermia in Maryland last year, state officials said. They urged residents to bundle up with hats, scarves and mittens instead of gloves, and to take care when using portable heating sources that can give off carbon monoxide.
Baltimore City Councilwoman Mary Pat Clarke suggested residents keep underground pipes running on a trickle to prevent them from freezing.
The Maryland Zoo in Baltimore said it would be closed Thursday because of the cold.
Extreme cold has been common in January, February and March in recent winters, but temperatures in Baltimore haven't fallen below 15 degrees in December since they hit 14 on Christmas Eve 2009.
Baltimore hasn't recorded single-digit temperatures in December since 2004, LaSorsa said.
Snow turning to sleet or freezing rain is forecast Friday night into Saturday morning, just as the cold gives way to warmer-than-normal weather.
Temperatures are expected to rise into the upper 40s by Saturday afternoon and into the mid-50s on Sunday.