Updated, 4:30 p.m.:

Snow, expected to accumulate as much as 12-15 inches,  started to fall earlier than expected  in the Merrimack Valley.

After a lull early in the afternoon, the wind began to pick up and the snow began falling again, at a steady pace, around 4 p.m.

The National Weather Service has issued a blizzard warning for Tewksbury that will remains in effect through 6 p.m. Monday.

According to the warning, 12 to 15 inches of snow is expected, accompanied by strong winds, resulting in drifting.

Driving conditions will be extremely dangerous and residents are being advised to stay off the roads if at all possible.

Gov. Deval Patrick issued a state of emergency and regional offices of the Massachusetts Emergency Management Agency (MEMA), including one on the campus of Tewksbury Hospital,were activated at 3 p.m. Sunday.

According to Peter Judge, MEMA communications officer, the each regional office acts as a clearing house for local emergency resource requests from approximately 100 communities.

"We handle requests for sandbags, heavy vehicles, help setting up shelters if need be, things like that," said Judge.

Light snow will become heavy into the evening, according to the weather service. Two to four inches of snowfall is expected during the nighttime hours.

Power outages are expected during the storm, according to the advisory, due to wind in excess of 35 mph knocking down power lines and the weight of the heavy snow.

The Tewksbury Police Department and DPW are reminding residents there is a street parking ban in effect and are urging residents to keep their park vehicles off the strets to allow plows drivers access to plow the roads and keep them as clear as possible.