Get ready to bundle up and pile on extra blankets.

Dangerously cold temperatures that haven’t been seen this winter are on the way, warns the National Weather Service. Overnight lows will drop below zero Monday and Tuesday and Tuesday’s daytime temperatures will be mostly in the single digits.

For Portland, temperatures on Monday will be in the 20s during the day, but plummet to -1 at night, said Sarah Thunberg of the NWS’s Gray office. On Tuesday, the high is expected to be 8 degrees with a low of -3.

Conditions will improve on Wednesday when temperatures rebound to a seasonable 25 degrees. The reason for the cold “is a really, really cold air mass coming in from the arctic,” Thunberg said.

Monday and Tuesday nights will be the coldest we’ve had this winter and may come as a bit of a shock because of the mild weather so far this season, she said.

While Portland will be frigid Monday and Tuesday, it will be worse in northern Maine, like Caribou.

Farther north, NWS meteorologist Angela LaFlash said Caribou’s expected temperatures will hover from 5 to 10 degrees during the day Monday and fall to -15 degrees Monday night. “Tuesday during the day we’re looking at 5 below for the high,” she said.

Then it gets worse. “Tuesday night it’ll be between 20 to 30 degrees below zero,” she said, much colder than normal. The average temperature this time of year for Caribou is between 5 and 22 degrees, LaFlash said.

During the cold snap, the Maine Center for Disease Control and Prevention recommends people avoid frostbite and hypothermia, which can happen quickly when outside, by wearing layers of clothing, mittens, scarfs, hats and proper footwear to reduce skin exposure, even if outside only briefly. Fingers, toes and the tips of ears and nose are especially vulnerable.

Dog owners should be careful of how much time they allow their pets outdoors, experts say.

Dr. Amy Hess of PetMedic Urgent Care Vet Clinic in Portland recommends bypassing dog walks in extreme cold.

“Just take them outside on a leash for a quick potty,” she said Sunday. “If you have a fenced in yard, go outside with them.” That way there’s little chance of forgetting the dog is outside or allowing he or she to stay out too long, she said. When dogs hold their paws up or shiver, they’re too cold.

“If they have short haircuts make sure they have a jacket on,” Hess said. “Another thing we see is a lot of injuries on ice, dogs slipping or they break their toenails.”

Limiting how long dogs are outside will minimize injuries, she said. “If it’s too cold for us, it’s too cold for them.”