The coldest days and nights of the year have arrived in Washington, blown in by an Arctic high pressure system that today is expected to deliver a double whammy--high temperatures in the middle 20s and winds of 15 miles an hour with gusts of up to 25 miles an hour.

"It's going to be really brutal," said Frank Rosenstein, a National Weather Service forecaster.

The outlook is for a warming trend to begin Thursday and Friday as the system moves out toward the Atlantic Ocean. Temperatures then may reach into the 40s, Rosenstein said.

Health officials warn that low temperatures pose special dangers for the very young, the very old and the unprepared. One of the hazards, they said, is hypothermia--abnormal low body temperature that can result in death. Another is frostbite.

"Temperatures have to be below freezing for frostbite, but certainly not for hypothermia," said W. Moulton Avery, director of the Center for Environmental Physiology, a nonprofit research facility here.

Both conditions can be prevented, Avery said. "The difficulty is that people are not aware of the danger--which is that if you are old, if you are an infant under one year of age or one of those individuals with a predisposing medical condition living in surroundings that are lower than 70 degrees, these conditions can be fatal if you are not dressed properly."

The number of deaths from hypothermia is uncertain, but it is estimated to be as high as 25,000 a year, Avery said.

He said older people who take prescription medicine can become hypothermic without ever feeling cold. The possibility of illness is even greater if they have turned down their thermostats in order to save on fuel bills, he said.

"We are not saying that they shouldn't take their medications or that they shouldn't turn back the thermostats, but they must check with their pharmacist or their doctor for advice and they must take precautions to dress warmly in the day and sleep warmly at night."

Here are steps that Avery and other health authorities recommend to live safely and comfortably in the cold weather:

* Wear a hat and scarf. "Half of the body's heat can be lost through the uncovered head and neck," Avery said. "So using a warm hat and scarf is like having insulation in the attic; it really is that important and it makes a tremendous difference in how warm a person feels."

* Use a layer system of clothing to trap warm air around the body. Mittens are warmer than gloves.

* Put on a windproof outer layer outdoors.

* Avoid alcohol. Drinking is "one of the worst things you can do if you want to stay warm," Avery said.

* At night, use hot water bottles, heating pads and electric blankets to keep your bed warm. Wear a nightcap.