A dangerous heat wave is expected to affect the Southern Appalachia region through early next week, with record high temperatures at or above 100-year marks, and oppressive heat indices up to 105 degrees, the National Weather Service said.

The temperature in Oak Ridge at about 5 p.m. Saturday was 104, and a heat advisory remains in effect through 9 p.m. Sunday in Anderson County.

Temperatures could continue to be close to 100 on Sunday and Monday, before dropping to 94 on Tuesday, according to a forecast by the National Weather Service in Morristown.

A strong high-pressure system and mostly dry conditions have led to record temperatures and dangerous heat, the Weather Service said.

Heat index values—a measure of how it actually feels—will be slightly higher than actual high temperatures. Afternoon heat index values could be as high as 107 in East Tennessee.

“Please consider reducing or eliminating strenuous outdoor activities during the heat of the day (late morning through the early evening) and remember to drink plenty of water,” the Weather Service said.

Heat is the No. 1 weather-related killer.

Weather officials said heat illnesses are possible, and people should take extra precautions if they work or spend time outside. Those precautions include rescheduling strenuous activities to early morning or evening, and knowing the signs and symptoms of heat exhaustion and stroke.

The Weather Service also advised wearing lightweight and loose-fitting clothing when possible.

Those working outside should schedule frequent rest breaks in shaded or air-conditioned environments. Anyone overcome by heat should be moved to a cool and shaded place, and 911 should be called if someone has a heat stroke, the Weather Service said.

Cable news television station CNN reported that the heat wave affected 100 million people across the country, and more than 1,000 temperature records have been broken this week.