The National Weather Service has issued a heat advisory for the region, with temperatures in the low '90s, soaring humidity and a heat index of over 100 degrees.

The service warned residents to stay out of the sun, check up on relatives and neighbors and be careful to keep pets in the shade and give them enough water.

The heat index combines the impact of air temperature and relative humidity in order to gauge the "apparent temperature" or and give a sense of what the weather actually feels like.

With a heat index in the hundreds, a car with its windows rolled up can quickly attain internal temperatures above 150 degrees, producing heat stroke or death, the service said.

The heat advisory is in effect at least until 6 p.m. Tuesday.

The weather has also prompted heat restrictions for portions of the Virginia Railway Express that uses tracks owned by the freight operator CSX, the Associated Press reported. Trains must go 20-miles-an hour slower than normal on those lines. The restrictions would affect the entire Fredericksburg Line, and the stretch of the Manassas Line between Union Station and Alexandria.

Actual temperatures in the area over the next 24 hours will range from a high of 92 to lows in the mid-seventies with relative humidity at about 83 percent.

The longer term outlook is for cooling temperatures by the weekend.