New York (CNN) -- A heat wave of historic proportions could strike some Northeast states, with prolonged triple-digit temperatures possibly creating "a dangerous situation," the National Weather Service advised Monday.

The Weather Service issued heat advisories for several northeastern cities over the next few days as humidity levels gradually increase, producing heat index values that surpass 100 degrees Fahrenheit.

An excessive heat warning is in effect for the Philadelphia, Pennsylvania, metropolitan area until 8 p.m. Wednesday, as record temperatures near 100 degrees are forecast for both Tuesday and Wednesday, the Weather Service said. There will be little relief in the overnight hours, the service added.

Tuesday morning low temperatures in the Philadelphia area should be between 77 and 80 degrees, the Weather Service said. For Wednesday the National Weather Service is predicting morning lows between 80 and 83.

In New York City and surrounding areas, a heat advisory is in effect until 7 p.m. Wednesday as high humidity levels and high temperatures will linger for two consecutive days, the Weather Service reported.

Heat advisories have also been issued until Tuesday evening for Boston, Massachusetts; Washington, D.C., Baltimore, Maryland; central Maryland, northern Virginia and eastern West Virginia.

The increase in humidity, matched by weather patterns, may prompt additional heat advisories for central New Jersey, northeastern Maryland and central and southern Delaware, the service said.

The National Weather Service issues both warnings and advisories in extreme heat, with slight differences in the definitions. But for both, temperatures are typically at or above 105 degrees for at least part of the day.

And for both, health experts say precautions are the same.

"People should take really strong measures to stay cool," said Tim Slater of the New Jersey Department of Health and Senior Services.

"Stay in the air conditioning during the peak hours, stay hydrated, (drink) plenty of water, and especially, avoid any physical activity during the peak hours of the day when the sun is the hottest, late morning through afternoon."

Frequent checks on elderly relatives, friends and neighbors are also strongly advised, said Slater.

"Make sure they have enough water and make sure their medications are up to date. It's a community effort," said Slater. "People have to look out for each other, not only in the winter when it's very cold, but also in the summer, when we have heat like this."

New York City residents are advised to call a help line at 3-1-1 to find cooling centers and obtain safety instructions.

Tuesday is expected to be the hottest day of the stretch, with highs in the Northeast ranging from 100 to 102 degrees and heat index values up to 106, the Weather Service said.

According to the Weather Service, in the 40-year period from 1936 through 1975, nearly 20,000 people died in the United States from the effects of heat and solar radiation. In a massive heat wave in 1980, more than 1,250 people died, the Weather Service said.

CNN's Shelby Lin Erdman contributed to this report.