New York (CNN) -- A heat wave of historic proportions could strike some Northeastern states as forecasters warn of prolonged triple-digit temperatures that could trigger "a dangerous situation," the National Weather Service advised Monday.

The Weather Service has issued an excessive heat advisory for several Northeastern cities to take effect Monday afternoon as humidity levels gradually increase, producing heat index values to persist and surpass 100 degrees.

Excessive heat advisories have been issued for the Philadelphia metropolitan area until Wednesday evening, and in New York City, a heat advisory is in effect from to 2 p.m. Monday until 6 p.m. Tuesday as high humidity levels and high temperatures linger for two consecutive days, the Weather Service reported.

The increase in humidity, matched by weather patterns, may force the Weather Service to issue additional heat advisories for central New Jersey, northeastern Maryland and central and southern Delaware.

The Weather Service advised people affected by the heat to "drink plenty of fluids," stay out of the sun and in air-conditioned rooms, and to check up on relatives and neighbors.

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New York City residents are advised to call 311 to find cooling centers and obtain "beat the heat" safety instructions, the Weather Service reported.

A heat wave is considered to be a stretch of more than 90-degree temperatures.

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

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