Heat index, or “apparent temperature,” forecast for Wednesday afternoon.

A late-summer heat wave is building in the Northeast this week. Temperatures will spike well into the 90s and, along with the humidity, heat indexes will push beyond 100 degrees. Overnight temperatures will stay high and, in some cases, may not drop below 80 degrees. In southern New England, this week’s heat could break records that date back to 1948.

The excessive heat will last several days — the next cold front is not expected until at least Friday.

The National Weather Service put 56 million people under heat alerts this week from Philadelphia to Portland, Maine, and into Upstate New York. That number does not include another 19 million people under heat advisories in the Midwest from St. Louis to Chicago, where the heat index will spike to 110 degrees Monday afternoon.

The National Weather Service issues a heat advisory when an extended period of extremely hot and humid weather is in the forecast. Particularly, the combination of heat and humidity will be stressful on the body and make heat-related illness more likely. Young children and the elderly are particularly susceptible to heat illness. The Weather Service recommends checking on elderly neighbors during heat advisories, especially if they don’t have access to air conditioning.

The Centers for Disease Control and Prevention recommends wearing light-colored and loose-fitting clothing to prevent heat-related illness such as heat stroke. The easiest thing you can do is stay indoors with air conditioning, but if you have to be outside for work or other strenuous activity, drink extra water and take extra breaks.

Signs of heat exhaustion or heat stroke include dizziness, nausea, headache, lightheadedness, weakness or fast pulse.

Tuesday

The extreme heat will begin Tuesday, when temperatures in the mid- to upper 90s combine with dew points greater than 70 to create heat indexes over 100 degrees. An excessive heat watch is in effect for Tuesday in Boston, where the heat index could reach 105 degrees. Boston is unlikely to break the high-temperature record Tuesday, which stands at 99 degrees set in 1948.

Forecast temperature (heat index in parentheses)

  • Philadelphia — 95 degrees (102)
  • New York City — 93 degrees (103)
  • Providence — 94 degrees (99)
  • Boston — 97 degrees (102)
  • Portland, Maine — 88 degrees (91)

Wednesday

Records will probably be broken Wednesday as temperatures push into the upper 90s in southern New England. With a 98-degree high temperature in the forecast, Boston is almost certain to break its record for the date, which stands at 96 degrees set in 1953. Providence could also break or tie its record of 95 degrees set in 1953.

Forecast temperature (heat index in parentheses)

  • Philadelphia — 95 degrees (101)
  • New York City — 94 degrees (101)
  • Providence — 95 degrees (103)
  • Boston — 98 degrees (102)
  • Portland, Maine — 86 degrees (90)

Thursday

Temperatures will remain elevated Thursday but will relax as a cold front drops in from the north. Peak temperatures depend strongly on how fast the cold front arrives. If it slows or stalls, temperatures could exceed the forecast Thursday.

Forecast temperature (heat index in parentheses)

  • Philadelphia — 91 degrees (96)
  • New York City — 89 degrees (94)
  • Providence — 90 degrees (92)
  • Boston — 90 degrees (93)
  • Portland, Maine — 78 degrees (N/A)

Heat advisories (orange) and excessive heat watches (red) are in effect across the Northeast this week. High pressure will build until Friday when a cold front is forecast to pass through. (National Weather Service)