Most of Maine is heading into a stretch of heat and humidity that won’t subside until Friday at the earliest, the National Weather Service in Gray said Sunday.

With a high temperature of 88 degrees recorded in Portland – the heat index for Portland hit 93 degrees Sunday – the weather service issued an excessive heat warning that remained in effect through 8 p.m. Sunday.

The heat index is a combination of actual temperature and dew point, more commonly defined as what it feels like outside. The heat index soared to over 100 degrees in some sections of New York, Vermont, Massachusetts and New Hampshire on Sunday.

A heat warning means that a prolonged period of dangerously high temperatures will occur and cautions people against participating in strenuous outdoor activity – especially during midday hours – and to drink plenty of water.

Some inland areas of Maine, such as Fryeburg, could find themselves in a heat wave, which is defined as three consecutive days of 90-degree temperatures. Fryeburg’s high Sunday was 92 degrees, with a heat index of 97 degrees. Communities along the coast will see slightly lower temperatures because of a sea breeze, the weather service said.

“We’re heading into a stretch of extreme heat and humidity,” weather service meteorologist James Brown said. He said a cold front will begin to move into Maine on Friday, with temperatures expected to drop into the 70s by Saturday.

But until then most of Maine, with the exception of coastal areas, should expect to endure extremely hot weather.

The early summer heat was also producing other negative side effects, such as a tornado warning issued late Saturday night for northern New Hampshire and a section of Oxford County near Mexico, Bethel and West Paris.

“We don’t have any confirmation yet that a tornado touched down,” Brown said, adding that the weather service may send an investigator to the area if there are reports of damage.

Three Portland television stations interrupted their regular Saturday evening programming to warn Mainers of the possibility of a tornado. Forecasters were concerned about a thunderstorm that was producing golf ball-size hail and damaging winds.

The prolonged heat forecast prompted a special warning Sunday from the Maine Department of Environmental Protection’s Air Quality Bureau.

“A significant heat event has been building and will be impacting the Northeast for the next 5 to 6 days. At the same time, conditions will allow precursors to form ozone very efficiently in the region. The airmass will not move out of the region until sometime next weekend,” the DEP warned on its website.

The DEP warning said that even moderate levels of ozone, combined with moderate levels of particle pollution and heat and humidity, will compound the harmful effects on people with medical conditions, the young and the elderly.

Monday’s forecast calls for moderate levels of ozone along the coast from Kittery through the midcoast.

Some towns, including Scarborough, were taking steps to help citizens who might not have air conditioning.

The Scarborough Police Department announced on its Facebook page that the town’s public library will serve as a community cooling center Monday and Tuesday. The library will not be open Wednesday, the Fourth of July.