BOSTON (WHDH) - The National Weather Service has issued an excessive heat warning for almost all of Massachusetts due to temperature index values that are expected to climb well over 100 degrees this weekend.

The warning is in effect for parts of Middlesex, Essex, Franklin, Hampden, Hampshire, Norfolk, Plymouth, Bristol, Worcester, and Suffolk counties from 12 p.m. on Friday to 8 p.m. on Sunday. The warning does not include the Cape and the Islands.

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Dangerous heat and humidity building in the Great Plains will move into the region and take hold of the Bay State on Friday. However, the worst will arrive on Saturday, resulting in possibly record-setting temperatures.

Actual air temperatures will be in the upper 80s to lower 90s on Friday. Conditions will change drastically come Saturday.

The “feels like” temperature on Saturday is expected to hit 112 degrees in some areas because of temperatures reaching 102 degrees and elevated humidity levels.

“We could tie or even break long-standing records with the heat,” 7Weather meteorologist Jackie Layer wrote in her latest weather blog.

Boston has only reached 100 degrees 25 times since temperature record-keeping first started nearly 150 years ago.

“If we do reach triple digits in Boston this weekend, that will be the first time since 2011,” Layer wrote. “It’s not common for Boston to hit 100+, we’ve only reached 100 25 times since record-keeping began in 1872.”

Highs between 96 and 102 degrees are expected across the state with “feels like” temperatures of 107-112 degrees.

Officials are warning that excessive heat and humidity may cause illnesses and stress during outdoor exertion or extended exposure.

Residents are being urged to drink plenty of fluids, stay in an air-conditioned room, stay out of the sunshine, and check up on relatives and neighbors.

Sunday will bring almost identical conditions.

It will not cool off much overnight, with lows in most areas only dropping to the mid-70s on Saturday and Sunday.

The heat and humidity will linger into Monday.

Tuesday will bring relief with highs in the low 80s and low humidity.

For more, visit the 7Weather page.

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