By Sarah Wroblewski

BOSTON (CBS) – A Heat Advisory is in effect for most of southern New England, besides Martha’s Vineyard and Nantucket from 11 AM to 8 PM Friday. Heat index values will be in the mid to upper 90s, but as high as 105 degrees!

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Extreme heat and humidity will significantly increase the potential for heat-related illnesses, particularly for those who have to be outside. Make sure to drink plenty of fluids, avoid staying in the sun and find a cool spot, preferably indoors in air conditioning.

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Understand the symptoms of heat related illnesses and make sure to check on neighbors and relatives as well as your pets. Young children and pets should also not be left in vehicles. The car interior can reach lethal temperatures in a matter of minutes with the extreme heat that is forecast.

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The combination of heat and tropical like humidity will also trigger the possibility of a couple thunderstorms Friday afternoon. The best chance of seeing a thunderstorm would be across the interior west of 495, with little to no activity expected near the coast.

The oppressive heat does look to break over the weekend as a cold front passes through bringing cooler and drier weather. The front is expected to slowly cross the region on Saturday, but not completely clear until overnight. So one more day of 90-degree heat with tropical humidity is possible on Saturday before dewpoints sink and much more comfortable air takes control on Sunday, bringing the heat wave to an end.

Mayor Kim Janey declared a heat emergency in the City of Boston beginning Wednesday, August 11 and lasting through Friday, August 13. A full list of cooling centers can be found at Boston.gov/Heat.

Stay tuned for updates to the latest forecast and timing on WBZ-TV, CBSN Boston and CBS Boston.com.

Sarah Wroblewski