Cat Goding of Boston tries to stay cool while sitting in the shade below Portland's Eastern Prom on Thursday, July 5, 2018. Goding is originally from Farmington.

This week’s hot, sticky weather is expected to break Friday as thunderstorms roll in, bringing with them a cold front and cooler temperatures for the weekend.

The heat index reached more than 90 degrees across the state Thursday, according to the National Weather Service, with the service issuing a heat advisory that lifted at 7 p.m. The heat caused “a significant increase” in people seeking treatment for heat-related illnesses at Eastern Maine Medical Center, spokesman Bob Potts said Thursday.

Customers at Bangor-area appliance store Dunnett Inc. bought 40 to 50 air conditioners just in the last day or so, said Jack Eisentrager, who works at the store.

But National Weather Service meteorologist Michael Cempa said thunderstorms will create cooler temperatures Friday.

That news is a relief at Acadia National Park, where workers have seen a few heat-related illness cases over the past several days, spokeswoman Christie Anastasia said.

Thursday’s temperatures set some Maine records for high temperatures set on this day, according to the weather service.

The 94-degree Fahrenheit temperature set records in Caribou and Houlton. The previous high in Caribou on July 5 was 92 degrees set in 1983; in Houlton, 93 degrees in 1983. In Millinocket, the high reported Thursday of 95 degrees exceeded the high of 92 degrees set in 1983.

Augusta’s recorded high of 93 degrees Fahrenheit Thursday bettered the old record of 91 set in 2010, according to the weather service.

People should always be careful on exceptionally hot days. Dr. Elise Fazio of Eastern Maine Medical Center said the human body is comparable to a car engine. Like an engine, the human body shuts down when its core temperature rises too high, she said.

Fazio and Anastasia had the same recommendations for those looking to bear the heat without getting struck down by it:

— Avoid being outside during the hottest part of the day, from 10 a.m. to 5 p.m. Groups particularly vulnerable to these illnesses include young children, the elderly and athletes, who tend to be outside practicing during the hottest hours of the day, Fazio said.

— Keep hydrated. A good way to know you’re drinking enough water: Take half your body weight, convert it to ounces and drink that much water every day. So a person weighing 140 pounds should drink 70 ounces of water daily, Fazio said.

— Protect yourself. Acadia officials recommend that park visitors wear sunhats and sunscreen to mitigate the heat’s worst effects, Anastasia said, as well as staying cool by going in the water, staying in shade or cooling down under a sprinkler.

— Avoid alcohol, which can be dehydrating, Fazio said.

Some groups have already taken preventative measures. The USA Track and Field 14-and-under developmental meet scheduled for Thursday afternoon at Old Town High School was postponed until Friday.

It was to be the second of six scheduled meets this summer for the program, which includes more than 400 participants from around Greater Bangor.

BDN reporter Caitlin Rogers contributed to this story.

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