Friday will be the hottest day so far this year, says the National Weather Service, which issued a rare heat advisory Thursday.
With the temperature expected to reach 94 degrees in Portland and dew points in the low 70s, it will feel like it’s 100 to 105 degrees, said the weather service office in Gray.
The weather service’s advisory, effective from 10 a.m. to 7 p.m., says Friday “will be dangerous for those sensitive to high heat and humidity and especially those without access to air conditioning.”
People who plan outdoor activities should bring extra water to drink and remain alert for signs of heat exhaustion, the weather service says.
Another unbearable day will extend a string of hot, muggy days that began Sunday, said meteorologist Steve Capriola.
Three consecutive days with temperatures 90 degrees or higher constitutes a heat wave. Portland’s high temperatures have ranged from the mid-80s to the low 90s since Sunday.
The temperature reached or exceeded 90 degrees Monday and Tuesday, but Wednesday’s high was just 84 degrees and Thursday’s was 83.
The heat has produced odd weather patterns, strained New England’s power grid and even slowed Amtrak’s Downeaster train.
The weather service said thunderstorms produced about 7,500 lightning strikes over a four-hour period in Maine and New Hampshire on Wednesday night.
Capriola said that was an unusually large number, but the weather service doesn’t keep records for lightning strikes.
The Downeaster, which carries passengers between Portland and Boston, issued an unusual warning on its website Thursday.
It said that “due to the excessive heat,” a speed restriction had been imposed, reducing the top speed of the Downeaster to 40 mph.
The speed reduction affected the Downeaster line from Portland to the New Hampshire-Massachusetts border.
Rail officials said the slowdown would cause trains to become “increasingly delayed,” arriving 45 to 65 minutes later than scheduled.
The warning did not indicate whether the reduced speed will remain in effect Friday.
The Associated Press said speed restrictions are imposed for safety reasons when internal rail temperatures exceed 120 degrees. The heat can cause the steel rail to expand.
The operator of New England’s power grid extended its request for electricity conservation Thursday, with demand for power expected to peak at near-record levels.
ISO New England warned earlier this week that the prolonged combination of heat and high humidity throughout the region could push energy demand close to capacity, possibly requiring power from other parts of the country.
On Thursday, it extended the request for voluntary conservation measures, such as raising air conditioning temperatures to 78 degrees.
Preliminary data indicate that electricity demand peaked Wednesday at 26,655 megawatts, making it the 10th-highest demand day on record in New England. Demand peaked at 26,840 megawatts Thursday, according to ISO New England’s website, and the demand was expected to remain high on Friday.
Demand for electricity goes up the longer a heat wave lasts because it takes more power each day to keep buildings cool.
New England’s record for electricity use was set on Aug. 2, 2006, with a peak of 28,130 megawatts. One megawatt of electricity can power about 1,000 homes in New England.
To watch power demand rise in real time, go to the ISO New England website.
Summer tourists and visitors were taking the heat in stride.
“It’s not too bad because of the proximity to the sea,” said Nawal Nasrallah, who drove to Portland from Salem, N.H., for a day visit. “If you walk here, the breeze off the water is very nice.”
Peter and Susan Stein of Long Island, N.Y., who are vacationing in Maine, were planning to visit the Portland Museum of Art but decided to go to the beach instead because of the heat.
“It’s the summer. If you don’t like it, move to the Alps,” Peter Stein said.
Roy Turner of Marianna, Fla., was enjoying the Maine Mall’s air conditioning. He and his wife spent 12 days visiting spots in Maine and Nova Scotia. The weather reminded him of his home state.
“We were surprised because we thought it would be cooler,” Turner said.
Capriola, the meteorologist, said cooler, less humid air is expected to move into the region Saturday night, with Sunday’s high temperatures in the upper 70s.
— Staff Writer Karen Antonacci contributed to this report.
Dennis Hoey can be contacted at 791-6365 or at: