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With a second heat wave closing out the month, Boston could see a record-setting June

Cousins Shaquille Leathers (left) and Jarell Leathers, 8, cooled off at the Artesani Playground Spray Deck in Brighton on Monday afternoon.Christiana Botic for The Boston Globe

This could be the hottest June ever recorded in Boston, according to the National Weather Service.

The average monthly temperature as of Monday was the second-highest it’s ever been. And with a scorching three-day heat wave expected to stretch into Wednesday, we’re moving into record-setting territory.

“We’re definitely in the ballpark of tying or surpassing the hottest June” since records began being kept in the late 19th century, said Weather Service meteorologist Torry Gaucher.

It makes intuitive sense. From June 5 through June 9, a five-day heat wave held the state in its sweltering grip.

For another seven days this month, the daily high temperature climbed to 85 or more, including this past weekend, when temperatures reached 86 and 89 on Saturday and Sunday, respectively.

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Only six days in June have seen temperatures below average, and that was just by a few degrees.

Now comes the capstone: another three days of oppressive heat. Boston hit 97 degrees at 3:11 p.m. Monday, setting a three-way tie for that date’s record high with 1901 and 1991, the Weather Service said.

Forecasters are predicting highs of 97 Tuesday and 97 Wednesday and are warning that, thanks to the humidity, it will feel like it’s more than 100.

So how is the monthly record determined? The monthly average temperature is calculated from the high and low temperature for each day of the month.

At Crystal Lake in Newton, Daniel and Leo Raffaelli, ages 4 and 7, respectively, got help with their fishing techniques from their grandfather Craig Lovett as they tried to stay cool in the day's heat.Suzanne Kreiter/Globe Staff

The monthly average temperature as of Sunday was 72.9 degrees. The normal monthly average for June is 67.5 degrees.

The record average temperature for June, set in 1976, is 73.4 degrees. But the heat wave will close out the month with three days when sauna-like daytime highs and lows in the 70s are expected result in daily averages in the mid-80s, which will boost the monthly average.

“Give it a couple more days and we’ll crunch the numbers each day,” Gaucher said. “You’re going to want to stay tuned on July 1 to get the official word.”

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Temperatures at other locations in Southern New England where the Weather Service takes measurements haven’t been quite as extreme, though Providence’s average monthly temperature for June is around the fifth-hottest, he said.

Mike Iacono, chief scientist at Blue Hill Observatory in Milton, said in an e-mail that June “will certainly be among the warmest on record at BHO.”

Why so hot? Gaucher said a vast area of high pressure to our southeast is spinning clockwise and bringing in warm air from the Southwest.

He said what’s happening bears some similarities to the record heat being seen in the Pacific Northwest, but it is a result of two different weather systems.

Gaucher said the heat wave — a heat wave is defined as three days in a row with temperatures of 90 or more — will end Wednesday.

Forecasters say there’s the possibility of isolated strong thunderstorms Wednesday afternoon, and the weather will transition to cooler than normal temperatures for Fourth of July weekend.

What’s the longer-term outlook?

If the next three days are as hot as expected, then Boston will have seen nine days of 90 or more in June alone. (The temperature also hit 91 on June 19.) The average number of 90-plus days per meteorological summer (June, July, and August) over the years has been 10.2, Gaucher said. So it might seem that we’ve used up our allotment.

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But don’t forget there’s a lot more summer ahead.

July and August are the warmest months for the Boston area, with average monthly temperatures of 73.0 and 71.3, respectively, Gaucher said.

The outlook from the Climate Prediction Center for the July-August-September period has Southern New England at a 60 to 70 percent probability of above-average temperatures, he said.


Martin Finucane can be reached at martin.finucane@globe.com.