Estimated temperature anomalies from Tuesday to Thursday reveal the core of the warmth centered over the Champlain Valley. (WeatherBell)

Montreal on Wednesday experienced its second-hottest day ever observed, a high temperature of 98 degrees measured at Trudeau International Airport at a time of year when highs are generally closer to 70. Dew points, a measure of humidity, soared to 66 degrees — a value more typical of the Carolinas in May. It’s uncharted territory for late spring in southern Quebec.

But it’s not just Montreal that’s baking. Much of the interior Northeast has been sweltering, with a number of cities in New England seeing records obliterated beneath an exceptional dome of impressive heat. Even Burlington, Vt., hit 95 degrees on Wednesday — 2½ weeks ago, it was snowing there.

Even more astonishing is Mount Mansfield in Vermont, which set not only a May record — it broke its year-round record high temperature before the official start of summer.

That heat is set to linger across the Northeast through the remainder of the workweek, with a dose of more-refreshing air sweeping southeast for the weekend. But until then, more records are likely to fall, the anomalous heat charging on.

Records fall across the eastern Great Lakes and Champlain Valley

The heat began to take shape over the eastern Great Lakes and Northeast on Tuesday, when much of Upstate New York spiked well into the 90s. Buffalo hit 93, breaking its previous record of 88 degrees, while Watertown hit 93 as well. On May 9, Buffalo had been down to 29 degrees, the temperature swings making May the city’s third-most-changeable month on record.

On Wednesday, the core of that heat pivoted east and intensified, baking the Champlain Valley with temperatures that approached the triple digits. Montreal just missed its all-time high of 100 degrees set in August 1975, falling short by two degrees. The other nine warmest days on record in Montreal’s top 10 occurred in July or August, making this particular May day the definition of extreme.

Montreal also didn’t fall below 71 degrees early Thursday, marking the city’s warmest overnight low temperature on record for May.

Environment Canada has issued heat warnings for portions of eastern Canada since Tuesday. On Thursday, they are focused in southern Ontario, southern Quebec and New Brunswick, and include Montreal.

In Vermont, Burlington hit 95 degrees on Wednesday and Montpelier 91. Both go in the books as records for May.

Buffalo, Burlington and Montreal all hit 90 degrees before Washington.

Mount Mansfield sets extreme record high

Perhaps the most remarkable was the feat achieved at Mount Mansfield, Vermont’s highest peak, at 4,393 feet elevation. In the town of Underhill about 20 miles east of Lake Champlain, the summit hit 85 degrees on Wednesday.

That may not sound terribly impressive, but that’s nearly a mile up in the atmosphere. The 85-degree reading is not only a record for the day and month — it is the highest temperature ever observed at that site. Readings there date to 1954.

Even more staggering is that the previous daily record was 76 degrees, set in 1978. Looking year-round, the summit hit 84 degrees in June 1999 and several times in July 2018.

It’s especially bizarre considering how early it is in the season for exceptional heat; Vermont’s warmest temperatures typically are entered around the third week of July, during the oppressive dog days of summer. As Brian Brettschneider, an Alaska-based climatologist, noted on Twitter, all major U.S. weather stations but one have an all-time maximum temperature that was recorded sometime in the year after May 30.

It is worth highlighting that Mount Mansfield has been very dry lately, and as meteorologist Eric Fisher of Boston’s WBZ pointed out, the lack of verdure atop the mountain could have played a role. Since there are no leaves to “evapotranspirate” and add moisture into the air, it’s easier for dry atmosphere to heat up beneath full sunshine.

The National Weather Service in Burlington was equally amazed at the reading, with plans to look into how and why the anomaly occurred.

“I think we need to look into [the peculiar time of year] also,” said Pete Banacos, a lead forecaster there. “Typically you’d expect some July day [to set a record]. But that’s what the sensor had. We don’t have any reason to discount it.”

The heat lingers late week

The heat will stick around through Friday in a sliver of the Northeast, its sphere of influence shrinking as an approaching cold front pushes east. Yet highs near 90 degrees could snake their way all the way up to Caribou, Maine, on Thursday, potentially making one of Maine’s northernmost cities the warmest on the East Coast.

Elsewhere, temperatures across Vermont’s North Country will remain very warm. Mid- to nupper 80s are likely in Burlington and Montpelier on Thursday and Friday, before a return to 70s on Saturday and lower 60s for highs on Sunday.

After an abrupt cool-down this weekend, Caribou could dip into the upper 30s to near 40 on Sunday night.

The winds make all the difference


The National Weather Service's high-temperature forecasts for Friday again reveal where the hottest place on the map is: Vermont. Note the cooler coasts; wind direction makes all the difference. (Pivotal Weather)

The secret to the heat has been the winds, an atmospheric squeeze play of sorts draping a tongue of warmth along the Appalachians. Clockwise-spinning high pressure to the east and Bertha’s remnant tropical low to the west behaved like two interlocked gears, conspiring to yield a persistent southerly flow in between that allowed warm winds to funnel their way up the Champlain Valley.

“It was the winds here that allowed us to stay well mixed at night,” Banacos said. In addition to Wednesday’s record high temperature, the morning low never dropped below 72, also a May record for warmest minimum temperature. That’s greater than the average late-May high in Burlington of 71 degrees.

“We had southerly winds the whole night between five and 10 miles per hour, up to 13 miles per hour,” Banacos said.

“We stayed well mixed all night long,” Banacos said, referencing how the light breeze overturned the air enough to prevent the ground layer from cooling much after sunset.

Those same winds between the two weather systems had just enough of an easterly component near the Atlantic coastline to generate a bit of an onshore flow, bringing in slightly cooler air off the waters and slashing any chance at record temperatures.

At the same time Vermont and southern Quebec were sizzling, Boston and Hartford, Conn., hit only 83 degrees Wednesday, while Providence, R.I., topped out at 79. Portland, Maine, sat at 77.

It’s likely that top-tier events like this will continue to be more significant and frequent thanks to the ambient warming associated with human-induced climate change. Average springtime temperatures in Burlington have warmed more than three degrees since the 1940s.