A cyclist rides in the day’s diminishing light, Wednesday, June 23, 2021, in Philadelphia,.
Matt Rourke / AP Photo
Heat Dome Triggers Scorching Start to Summer Across Pennsylvania
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Asia Tabb
Matt Rourke / AP Photo
A cyclist rides in the day’s diminishing light, Wednesday, June 23, 2021, in Philadelphia,.
Aired; June 26th, 2025.
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As summer kicks off, much of the East Coast — including Pennsylvania — has already endured a punishing stretch of extreme temperatures, thanks to what meteorologists are calling a “heat dome.” According to ABC 27 Meteorologist Dan Tomaso, the phenomenon is a classic — but intense — example of high-pressure systems trapping heat across large regions.
“We get under what’s called a heat dome, which is basically a supercharged area of high pressure,” Tomaso explained during an interview on The Spark. “It expands through the entire column of air and keeps out thunderstorm activity, which you actually need cooler air to form. So instead, the heat just builds and builds.”
Tomaso emphasized that this persistent weather pattern is difficult to break and that the combination of high heat and humidity this time made the dome especially potent. “You would think we couldn’t get as hot because a more humid atmosphere takes longer to heat up,” he said. “But we had the double whammy — the heat and the humidity — and it led to near-record numbers, not just here in Pennsylvania, but across the board.”
This recent dome shattered expectations across the state. “Even in far northern Pennsylvania, where people used to escape the central PA heat and humidity, there wasn’t much of a break this time around,” Tomaso added.
While summer had a slow start — with rainy, milder weather early in June — conditions changed quickly. “We were on the eastern side of this building high-pressure ridge,” Tomaso said. “As that cooler weather lifted, it allowed the heat dome to expand eastward. By midweek, we were directly under it.”
According to Tomaso, the signature of a heat dome is visible well above ground level. “We look at the upper-level weather patterns — not just what’s happening at the ground, but at flight levels for airplanes and halfway up between the ground and sky,” he explained. “When you see a little hill form in the atmosphere on our maps, that’s how we know we’re under it. And we’ve been underneath it for most of the work week.”
