‘Abnormally dry’ weather in the Philly region is giving temperatures a boost as city extends heat emergency

Brittle leaves are littering some of the dormant grasses and cracked soils, and the fact that the government has identified parts of the region as “abnormally dry” speaks to one reason that Philadelphia might reach 100 degrees on Sunday for the first time in 10 years.

While the heat index got as high as 104 degrees in Philly on Thursday, when the high reached 96, the weekend promises to be even more blistering. The city announced it was extending its “heat health emergency” through Sunday.

“The weather is dangerously hot, so I implore Philadelphians to continue to look out for their neighbors and loved ones to make sure they’re safe,” Philadelphia Health Commissioner Cheryl Bettigole said in a statement. “You literally can save someone’s life this weekend.”

The temperature run-up has something to do with the rather-sudden dryness, meteorologists say.

Leading up to the heat wave that began Monday, forecasts were liberally sprinkled with shower possibilities, but through Wednesday a mere 0.13 inches of rain has landed at Philadelphia International Airport since July 7. Isolated showers popped up Thursday afternoon, but most of the region was subject to another nonstop sun-baking.

Overall, rainfall in Philly, as well as in Bucks and Montgomery Counties, has been about a third of normal in the last month, according to the Middle Atlantic River Forecast Center, and the U.S. Drought Monitor on Thursday placed portions of Bucks, Burlington, and Camden Counties in its “abnormally dry” zones.

» READ MORE: Philly's heat waves have been shorter, even though it's hotter

The harvest of dust has contributed to the heating, said John Feerick, senior meteorologist with AccuWeather Inc. When the ground and foliage are moist, some of the sun’s energy is diverted to evaporation. When it’s bone dry, the sun can cook away.

If it does reach 100 degrees on Sunday officially in Philadelphia, it would be the first time that’s happened since a particularly dry July in 2012. “It’s hard to get to 100,” said Feerick. The record for a July 24 is 98 degrees, set in 2011.

In records dating to 1874, the temperature has reached 100 degrees on average every four years — five times in 1988 — but not once in the last 10 years.

Feerick and other meteorologists theorize that although summers have been warmer, increased atmospheric moisture around here has been putting a cap on daytime temperatures. That coincides with a climate-change symptom: Warmer air can hold more moisture.

» READ MORE: Philly summers are 3 degrees hotter on average since 1970, new data show

More moisture, which would increase the chances of heat-wave-breaking thunderstorms, also might explain why the region has avoided long-enduring hot stretches, they say.

Another factor holding readings below the century mark could be the siting of the official thermometer at Philadelphia International Airport, which is near a swamp and the Delaware River.

» READ MORE: Where is temperature measured in Philly, and why?

While rain has been generally scarce recently, the region remains generously coated with atmospheric moisture. That’s life in an area situated close to the Atlantic Ocean and Delaware Bay.

That humidity is having a powerful and potentially dangerous effect at night. Water vapor at night inhibits the release of daytime warmth.

Overnight warmth is especially hazardous for elderly and vulnerable people who live alone, particularly those living in rowhouses that can heat up in a hurry after sunrise if they don’t have a chance to cool off at night.

Thursday morning’s low, 79 degrees, came close to a record for the highest minimum temperature for the date, and it’s not going to get better for awhile in what is likely to be the longest heat wave in nine years.

“The duration and overnight low temperatures are a big concern with this event,” said Sarah Johnson, warning coordination meteorologist at the National Weather Service Office in Mount Holly. “Overnight lows within the city are not forecast to get below 75 degrees throughout the weekend,” and perhaps not below 80 early Monday.

Monday will be a “transition day,” said Johnson’s colleague, Amanda Lee. “We’ll have temperatures becoming a little cooler ... relatively speaking,” she said. On Tuesday, readings are expect to fall short of 90.

They may make it back to 90 Wednesday and Thursday, but with “definitely much lower heat indexes,” she said.

As for a decent watering, showers are possible Monday, but given the recent rainfall luck, don’t get your hopes too high, AccuWeather’s Feerick said.

“The old saying goes, when in a drought, forecast a drought,” he said. “Right now it’s only a short-term dryness. The ground water is fine.

“I hope it stays a short-term thing.”