Another heat wave gets underway in Philly, and trees are losing their leaves too soon

The premature leaf fall that has littered some of the region’s lawns, driveways, fields, and pathways with harbingers of October hasn’t escaped the attention of Brian Haines.

“I noticed that on the side of the road when I was going to work today,” said Haines, who as the science and operations officer at the National Weather Service Office in Mount Holly would be well aware that it sure doesn’t feel like October. And right about now, a whole lot of people probably wouldn’t mind if it did.

Those defeated leaves are part of the fallout of a quite-remarkable and unpleasant run of summer heat.

And after a one-day break, another heat wave got underway Tuesday when temperatures reached 93, the 27th time this season it has reached 90 or better officially in Philadelphia. Normal for an entire year is 30, and August is just beginning.

» READ MORE: Philly declared its first heat emergency of year two weeks ago

On Thursday a record could fall, and the National Weather Service has issued a “heat advisory” as the heat index is forecast to reach 100. Philadelphia has recorded six heat-related deaths so far this season, and it’s possible the city will declare the season’s second heat-health emergency later this week.

After a July that roasted Philly with its second-warmest month on record and a three-week run in which the temperature never got below 70, perhaps August got jealous.

For the trees this has been an especially punitive period. The combination of 90-plus days and lack of rain constitutes “a tree’s worst nightmare,” said Bill Cullina, executive director of the University of Pennsylvania’s Morris Arboretum, in Chestnut Hill.

Rainfall in Philly the last 30 days has been about a third of normal, he said, and the latest U.S. Drought Monitor map has much of the region in its “abnormally dry” zones.

The trees are shedding leaves in response to the heat and dryness, Cullina said, and a tree can develop a “vascular embolism,” in which air bubbles form and interfere with its efforts to draw water from the soil.

Said Ryan Reed, a program specialist with the Pennsylvania Department of Conservation and Natural Resources, “Dropping leaves is just a water conservation measure,” adding that it isn’t all that uncommon in summer.

The dryness also has helped bump up temperatures, since the sun is able to direct more energy toward heating, rather than evaporating water.

If heat does have an upside, thankfully, it might be too hot to rake.

And the summer dryness could pay dividends in the fall, Reed said. “Drier conditions are less hospitable to leaf fungi ... which can really put a damper on fall color.”

The heat will pick up steam the next few days with a high of 98 degrees forecast for Thursday, which would match the record for an Aug. 4.

However, this siege might end up being only half as long as the eight-day heat wave that ended on July 25 and prompted the city to declare its first heat emergency of the season.

When the city declares an emergency, it takes measures that include opening cooling centers. The Philadelphia Corporation for Aging also activates its Heatline, with extended hours.

While Thursday’s heat is looking like a sure thing, Friday’s outlook is more problematic, said Paul Walker, senior meteorologist at AccuWeather Inc. The forecast calls for highs in the mid-90s, but a front will be approaching with hit-or-miss thunderstorms that could put a lid on the temperature.

» READ MORE: Summers are hotter in Philly, data verifies

“It depends on the timing of the storms,” he said. In any event, on Saturday, when temperatures are due to hold below 90, the storms will be “more generous,” he added.

With an official average temperature of 82.1 degrees in Philadelphia, last month was the second-warmest July in 151 years of record-keeping and the No. 2 warmest among all the summer months, according to the Northeast Regional Climate Center.

The temperature hit 90 degrees on 19 different days, and on 15 dates, it didn’t get below 75, tying a record for any month.

Throughout the Northeast, 20 different sites experienced July temperatures in the top 20 for warmth, the center said.

» READ MORE: This has been one of the warmest Julys on record in Philly, and hot across the country

If the government’s long-range forecasters are right, October will have to wait. The Climate Prediction Center’s updated outlook for August favors above-normal temperatures in about 70% of the contiguous United States, with a strong likelihood throughout the Northeast.

In short, August should feel like August — only a little warmer.