October 30, 2024
Thom Carroll/for PhillyVoice
Philadelphia hasn't gotten measurable rain in about a month, and the dry weather can cause the fall foliage season to speed up.
The record for longest streak of dry days in Philadelphia was broken Tuesday — as the city hit 30 consecutive days without rain — and the lack of water could have dire consequences for the fall foliage.
Philly has not seen "measurable precipitation" since Sept. 28, with only small traces occurring on Sept. 29 and Oct. 24, according to the National Weather Service Philadelphia/Mount Holly. The previous record of 29 consecutive dry days was set 150 years ago, when not even a trace of precipitation was recorded in the city between Oct. 11 and Nov. 8, 1874. Precipitation records stretch back as far as 1871.
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This extended dryness can have an effect on fall foliage, speeding along the leaves' color-changing process.
"In the fall, deciduous trees — meaning trees that shed their leaves — understand ... that it's no longer economical to produce chlorophyll, which is the pigment that helps leaves harvest the energy of the sun, and that energy is used in the process of photosynthesis, which relies on water," said Ryan Reed, a natural resource program specialist with Pennsylvania's Department of Conservation and Natural Resources. "So when the water is shut off, there's really no way to do photosynthesis anymore, and so the green pigment, chlorophyll, ceases to be produced in the chloroplast."
From there, the leaves' underlying pigments — like yellows, oranges and reds — begin to show through for a couple of weeks and then those colors degrade, the leaves turn brown and the trees eventually shed them.
"When it's very dry, it tends to speed up the process a bit, the coloration process, and then the eventual turning brown," Reed said. "When it's extremely dry, it does lead to a pretty quick reaction, and so you see the leaves turn color pretty rapidly. And that is, I suspect, why we had a little bit of an earlier fall foliage than what we've had, say, in the past decade. I did notice leaves colored up a little sooner this year."
Since late September, the DCNR has been releasing weekly guides — generated by Reed — to the progression of colorful autumnal leaves across Pennsylvania's 2.2 million acres of state forestlands and other public viewing places. The final guide of the season will be published Thursday, Reed said.
Reed said that dry weather can be a blessing and a curse of sorts when it comes to its effect on fall foliage. He said that despite the fact that it speeds up the process, dry autumns have produced some of the most vibrant foliage he's seen, and this year has been no exception.
"I say it can be too much of a good thing," he said of dry weather. "So if it's a dry fall, then you're setting up for a pretty nice fall foliage season because dry does not allow for the proliferation of fungus. There's a lot of leaf fungus that thrive in the fall and when it's really wet. So if it's drier, there's less fungus. And generally speaking, dry is a stimulant that brings forth color in the leaf. So if it's a pretty dry fall, you're going to have generally a pretty vivid fall, but if it's too dry, then you're going to have that process happen really fast. (You'll) see some more browns mixing in, and you're going to see the leaves shed more quickly."
As for this fall, despite this year's "bizarre" weather, which Reed described as a "yo-yo" between wet and dry spells, he said the foliage should continue triumphantly at least a few more days — as long as a wind doesn't knock the weakened leaves off the trees.
"I think there'll be still some really nice color to see around Southern and Southeastern Pennsylvania for at least another week, maybe even up to 10 days," Reed said.
The latest map from the U.S. Drought Monitor has most of Philadelphia in a "moderate drought," with a small portion in "severe drought." But while New Jersey issued a drought watch earlier this month, Pennsylvania had not yet taken that action.
Regardless of whether it's officially being called a drought, the reason for Philly's extended lack of rain has to do with air pressure.
"Basically, the way the weather pattern's setting up, high pressure has been kind of dominating the weather across the area," said Lee Robertson, a meteorologist with NWS Mount Holly. "And any weather systems that do move into the area, like any cold fronts or other weather patterns, basically dry up as they approach our area. Then the high pressure re-establishes itself and keeps the dry weather across the region. So it's just been an endless pattern of that dry, continuous weather."
There is a chance of rain Friday, but Robertson does not predict there to be much precipitation so the dry streak could continue.
"Hopefully things will begin to change here soon, and we'll get some wetter weather, but that does not look like a good possibility for at least the next week or so," Robertson said.
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