Fall arrives in Philly Sunday. Will the foliage and frost come early? And other seasonal questions.

Judging from the precocious leaf fall, it’s reasonable to believe that the trees have bought into the pumpkin-spice, Halloween orange-and-black seasonal creep movement.

The astronomical fall — when the sun will beam directly on the equator — doesn’t occur until 8:44 a.m. Sunday.

But not only have the trees been littering lawns with brittle cinnamon leaves in an amazingly dry period, they have been dropping clues about the course of this year’s foliage show — nature’s annual compensation for erasing more than 2½ hours of daily sunlight between Sept. 1 and Halloween.

Just how the foliage season will play out is one of the cosmic questions we address as the sun’s direct light begins its ponderous southward droop.

The foliage show may be jumping the gun in the Northeast, according to foliage experts, and that’s all about this dry spell. Officially, Philadelphia has had just 0.36 inches of rain this month, and that’s more than Bridgeport, Conn., or Bangor, Maine.

Jim Salge, a meteorologist in New Hampshire who is the veteran foliage forecaster for Yankee magazine, said he expects an earlier-than-usual color change, at least in northern New England, and a short season.

Dryness in Pennsylvania and cool nights earlier in the month “may speed up the coloration process,“ said the state forestry bureau’s Ryan Reed, author of the state’s weekly foliage reports, and may shorten the season.

But both Salge and Reed agree that the leaf peepers in the Northeast’s prime viewing areas should be in for quite a foliage bouquet. Said Reed, “Dryness can actually increase the vividness of the colors as long as the leaves don’t dry out too much.”

No deficit-ending rain is imminent, but showers are possible Tuesday night through Thursday, and NOAA’s Climate Prediction Center has the odds favoring above-normal precipitation next week.

Rain-deprived Septembers aren’t all that unusual around here. For example, only 0.21 inches of rain was measured in Philly in 2005. The normal total for the month is 4.4 inches.

Historically, however, September has been one of the wetter months of the year, and that has a lot to do with tropical storm remnants, which wrung out 10-inch-plus amounts in 1999 and 2011.

Some of the best minds of meteorology have been asking the same question.

Forecasters had warned confidently that this could be a record-breaking Atlantic tropical storm season with ultra-warm waters in the hurricane basin. Yet, so far only seven named storms have formed, three less than normal for this point in the season.

For the Philly region, this hasn’t been the year of the remnant, but bear in mind that the season doesn’t end until Nov. 30.

It is well-established that growing seasons are getting longer and official first freezes — when the temperature drops to 32 degrees — are coming later. On average they have arrived in Philly the first week in November.

However, predicting a first frost is more problematic, since it is so tied to terrain features, and frost can occur at temperatures above freezing, around 36 degrees, says Lee Robertson, meteorologist with the National Weather Service Office in Mount Holly. Surfaces where frost forms can be cooler than the air, think of car roofs.

Unless you live in Center City or right along the Delaware, it’s a good bet that it will fall to 36 before it does in the tropics of Philadelphia International Airport, where the records are kept.

Officially, Philly has recorded its first 36 as early as Sept. 25, in 1963, and readings of lows of 36 or lower have been recorded at least once on every date from Oct. 8 on.

The average date for a first measurable snow in Philly is Nov. 21. Normally, just over 2 inches of snow has fallen upon Philly officially before the winter solstice. The earliest snow, 2.2 inches of it, occurred on Oct. 10, 1979.

The latest? It didn’t happen at all in the winter of 1972-73.

Expect to see a flurry of winter outlooks in the next few weeks, but the hurricane season suggests a certain fallibility in the long-range forecasting business. Don’t believe anything until you see the whites of the flakes.

Predicting the behavior of the chaotic atmosphere remains a challenge, but not quite in the league with predicting the behavior of the sun and the solar storms that ignite the northern lights, in the view of Space Weather Prediction Center’s William Murtagh.

He has observed, “I remind the meteorologists, ‘You guys got the first 10 miles. I’ve got the other 93 million.’”

But the solar storm activity has reached a 23-year peak and this may go on for a while. The center’s Rob Steenburgh says it is entirely possible that some night the aurora will appear over the Philly region.

“We still have a shot —or shots,” he said. If they do happen, expect no more than a few hours’ notice.

Any night now, astronomers tells us, a new star as bright as the North Star will appear over Philly, the result of a cataclysmic explosion that occurred 3,000 light-years ago on a distant nova. It’s just a matter of when the light finally appears. We’re waiting.

More predictably, the second supermoon in two months reaches the instant of fullness on the morning of Oct. 17. It will be the closest-approaching full moon of the year, only about 222,000 miles from Earth.

Not only will it appear larger and brighter than other full moons, it will be all but technically full when it rises before 6 p.m. on both the 16th and 17th, so it will reach peak brightness when most people are awake.