PEORIA — By the days on a calendar, fall officially begins Friday at 3:02 p.m. By the laws of nature, it is already well underway.

Just look outside. There's color on the bluffs.

"Obviously this is the time of year when trees begin their change," said Mike Miller, the former chief naturalist at Forest Park Nature Center who is now a supervisor with the Peoria Park District. "But because of the dry conditions in the region, I'd say we are are seven to 10 days ahead of the norm."

It's the kind of early foliage season that begins with a vague notion of change in the first two weeks of September. That feeling then evolves into a "Huh? The leaves seem to be turning earlier than normal this year"-articulated realization just before the start of the autumnal equinox.

Here's why:

Lack of rain.

Rainfall in the months of June, July and August was a combined 3.38 inches below normal. September continues to be dry. Even with a half-inch rainfall Monday night, the month's precipitation is 1.42 inches below normal. The area is not in official drought, but it is close.

"The classification 'D1' is a moderate drought," Chuck Schaffer, a meteorologist with the National Weather Service in Lincoln, said this week. "Peoria is D-0, which means abnormally dry."

While there is almost no rain in the extended weather forecast, neither are there any fall-like temperatures. The September heat wave — call it the equivalent of a March blizzard, neither rare nor welcome — will extend through the weekend with highs in the 90s. Meanwhile, the chlorophyll production in trees continues its annual shutdown despite the summer-ish temperatures.

"The top layer of soil, the top eight inches, is very dry right now and that is where the majority of a tree's root system is," Miller said. "Trees go dormant early in dry soil and that's when you see the colors emerge."

Trees produce sugars to produce chlorophyll to produce green leaves. It takes water to fuel that process, Miller said. When water is lacking, the tree starts shutting down in preparation for the winter section of its life cycle.

First to change are trees with compound leaves — locust, walnut, buckeye, American ash. The next wave includes the reds and oranges of maple trees and others. Miller said the hot, dry conditions should not affect the brightness of the colors.

Daylight also has a direct effect on the end of the summer life cycle.

"As the cycle edges closer to 12 hours of light, 12 hours of dark, it's another trigger that things have wound up and it's time to finish," Miller said.

While acknowledging that he is no arborist, meteorologist Schaffer is a keen observer of outdoor events.

"I had noticed the trees seemed to be changing," Schaffer said. "A drier summer tends to lead to an earlier than normal fall."

So, there you go.

Scott Hilyard can be reached at 686-3244 or by email at shilyard@pjstar.com. Follow @scotthilyard on Twitter.