SAM DEAN The Roanoke Times

Partly painted crimson by the approach of autumn and partly still clinging to summer green, a leaf just off the Blue Ridge Parkway hangs in the balance.

Kevin Myatt is The Roanoke Times' weather columnist.

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"It should be great."

That's the fall foliage forecast from John Seiler, a professor of forestry at Virginia Tech who specializes in tree physiology.

Every year, I get questions about fall foliage and how the weather we've been having will affect it. And, every year, I am at somewhat of a loss to answer the questions.

Some of that is because I haven't really studied biology much since my sophomore year in high school.

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A big part of it though, is that during my hiking and weather observation over the years, I have seen opposite weather conditions produce similar foliage results, and similar weather conditions produce very different results. Of course that's based on subjective judgment and memory, so I hardly have the facts on hand or the expertise to explain those occurrences.

That's why I found a foliage expert -- with the advice of a commenter on my blog -- to help me answer questions this year.

So what will make this year so great? In e-mailed answers to my questions, Seiler gave two big reasons.

n "Clear sunny days and cool nights are best for color development." Check.

That exactly describes the weather we've had most of this past week, and after a few showers on Saturday, that kind of weather is expected to resume early next week -- though maybe not quite as cool as Thursday morning's low temperatures.

n "Sufficient soil moisture is also important so that the leaves do not fall off prematurely." Check.

Spring provided widespread and abundant rain in Southwest Virginia. The summer rains were more spotty, with some areas getting flooding rains and others coming out somewhat less than normal. Still, Southwest Virginia as a whole is nowhere close to being in a drought as it has been the past several years.

"Long droughts are probably the worst problem," Seiler said. "When the ground is dry the leaves often come off early with little color."

Even a few dry weeks, like we had from late August to just a few days ago, were cause for concern. "I was getting a bit worried about how dry it was getting but last weekend's rain should have helped that a lot," Seiler said.

Widespread rainfall of 1 to 3 inches across most of Southwest Virginia, with some locally heavier amounts, has soaked the ground.

This will allow leaves to continue to draw moisture from the ground on those warm, sunny days even as the longer, cool nights coax the leaves into producing hormones that wind down their life cycle and lead to color changes. So the leaves won't just cut loose and fall off, but will gradually change color.

That is, if we can avoid lots of windy, wet weather, which can pull leaves off the trees prematurely.

One of Seiler's answers challenges a widespread belief. He described the occurrence of a first frost as "not important" in the development of colorful foliage.

"The primary trigger is shortening days and that happens whether we get a frost or not," Seiler said.

Being a tree biologist, Seiler can find the beauty of trees in any season, and in any fall regardless of whether people driving the Blue Ridge Parkway or hiking the Appalachian Trail deem it a "good" color year. Seiler believes the reason some autumns get labeled as bad color years is more because "no one takes drives or hikes" in rainy weather, and the sunshine may not be reflecting favorably off the color that's there on cloudy days.

"In my opinion every year is a good year," he said. "Simply due to bad weather weekends (rain and wind knocking off leaves) certain years become known as bad years."

Weather Journal appears on Monday, Wednesday and Friday.