Scotty Johnston has been leading fall foliage tours for more than 40 years, becoming so conversant with the whys and wherefores of turning leaves that he has been dubbed the nation's only "foliologist." But his prediction of a beautiful and colorful fall foliage season is based only partly on science.

"I am confident for one reason — we've never had a bad one," says Johnston, who works for Tauck World Discovery, a tour company with a full slate of foliage tours. "Even what is an average year for us who live (in New England) is eye-boggling, especially to the visitor."

And though it's still full-bore summer, the leaf-peeping season is just a few weeks away from beginning in Canada and parts of the Northern United States, where trees start turning rich autumn colors as early as the beginning of September.

Johnston does bring some science to bear on his annual color predictions, which he publicizes in a toll-free hot line suggesting hot spots for foliage tourists. But he says the factors that combine for the best color — adequate springtime rain, cool, sunny fall days, low wind — don't make much of a difference from year to year to affect the experience of the average tourist. More important is knowing when and where to go, he says.

Choosing the optimum week or two can be a bit of a gamble — just slightly easier than trying to hit Washington, D.C., at the right time for the cherry blossoms. Most important to keep in mind is that both latitude and altitude play a role. The northernmost areas with the highest elevations turn first, with the color changes moving southward at the rate of roughly 25 miles per day, he says. Inland areas, with chillier temperatures than those on the coast, also turn earlier.

According to this timetable, the first areas to enjoy fall colors are the Canadian and Colorado Rockies, which begin the North American foliage season as early as the first weeks of September, Johnston says.

Next comes eastern Canada, such as Montreal and Quebec, in mid-September. By late September, look to the highest and farthest north portions of New England, in spots such as Vermont, New Hampshire and Maine.

A week or two later, in early October, the prime spots have moved south to the lower portions of those states and neighbors in Massachusetts and Connecticut. The New England season is pretty much finished by the third week of October — about the time for peak color in such states as Pennsylvania, North Carolina, West Virginia, Virginia, Tennessee and Kentucky. Johnston considers the last good color to appear in Missouri's Ozark Mountains and Arkansas' Ouachitas in late October and early November.

There are exceptions, of course. Nova Scotia, for instance, enjoys its peak color in mid-October — it turns later than its fellow Canadian sites because no part of the land mass is more than 35 miles from the Atlantic Ocean, which provides a moderating influence on both temperature and color.

And Texas, of course, is on its own timetable.

Howard Rosser suggests that Texans keep their eyes on southern Arkansas and Oklahoma; Texas leaves traditionally start turning a week or so after those areas, usually by late October. The season extends into late November in the southernmost regions.

Rosser says good spring rains and a summer that hasn't been as severe as usual should produce a good fall season. Traditional highlights are Daingerfield State Park, Big Thicket and Lake o' the Pines. Rosser's association can suggest a number of scenic drives, with natural stopping points in such towns as Pittsburg, Sulphur Springs and Marshall.

Other Texas spots to watch:

Lost Maples State Natural Area, in the western part of the Texas Hill Country near Bandera, is perhaps the best-known (and most-visited) Texas site for foliage. Leaf-lovers congregate in this area, usually beginning in late October, for hiking as well as driving tours.

Canadian, up in the Panhandle, is usually the first area in Texas to turn. in early to mid-October; hot spots include the natural area at Lake Marvin, part of the Black Kettle National Grasslands.

Caprock Canyon State Park, the lesser-known neighbor of Palo Duro Canyon in western Texas, puts on a good natural show, usually beginning in mid-October.

Guadalupe Mountains National Park, way out in far West Texas, about 100 miles east of El Paso, offers foliage in a spectacular if remote setting; the star of the show is McKittrick Canyon, where the most vivid colors are present from roughly mid-October to mid-November.

But whether you're headed to New England or East Texas, Johnston offers good advice for would-be scenery spotters: Use good maps. Plan a route judiciously, not trying to cover too much ground in one day. Allow plenty of time, perhaps twice as much as you think you'll need; the best views are often along narrow, two-lane roads likely to be crowded with other tourists.