By Joe Holley
Washington Post Staff Writer
Wednesday, October 10, 2007; B01

With autumn-like weather expected to sidle into the mid-Atlantic today, perhaps Washington-area residents will at last begin to see fall itself -- the glorious changing of the leaves. Or maybe not.

The drought and summer heat that won't leave -- another record was set yesterday -- might result in a less-than-spectacular fade to brown.

"We have had such drought, and the woods are completely green," said Joel M. Lerner, a Capitol View Park landscape designer and a regular Post gardening columnist. "Even the tulip poplars are still in full leaf. I'm flabbergasted. Plants should be starting to color just about now."

"We're seeing very little color change in the mountains," said Chris Thomsen, an assistant regional forester with the Virginia Department of Forestry. "It may not come until early November, and it may be a one-weekend-type deal, with mostly browns, reds and purples."

Predicting the leaf change is an inexact science, but colors in the Washington area are usually at their most spectacular between mid-October and early November, depending on a mysterious combination of temperature, sunlight and moisture.

What's actually happening is that photosynthesis is slowing, which causes trees to lose chlorophyll. The fading chlorophyll exposes other color pigments, called carotenes (oranges) and xanthophylls (yellows), that were in the leaves all summer but hidden by the green chlorophyll. Anthocyanins also are forming, which accounts for the red. Tannic acid in some trees causes leaves to brown and die instantly, without a striking blaze of color.

The ongoing regionwide drought could have a similar effect on the autumn palette. Or it could increase the percentage of reds. Or delay the onset of the transformation or shorten its duration.

"Trees around town are pretty tired," said Richard Olsen, a research geneticist at the U.S. National Arboretum. "They don't care how they get into fall; they just want it to be over with."

He pointed out that ideal conditions for striking fall foliage include adequate moisture, moderately warm days (highs in the 70s) and cool nights (low in the 40s). "We haven't been getting those," he said.

Olsen noted that isolated pockets across the region could still be spectacular, but in general the vibrant colors won't be there -- even if healthy rains and cooler weather begin immediately.

Cooler weather is supposed to move in today; showers started yesterday, but by late last night, only Annapolis and a few other areas had recorded as much as one-tenth of an inch of rain.

Temperatures rose to 94 degrees at the region's airports yesterday, setting records for the day at each. At Reagan National Airport, the reading was four degrees above the old record, which was set in 1939.

Yesterday marked the third successive day of 90-plus temperatures -- a phenomenon, at least since the National Weather Service started keeping track, that has occurred during the month only once before: Oct. 4-7, 1941.

Michael Halpert, head of forecast operations at the Climate Prediction Center in Camp Springs, told the Associated Press yesterday that the outlook for winter is warmer weather for most of the country, with continued dry conditions in the parched Southeast and Southwest.

That's why Thomsen, who works out of the forestry department's Salem regional office in Virginia's southwestern corner, was worried more about the danger of forest fires than he was the dearth of color.

In the mountains, change will come, he said; it will just be more subtle. "I've been in this business 26 years, and I have never seen an ugly fall," he said.

Staff writer Martin Weil contributed to this report.