Rain totals in Philadelphia for the last six days now are pushing 6 inches, with more elsewhere in the region. Therein resides a certain symmetry. Before the rains started late last week, the accumulated rainfall deficit since April 1 was right around 6 inches. The effects of the dryness and the summer's record heat were evident on the browning trees and on all the lawns covered with premature, brittle leaf-fall. Among the stressed-out species were sugar maples, tulip poplars, and bald cypresses, according to William Elmendorf, a forestry specialist at Penn State. Some of the damage may be irreversible in terms of taking leaves out of play for the annual fall-color extavaganza. "In some cases, the die may already be cast," said Paul Meyer, executive director of Penn's Morris Arboretum, in Chestnut Hill. But it won't be a complete wash-out. The late-breaking rains should help apply the brakes on the leaf-fall. "It will slow it down some," he said. Elmendorf believes that the trees will get a jolt from all the fresh rains, but that they may not reap that harvest until the spring. In any event, Meyer assures, fall will be fall, and do expect plenty of color. Foliage connoisseurs can look to 1995 for encouragement. That year, heavy October rains helped break a serious drought, and leaves were hanging tough deep into November. That is not to say it will happen again. Like love and the atmosphere, the onset, duration and vibrancy of fall colors are subject to maddening variables. Said Meyer, "I don't think anyone really knows what's going to happen." But it almost certainly will become more colorful around here by the end of the month.