With rainfall totals radically below normal during the last month, the government's Climate Prediction Center says drought conditions are "likely" this month throughout most of the Mid-Atlantic region into southern New England.
While shower prospects are decent for Sunday night and Monday, Sarah Johnson, a lead forecaster at the National Weather Service in Mount Holly, said Wednesday it was impossible to know how generous they might be with rainfall.
Meanwhile, officially no measurable precipitation has fallen in Philadelphia in two weeks, and in the 30-day period that ended Wednesday, rain in every county in Pennsylvania, New Jersey, and Delaware has been significantly below normal, according to the Middle Atlantic River Forecast Center. Absent any tropical-storm remnants or juicy frontal systems, rain has been about a third of long-term averages in the major river basins, and only 25 percent normal in Philadelphia's neighboring Pennsylvania counties.
Johnson said the region has been capped by a persistent ridge of high pressure, or heavier, sinking air that has discouraged the rising currents needed for rain — "quite a contrast" to the active, wet pattern of the summer.
Along with the harvest of dust in backyards and walking trails, one symptom of the recent dryness has been showing up in the daily pollen samples examined by Dr. Donald J. Dvorin of the Asthma Center.
"I'm seeing a lot of leaf debris and particulate matter in the air sample," he said. We have noticed a rather robust early – and quite brittle – leaf fall."
Given the generous leaf-nourishing rainfalls of summer, Ryan Reed, the foliage forecaster at Pennsylvania's Department of Conservation & Natural Resources, had foreseen a "spectacular" foliage season. He has tempered that a bit.
"Indeed, the lack of rainfall for the last month has conspired to downgrade the outlook somewhat," he said. His outlook will be updated Thursday.
Sunny days and cool nights help stimulate the biological processes that produce the colors, although the recent widespread warm spell evidently was an impediment to progress.
Up in the mountainous far-north country, which is approaching peak season, the curtain was raised a little earlier than usual, but the September heat "put the brakes on the fall foliage transitions," said Visit New Hampshire's Kris Neilsen.
But the show will go on. Reed said the lack of rain likely would apply only to the sugar maples, which would tend to mute the red pigments.
Maples notwithstanding, he said, the state has "140 species of deciduous trees, shrubs, and vines; one realizes there is still much to be excited about."
Colors along the northern tier of the state and the Poconos could peak during the Columbus Day weekend.