Why Fall Color Is Delayed in the Eastern U.S.

By early October, fall foliage should be approaching peak levels for most of northern New England into central New York and northern Pennsylvania, as well as in the Upper Midwest. In the West, most locations in the higher elevations  experience peak fall foliage in late September and early October.

(MAPS: Current Fall Foliage)

This year, however, fall color is behind schedule, particularly in the Northeast. In northern Maine, many trees were still green as of Wednesday, as apparent in the photos below comparing foliage color in early October over the past several years, according to the National Weather Service in Caribou.

This delay in the leaves changing color extends southward as well.

The Blue Ridge Parkway in Asheville, North Carolina, should be seeing some fall color with color in mid-October. 

One big factor in the late arrival of fall color is the heat in the Northeast and Southeast, where above-average temperatues have many asking when fall is going to arrive. A persistent ridge of high pressure has allowed the warm temperatures and high dew points to surge northward at times.

How the Weather Impacts Fall Color

The weather before and during the time chlorophyll in leaves decreases affects when leaves change and how vibrant their colors are, according to the U.S. Forest Service.

But the weather is just one factor that influences their color. The shorter amount of daylight is the primary trigger for the color change. As the night's become longer and colder, chlorophyll production slows and eventually stops. Without chlorophyll, the yellow and orange pigments of the leaves become dominant, resulting in the many beautiful colors of fall.

This summer and in September southern New England through the mid-Atlantic and parts of the Midwest experienced record or near-record warm temperatures and rainy conditions.

Low temperatures have been well above average. Boston, for example, saw its warmest low temperature for September with an average low of 62.8 degrees, more than 5 degrees above average. Asheville, North Carolina, also saw its warmest September in terms of low temperatures with an average temperature of 65.9 degrees, just over 10 degrees above average.

Areas farther west, including Chicago and the Twin Cities metro areas, saw a top-10 warmest low temperature in September. 

The stretch of warm nights since autumn began affects when the leaves change color and how bright they will be when they finally change.

The bad news for leaf peepers is that warmer-than-average temperatures are expected to extend into at least mid-October, especially for areas east of the Mississippi River. Additionally, strong winds and heavy rain later in October could cause leaves to fall off trees and limit the time fall color is visible. 

In the West, colder-than-average temperatures are expected the next two weeks. While a ridge of high pressure has dominated the East, a southward dip in the jet stream or trough has developed over parts of the West and has allowed colder temperatures to slide south.

This colder pattern has developed in the northern Rockies and snow has accompanied the chilly conditions. East Glacier National Park received an estimated 18 inches of snowfall as of midday Wednesday.

Heavy, wet snow can bring down trees or cause them to shed their leaves.

Even without snow, freezing temperatures destroy the ability of leaves to manufacture red and purple pigments. As a result, an early frost or freeze can cause leaves to fall from trees sooner.