It still feels like summer across Pennsylvania, but the weather over the next several weeks will help determine whether this fall brings brilliant foliage or a disappointing display.

The annual change actually begins with something weather cannot alter: shortening daylight. As days grow shorter, trees stop producing chlorophyll, the pigment responsible for their green color. As chlorophyll disappears, other colors become visible.

Weather, however, plays a major role in how intense and how long those colors last.

One of the biggest factors to watch through late summer and September is rainfall. Prolonged drought stresses trees, which can cause leaves to turn brown and fall before producing their usual autumn colors. Young and recently planted trees are particularly vulnerable and can benefit from deep watering during dry spells.

Temperatures will matter more as September progresses. Cool nights in the 40s, but above freezing, help trap sugars in leaves and encourage vivid reds. Persistently warm nights can result in less vibrant yellows and browns.

Sunshine is another ingredient in a colorful fall. Bright, sunny September afternoons allow leaves to produce and store sugars, while cool nights help keep those sugars in place. Long stretches of cloudy, rainy weather can produce a more muted display.

Even after brilliant colors develop, the weather can quickly end the show. A poorly timed episode of heavy rain or strong winds can strip trees of their leaves in a matter of hours.

That combination of variables also makes predicting peak foliage months in advance difficult. The timing can shift by a week or more depending on rainfall and temperatures during late summer and early fall.

Source: The Weather Channel