There are a number of factors influencing the 1-month and 3-month Outlooks for September and September through November. These include a less active jet stream, approaching peak hurricane season, the demise of El Niño, current soil moisture, and long-term trends.
From a climatological perspective, the fall months are usually fairly quiet with a relatively low percentage of annual precipitation for many areas. The jet stream is often weak and frontal passages typically occur with lower amounts of precipitation compared to the spring.
Hurricane activity peaks in September. In the image at right (percentage of monthly rainfall), note the higher percentage over the Southeast, which reflects enhanced tropical activity.
The past weak El Niño has faded and should not have any impact on fall weather, and probably winter weather either.
Many long-lead climate models, including NMME, seem to pick up on wet soils across the North Central U.S. This results in a continuation of above-normal precipitation within these areas.
Outlooks, especially later in the forecast period, align closely with long-lead climate models, especially the North American Multi-Model Ensemble (NMME).
The Fall outlooks are also based on long term trends, especially above-normal temperatures nationwide and the possibility of above-normal precipitation along the Mid-Atlantic region.