Yesterday, I introduced the general meteorological framework for the start of autumn 2015. Today, we’ll be looking at some specifics.
Computer models continue to showcase an iterative evolution of a large-scale upper level trough pattern across the United States by the start of autumn. A series of low-pressure systems will move across the northern U.S. and southern Canada. Each will allow a cool, Canadian air mass to infiltrate the U.S. Over time, the combined effect of the repetitive weather pattern will lead to an upper level trough over the U.S. and an associated cool weather pattern, at least across much of the northern two-thirds of the Nation.
By the first day of autumn, here are some forecast daily average temperatures compared to seasonal averages for the day:
Fig. 2 shows computer model predictions of weather map features, precipitation and temperatures (red lines). Note that parts of the central Appalachians and much of the interior Northwest will have morning lows below 50 degrees.