Autumn arrives officially at 4:21 a.m. E.D.T. on Sept. 23, 2015. Astronomical winter, that is. Meteorological autumn began on Sept. 1, 2015. This disparate shift between the two events is linked, in large part, to how weather data is collected and organized (by months) versus how the tilt of the Earth’s axis sits at specific times during the year. Notwithstanding, the arrival of astronomical autumn 2015 is now expected to be meteorologically autumnal, as well, across large parts of the Nation.
Currently (Sept. 7, 2015), an upper level trough (also referred to as a “dip in the jet stream”) is in place across the northwest U.S. This u-shaped part of the upper level wind pattern is often linked to cool air intrusions southward on its western flank and surges of warm air northward on its eastern side. This was the weather system that helped bring some relief to Pacific Northwest firefighters a few days ago. Here, an upper level ridge (an upside down “U” also marked with a saw-toothed orange-yellow line) lies to the east of the upper level trough (Fig. 1).
However, as the next two weeks unfold, according to at least one computer model, the trough pattern intensifies and the upper level ridge weakens. A second trough, albeit weaker than the one in the Pacific Northwest, is expected to replace the upper level ridge (Fig. 2).
This means that except for Texas and parts of the Desert Southwest, seasonably to unseasonably cool air will be in place just as astronomical autumn arrives. The coldest air will lie across the Northwest and Northeast. Figures 3 and 4 show the upper level and ground level weather patterns expected at the onset of autumn. The air will be cold enough across the Northwest for some mountain areas, especially at higher elevations, to see sub-freezing temperature readings.
This pattern will be evolving during the next two weeks. It will not just develop and move into place two weeks from now. As always, specific events this far into the future are open to error. However, right now, it is looking as though summer may be taking a vacation from, well, summer, at least in many parts of the U.S.