An extremely strong Polar Jet was directed at the PNW (this is the kind of jet stream pattern that produces the occassional snow to sea level in Western Washington and Oregon) which brought in a push of very cold but very dry air. As a result, although it was clear and sunny, day times highes in the region were around 65 and it was close to freezing in many locations (Frost in September in the Willamette Valley is unusual).

This culminated with Eugene setting the all time record low for the month of September on Sept 28.

This jet stream pattern quickly vanished and warmer conditions rapidly ensued. Indeed, Sept 28 witnessed a remarkable 47 degree dirunal variation (31 --> 78).

And, in fact, the persistent Northern flow throughout September, while resulting in clear sunny days, produced and average minimum temperature of 42.6 degrees, the coldest on record does this portend a real winter this year?