To help inspire vertical motion, plentiful moisture at the mid-troposphere level is helpful to create an environment supportive of tropical cyclone development. However, during early September, relative humidity at 600 MB across the deep tropics in the North Atlantic basin is arid. Plentiful moisture at 600 MB is too far to the north to help inspire TC activity.

As the polar region turns sharply colder due to loss of sunlight mid-to-late autumn, cold air is swept across an increase in open ocean water (due to a constricting polar ice cap) that leads to advection snows bringing above normal snowfall to the northern continents most notably Northern Russia.

The unexpected quiet period across the tropical/subtropical North Atlantic basin may be about to end. Tropical waves are stronger as they move across tropical Africa and into the eastern North Atlantic tropics. However, the ECM ENS 15-day rainfall forecast indicates extreme rain across tropical Africa as tropical waves and their attendant convective energy are about to intensify dramatically.

As early September arrives, most striking, is the cool weather set-up in the Midwest/East U.S. centered on the 6-10/8-12-day period. At that time, both the arctic oscillation (AO) and North Atlantic oscillation (NAO) shift to negative phase supporting a high latitude blocking ridge and amplified polar vortex centered on Ontario and the Great Lakes.