It now appears the end of September and much of October will end up cooler than normal for multiple portions of the nation.
The European ensembles develop an Omega Block across much of Canada and far northern portions of the Plains. The Omega Block involves a body of high pressure being sandwiched between two areas of negative height anomalies. If you look closely at the height contours, you may be able to see what looks like the Greek letter Omega (Ω). The Omega Block pattern typically brings hot weather to those in the positive height anomaly region, and cool temperatures to those in the negative height anomalies. The Northeast, Ohio Valley and Plains will likely receive the negative height anomalies, while Canada and Greenland are inundated by positive height anomalies. Should this pattern stay in place for a prolonged period of time, it would not be impossible that we see a cooler than normal September over portions of the country that would be affected by this Omega Block. I do believe that it will be somewhat difficult to maintain this Omega Block, but I do not find it impossible that we see at least a form of a blocking pattern bring cool weather to parts of the United States.
This cool prognosis for September and October is supported further by CFS model projections of a negative North Atlantic Oscillation (bottom image) and negative Arctic Oscillation (top image). These two indexes are closely related, so it is no surprise to see these two going negative at the same time. It is epxected that the NAO and AO remain negative until at least October 10th, when the CFS starts to bring the two indexes to more neutral territory. But even in that situation, the CFS has lowered the projection from neutral to slightly negative in past model runs, which could mean we continue to see a negative AO and NAO past October 10th. Such a development would lead to a colder United States if the rest of the atmospheric pattern cooperates (indications are it would).