Penn State E-Wall. In a typical pattern we have a ridge, trough, ridge, trough and so on. However, vorticity (a measure of spin) from Ana is noted in the model and can be seen as the small "kinks" in the area circled. While Ana will not have a noticeable effect on weather over Montana and points south, it will affect the pattern at upper levels for the eastern half of the country, which will have an effect on temperatures across the region by the end of the week.
Here are the 850mb temperatures off the GFS (left) and European (right) computer models. The robust GFS has temperatures of 10-14C below zero over the Great Lakes, which translates to high temperatures in the 30s. The Euro has highs in the 40s and 50s for the same region. No matter what model proves to be fully accurate, a below normal Halloween should be expected. I would expect the Official CPC 6-10 Day Outlook to be updated in the coming days to reflect this shot of colder air.