The strong cold front stretching from northern Mexico to a large storm in the Ungava Peninsula will be the first in introducing polar air masses into the lower 48 states. The process is slow, since actual cold air presence in northern Canada is limited. It is better to call temperature trends “cooler” and not “colder”, since the incoming regime is polar (cP) as opposed to Arctic (cA) in nature.
Another issue to watch will be in the medium range. The next frontal structure moving around the upper low and trough complex in eastern Canada will interact with a tropical feature (possibly a hurricane) turning north from the equatorial Pacific Basin below Mexico. Initial impacts will move from the Mexican coastal resorts to the Rio Grande Valley, then into the Mid-South. This merger could produce widespread torrential rainfall along the way, with severe thunderstorms that will spread through Appalachia into the Eastern Seaboard.
You can easily see that we have a pattern setting up, with each storm drawing down cool air. The warmth under ridging in the West and the Prairie Provinces should not generate much cooling demand (but CAISO districts may see some spikes due to hot/dry afternoons). The question then becomes: does the ridge axis stay put, or give way to an El Nino inspired split flow with precipitation hitting California? The various ensemble platforms say no change in this amplified, but contiguous, upper air pattern through October 21.