On the heels of one July polar invasion, yet another major cooldown is here to wrap up July.

A large southward dip in the jet stream, also known as a trough, has pumped in cool air from Canada into much of the central and eastern U.S. Forecast high temperatures will be as much as 25 degrees below average for this time of year in some areas, and most of the region will see at least one or two days with temperatures 5 to 10 degrees below average -- a noticeable difference at a time of year when many of us spend the most time outdoors.

(MORE: Coldest July Days in U.S. History)

What was notable about this cooldown is not just the fact that it took place in mid-summer, but that it resulted from a pattern that seemed to be frequently repeating.

The same jet stream pattern this week showed up not only in mid-July, but during much of the January-June time frame as well. As a result, much of the country east of the Rockies was much cooler than average for the first half of 2014, while the West was unusually warm.

(MORE: January-June 2014 Coolest in U.S. In 21 Years)

That's a temperature regime we'll see again this week.

http://www.weather.com/news/weather-forecast/late-july-summer-cool-down-20140724