I live in SD, and today’s high is supposed to be 64. The average is 85. There has been on and off cold weather ever since April. The last one until yesterday was in early July (the first fireworks show that I went to in a coat) and it seemed like it was over. Nope. I noticed that the jet stream that usually stays up north during the summer months is now south, and that is what it was doing before. It is keeping the cold air here. Just a couple days ago, it was nice and warm. Now it’s not. Why the unusual cold weather and unusual position of the jet stream?
Wildass guess:
Global warming has destabilized the jetstream so that we get winterlike weather patterns during the summer?
Seriously, I think it’ll be next winter before the NOAA figures out what happened this summer.
It’s that damned barb wire fence on the border. It can’t even keep those Canadian cows from crossing the border. Do you really think the jet stream pays it any mind as well?
I don’t know why the jet stream is doing what it is doing, but I’ve found that the easiest way to find out is to turn on the 6:00 KELO newscast, and just wait for Jay to explain it. As I recall, he was often good about throwing in the “why” at some point, if the weather was weird.
According to this link, eastern North Pacific sea surface temperatures have been above normal since around the beginning of May. So, this could have contributed to the large scale ridge of high pressure that has been present for most of the summer over the western U.S. With this ridge in place, most of the central part of the country has been under northwest flow since May, starting with a normal summertime flow pattern back in the middle of spring.
Anyway, since the upper-level flow has been out of Canada along the northern tier of states, the storm systems will be moving from that area, meaning relatively cool weather for much of the summer. So, this is possibly the reason why it has been so cool through much of the eastern two-thirds of the country.
As to why the eastern Pacific water temps are warmer than normal? It could be any number of reasons, from normal climate variability, the timings of the Pacific Decadal Oscillation (PDO, which I haven’t looked into much), the location of the Walker Circulation in the Pacific Ocean, or even the anomalies in the North Atlantic. There is even some evidence that the water temperatures of the North Atlantic are playing a bigger role than anyone would have guessed in the grand scheme of things, and after many years studying ENSO (the El Nino/Southern Oscillation phenomenon), we are only beginning to understand the changes in the North Atlantic.
So I guess it could be summed up as: who knows? 
Vis
Like maybe causing the 30’s dustbowl:
Well, we have been getting a lot of rain this year. A lot more than the last several years that were droughts. But every rain comes with cold weather it seems.