A few climate scientists still resist the Rossby Theory explanation for blocked weather patterns, claiming that no mechanism has been proposed. In Mr. President: Regulate Our Emissions Now or We Face Extinction, Thom Hartman presents a mechanism. As the Jet Stream slows down, mountain ranges have more impact on the Jet Stream's direction, just as a near-surface rock will divert flow as a stream flow lessens.
In other words, the Jet Stream has slowed enough for the Rocky Mountains to cause the Ridiculously Resilient Ridge of high pressure responsible for the West's drought.
There used to be a huge temperature difference between the US and the arctic, and that temperature difference – and the pressure difference associated with it – used to maintain a giant wall of air north of us that kept the Jet Stream in place. The Jet Stream – a river of air that runs around the world just south of the Arctic – used to keep cold polar air over the north pole. It did so because of the big difference in temperatures and pressures between us and the Arctic.
But now that the arctic has warmed so much – that 27 degrees F measured just a few weeks ago – that difference in temperature and pressure isn’t so great.
The Jet Stream is going to keep rolling along – in part it’s doing so because of the rotation of the earth, which hasn’t stopped – but without that giant wall of air to hold it in place, the main thing that’s controlling where it can go are mountains. And we have a big chain of them that runs from Alaska down to New Mexico that we generally refer to as the Rockies.
Which may well explain why it is that the Jet Stream is being kept above the area from Anchorage to Los Angeles – keeping that area very warm – while just east of the Rockies it’s drooling all the way down to Texas and Georgia. And when the Jet Stream falls, it lets the cold polar air slide off the North Pole like a man’s hat falling over his face, and that cold polar air makes it insanely frigid here. [emphasis mine]
Since fossil fuel use is actually increasing and the Rockies aren't going anywhere, prolonged drought in the West and Polar Vortex incursions into Central US are likely to become our new normal.
You can see the Ridiculously Resilient Ridge.
The brown to red feature in the Pacific shows how much higher the 500 millibar pressure layer is - as you can see by the color key up to 70 meters higher. "Increased geopotential height deflects storms..."
... scientists a decade ago not only predicted the loss of Arctic ice would dry out California, they also precisely predicted the specific, unprecedented change in the jet stream that has in fact caused the unprecedented nature of the California drought. Study co-author, Prof. Lisa Sloan, told me last week that, “I think the actual situation in the next few decades could be even more dire that our study suggested.” [emphasis mine]
This article doesn't mention the mountains, but attributes the jet stream changes to sea ice reduction creating rising columns of warm air.
“Where the sea ice is reduced, heat transfer from the ocean warms the atmosphere, resulting in a rising column of relatively warm air,” Sewall said. “The shift in storm tracks over North America was linked to the formation of these columns of warmer air over areas of reduced sea ice.”