The weather pattern across the globe is fairly exaggerated right now, and that typically means quite the extremes of weather happen.
We have two very strong ridges of high pressure -- one centered over the heart of Wyoming and another out in the north central Atlantic Ocean. But in between those ridges, you can get some real cold snaps.
That's because the ridge will drive air way up to the north into the arctic regions, then drive it south on the backside of the ridge. To wit: Some very cold air was being pushed down from northern Manitoba and into the northeast and mid-Atlantic states, bringing their first snow of the season. As much as 12-14" fell in some spots, and there were nearly 100,000 people without power across the northeast.
Snow even fell in Ashville, North Carolina.
Here's a snapshot of the current set up over the United States. Blue is cold, red is warm (No, bleary-eyed political news junkies, that's not an electoral map -- although cool down the west coast a bit and...)
But then, there's the second ridge over the Atlantic, driving up some warm air across southern Greenland and pushing the jet stream way up into the arctic, then driving it south again into the UK and western Europe.
Here's a similar map there:
Tuesday, London received its first October snow since 1934, and parts of northeastern Spain had their first October snow since records in the early 1900s. (Thanks to Ed Blanchard at the UW Atmospheric Sciences Department for finding that tidbit.)
In fact, it was warmer today in Kulusuk, Greenland (46 degrees F) than it was in London and Madrid.
This is the type of pattern when you see something referred to as a "long wave" -- i.e., the ridges and ensuing troughs are exaggerated, and thus there are just a few of these waves going around the globe.
And as I mentioned, you tend to get extended warm, dry spells and extended very cold, rain/snowy spells in spots in this situation. The Northwest just happens to have been under the influence of the big ridge that's currently over the Midwest, and that's why it's been so dry here for the past week or so.
Actually, now that I think about it -- this image might help explain long waves too. This is the current pattern, as seen from standing way atop the North Pole looking at the entire Northern Hemisphere:
You can see the U.S. in the bottom center, outlined in white. There's basically four big ridges right now -- the two I mentioned earlier over Wyoming and the North Atlantic, and then another over the Western Pacific Ocean and a fourth over Siberia. Looks like some unseasonably cold weather over northern Japan (upper left -- I know, hard to read since Japan is upside down in this image, but hey, think of it as a real challenging geography quiz. You won't even need to do your Brian Age games today!)
On the flipside, we can have "short wave" patterns, where the ridges and troughs are fairly subdued but more plentiful. This tends to be more changing weather, but not as extreme one way or the other.
Snow fans need to hope for a long wave pattern such as this, but with the big ridge to set up shop out in the Pacific Ocean at about our latitude but about due south of Anchorage for longitude. That would send the air way up into the arctic regions of Alaska and then drive it down through B.C. into our neck of the woods. No sign of that now, but now you know what to root for :)