It was one my favorite statistics from Monday's storm -- in the middle of the day it was 28 degrees in Seattle -- and 34 in Barrow, Alaska. You'd be hard pressed to get any closer to the Arctic than Barrow, one of the most northern points in the U.S. Yet here we are some 2,000 miles of Barrow and somehow 6 degrees colder. (And yes, the 34 was a record high temperature for Barrow.) Dillingham, which is out near the start of the Aleutian Islands, was up to 41 on Monday.
As you might suspect, the two weather extremes are related. Typically when Seattle is at its coldest, Alaska is at its warmest.
To get really cold arctic air into the Northwest, we need a very strong ridge of high pressure to form out in the Gulf of Alaska. Not only does this send warm air well up into the Last Frontier State (yes, that's their unofficial nickname. I looked it up) but it sends the jet stream way to the northern reaches of the Arctic Circle, where it picks up some really cold air.
The jet then zooms south along the east side of the high pressure ridge, typically hugging the B.C. Coastline and then crashing into the Northwest, where it turns back to the east and moves on its way into the U.S. interior. This acts like a conduit to transfer cold air from the Arctic Circle down into the southern interior of British Columbia. That's how we get the cold air.
Here is an upper air chart showing Monday. It doesn't go all the way to the top of Alaska, but you get the idea. Note the warmer colors in Alaska, and how the cold air just rolls down into the Northwest:
Think of it like a pitcher of cold air that sits high on a shelf. The ridge is like a broomstick that tips over the pitcher and spills its cold air over and down -- in this case, into Canada and the Northwest.
And that is how Seattle can end up colder than the northern reaches of the Arctic.
But for snow, we also need moisture, and that's where the jet stream comes into play again. If you think of the jet stream as the storm track, you can see how this presents problems. Storms go waaay up into the arctic, get really, really cold then get funneled south -- as what happened Monday. The storm followed the jet south along the B.C. Coastline. The amount of moisture the storm gathers is dependent on how much time it spends over water.
If it tracks over land, it's usually colder but not as moist. If it goes too far over water, the storm gets wetter but warms a bit from being over the 50 degree Pacific Ocean. Best case scenario is a mainly land track that curves out over the water to pick up some moisture then turn inland before it has a chance to warm up much.
Having the storm come in just to our south is crucial, because that lower pressure will draw the cold air from B.C. through the Fraser Valley into Western Washington. If the storm goes to our north, we stay on the warmer, south-wind side of the storm.
In Monday's storm, the issue was that the models had the storm staying out offshore as it passed Washington and was supposed to curve inland into southern Oregon keeping the snow well to our south. Instead, it came inland around Forks and voila, it's #snomg. Then it did all crazy things like making Convergence Zones plus draw in that strong north wind that sent temperatures well down into the low 20s.
Of course, to get snow, we don't need to have the storm come in from the north. In fact, some of our greatest snows are when the storm comes in from the west. For that setup, we need to have the arctic air engine happen first, which pools the cold air in Canada, then a big storm come in -- again just to our south -- that draws in the cold air, then slams a big what-would-be-rain-storm on top of it, and now you have a big snow event. This scenario happened a few times in Dec. 2008 and was the primary reason for the Dec. 1996 big snow.
Long range forecasts show the arctic chill returning to Barrow, which in turn means temperatures will begin to moderate here, and probably none too soon for many. But next time you see such a frigid forecast for Seattle, feel free to head to the warmth of Alaska.
Or, I suppose Hawaii works too...