Arctic Blast No. 1
As indicated in my long-range winter forecast, the super-mild December of last year will NOT repeat itself. In fact, December 2016 should end up with temperatures below normal (last December was 13 degrees ABOVE normal). The first of the Arctic blasts will be moving in for this weekend, with another, even colder one, coming later next week.
The first blast has already affected much of the country to our west. This map from today shows a huge area of waaaay below normal temperatures that entered the country across the Northern Rockies and Plains. Those are the areas in pink, with the edge of the Arctic air in purple.
Now, look at what happens to the Arctic air by Saturday morning: the entire East Coast gets slammed with January-like temperatures.
When you add in the wind, it will feel close to 10 degrees Friday and Saturday mornings. That’s plenty cold.
Arctic Blast No. 2
But this weekend’s chill won’t be as extreme as the next blast. Take a look at the same type of map as the one above, and see the difference. And look at the scale on the right-MUCH colder.
By next Thursday, daytime temperatures may not get out of the 20s, and wind chills could be near or even below zero. Now, that’s cold!
Another Polar Vortex Invasion?
Now, take a look at the differences in the upper-air maps. For this, we need to look at all of North America to see the full pattern. First is the current map, and then the map for next Wednesday.
The dark blue blob on the bottom map represents what is known as the “Polar Vortex.” It’s an area of waaaay below normal pressures high up in the atmosphere. The Polar Vortex is not a new discovery, and is always on the map somewhere in the world. It just became a commonly used term a couple of years ago, when one of this things came all the way down to the Great Lakes, leading to a super-cold outbreak. The one next week doesn’t look quite as extreme.
The other important feature is the big area of red and purple in Alaska (upper left of the picture). That’s an area of pressures waaaay ABOVE normal. The combination of that giant HIGH and the Polar Vortex allows air from the North Pole, and even Siberia, to come down into the U.S. As long as this type of pattern holds, it will be extremely cold, especially in the middle of the country.
What About Snow?
This is not the type of pattern that leads to big snowstorms in our part of the country. But it does bring in enough cold air to give us chances for snow at times. In general, the farther north you go in our area, the greater the snow chances. But with Arctic air around, the rain/snow line can shift pretty far south.
Here are three different computer model maps for late Sunday/Sunday evening. All three suggest at least a chance of mainly light snow.
In these cases, warmer air moves in for Monday, so any snow would change to rain (at least from the Philadelphia area and south).
Then, another weak system looks to move in around Wednesday, ahead of Arctic blast No. 2.
We would never expect identical “solutions” that far ahead of time, but the above two models are similar. And, as of now, the Wednesday threat appears to have more potential for at least some snow accumulation.
Winter is here, folks. Don’t let the calendar fool you!