EARLY ARCTIC AIR-WILL IT LAST?
Unseasonably cold air is hitting our area for the middle of this week. In fact, it’s hitting the ENTIRE country east of the Mississippi River. It’s also unseasonably mild in the ENTIRE western half of the country. That’s quite a “half-and-half” picture!
Now, take a look at the world’s leading model-the European-for exactly a week after the map above. It looks almost the same (except for a much warmer Florida):
So, does this mean we’re going to be stuck in the Arctic air for the rest of December-and beyond? Probably not. For the answers, we have to look much higher up in the atmosphere.
UPPER-AIR CHANGES COMING
As we’ve discussed many times before, meteorologists look to a level nearly 20,000 feet up-known as 500 millibars (500mb). This is the level best used to track overall weather pattern changes.
Here is the 500mb pattern this Wednesday:
The bluer the color, the more extreme pressure BELOW normal. And the redder the color, the more extreme pressure ABOVE normal. So, we have a major low pressure “trough” in the Eastern U.S., and a major high pressure “ridge” along the West Coast. Seeing this developing pattern a couple of weeks ago led me to confidently predict our current cold and first snow of the season. But that ridge in the west is just as important. It forces any air coming across the Pacific way up into the Arctic. The air then comes down from the Arctic into the Eastern U.S. Just follow the lines: the air at this level flows along the lines.
Now let’s see what happens a week later:
There is still a general trough and low pressures in the Eastern U.S. But what happened to the western ridge? It’s gone. The upper-air flow is now coming across the Pacific and pretty much straight west to east through much of the U.S. We no longer have a blocking pattern that brings our air from the Arctic. At least for a few days after December 20th, our cold pattern should ease.
The above maps are known as “ensemble maps," which is the average of the 51 times the European model was run. Each one starts with slightly different initial conditions. This helps eliminate “bad” data, and the natural variation of computer models the farther we go out in time (“chaos”). So, we’re talking about the best solution from the best model-the European. There is no model, or ensemble of models anywhere in the world, that consistently beats this. It’s far from perfect, but it’s the best solution available.
AND AFTER DECEMBER 20TH?
Here is where it gets tricky. Some models, including the EURO, re-develops the West Coast ridge by Christmas Day, the 25th. This is known as the pattern “reloading," and suggests a return to the cold east/warm west pattern.
The only real difference between the maps from December 12th and the 25th is that the eastern trough and western ridge are both a bit farther west around Christmas. That slight change could lead to:
1. Less extreme cold for us
2. A wetter pattern for us
That combination could give us at least a decent chance of a White Christmas, or possibly even an icy Christmas. Stay tuned for any changes (small changes in the upper pattern can mean significant changes in our weather).
THE NEW MAPS
You might notice that the last three maps come from weather.us which is a new and incredible website. The brilliant map creator, Dr. Ryan Maue, is giving unprecedented access to many weather model maps and data that we’ve never seen before. I am in awe of this site, and what it provides. May it stay available (and free for all) forever!