December seemed to have so much potential! Arctic cold had been dislodged from the Arctic, snow cover in North America was well ahead of last year, the Pacific jet stream appeared willing to cooperated. But then everything said “hell no” to winter’s arrival. This is how a friend of mine who’s a meteorologist put it on Twitter.
That was followed by this.
We all know that’s true. While I rarely forecast beyond day 10, at least to the public, medium and long range forecasting is a challenge. Hell, short term forecasting can be quite challenging as well (see the snowstorm from earlier in November!).
The month will start on the cold side. However, a chilly December 1st will quickly transition to torchville by December 2nd. Through next week the GFS MOS has temperatures solidly above climo.
Hideous. But why so hideous? Here’s a look at the day 1-5 Euro Ensemble 500mb mean heights. It’s a pretty instructive chart for why we’ll be torching.
The first thing to note is the huge ridge (above normal heights) from the Aleutians through the North Pole and even into the North Atlantic near Iceland. This Bering Sea block has effectively become a ridge bridge traversing the pole and into the North Atlantic. This correlates to a strongly negative Arctic Oscillation (AO) and can be favorable for cold in the eastern U.S.
However, there is one feature overwhelming everything else in the U.S. and that is on the west coast. An exceptionally powerful Pacific jet stream is flooding the country with warm, maritime air. Jet streams form due to a gradient in temperature and you can see that gradient between above normal heights around the 4 corners and well below normal heights with the storm to the northwest of Seattle. This is an UGLY pattern.
The question is, however, where do we go from here? Here is day 6-10 on the Euro ensembles. Same map as above with 500mb height anomalies colored.
Same deal here. Large negative height anomalies over British Columbia and the Pacific Northwest and Bering Strait block remains though it has retrograded a bit toward Russia. This is a somewhat more favorable pattern for us for wintry weather but not really for cold. Here are the 850mb day 10 Euro Ensemble temperature anomalies.
The cold has begun to invade the northern Plains while the warmth hangs on in New England. This is not a terrible pattern for getting some storminess around after a dry stretch. If we can keep the storm around the 10th from being too strong and too far west maybe we can squeeze out some wintry weather? The best chance would be to our north, however. I’m a little worried about the lack of NAO ridging for any storm around the 10th. It could turn into a mild cutter pretty easily.
Past the 10th the models show a period of chill and maybe a wintry event for mid-month. While the pattern isn’t great it’s better than what we’re looking at for the next 10 days. The cold will ooze east from the northern Plains across then northern tier of the U.S. and probably provide a boundary capable of producing some snow.
Will it last through Christmas? I have my doubts. It appears the -AO, that at one point was signaling a really wintry start to December, will get replaced by a +AO. The Bering Strait ridge that helped dislodge the cold appears to retrograde into Kamchatka (it’s like playing Risk with weather features!) and then mainland Siberia by Day 15. In the process it gets eaten away and eventually disappears.
The bottom line is that I do think we’ll see a period between 12/10 and 12/20 with the potential for wintry weather. In addition, I do think we’ll see a period of modest winter chill. The chances for snow aren’t by any means a slam dunk. A shift in storm track to the west could leave winter lovers tying nooses with strands of Christmas lights here in New England.
There are some signs we begin to warm back up by Christmas as the AO switches to positive and the northern Pacific remains rather hostile (low heights in Gulf of Alaska). That said, we’ll have to see if the MJO decides to wake up after a dormant period – that could reshuffle the Pacific and we’ll have to see if the NAO wants to play nice. Either of those could provide a mid to late December surprise.
Special thanks to Alan Huffman’s model site for the ensemble anomaly images. Awesome site… check it out!