December is here and the holiday season is in full swing! In November, we saw a major pattern change in as we went from record warmth in the first third of the month to below normal for the middle third followed by a warm up at the end of the month. Even with the change, November ended up above normal.
December has started off with a warmer than normal regime, but major change is coming. This is a special discussion on what I'm seeing, and what the implications may be as we get toward the middle to end of December. Before diving into the pattern discussion, let's talk next week briefly.
The Week Ahead
It seems counterintuitive, but as the pattern changes in the upper levels, we will see a warming trend through the week. There is significant uncertainty regarding a late week storm system that is likely to develop. The GFS brings a significant coastal storm to New England that is mostly rain, while the Euro is further south and bring little precipitation. This is something to watch as there is a lot of uncertainty in the guidance. I do think a snowy event is unlikely.
Monday: Seasonable and sunny with increasing clouds. Highs in the upper 40s.
Tuesday: Mostly cloudy with rain showers as the day progresses. Highs in the low to mid 50s. Chance of rain 50%.
Wednesday: Mostly cloudy with rain. Highs in the mid to upper 50s. Chance of rain 80%.
Thursday: Partly cloudy. Highs in the low to mid 50s.
Friday: Mostly cloudy with rain. Highs in the low to mid 40s. Chance of rain 50%.
Saturday: Mostly cloudy with rain showers. Highs in the low 40s. Chance of rain 40%.
Sunday: Partly cloudy. Highs in the mid to upper 30s.
There is a lot more to discuss, so let's dive in.
That's very basic, but it will be a reference point for the coming discussion.
What Pattern Change is Happening?
I use the word "happening" because we are already seeing major change take place in the upper levels. I prefer to look at 500mb analysis rather than numerical indices because the orientation and position of teleconnections, the elements in different parts of the hemisphere that make a pattern, matter. A lot.
Just looking at the image above, I see a mixed bag. There's a strongly negative North Atlantic Oscillation (-NAO) and negative Arctic Oscillation (-AO). For a winter lover, this is excellent. A -NAO has been hard to come by, especially in December. The Atlantic looks great already. However, looking at the Pacific, the pattern does not look as favorable with a near neutral Pacific North America (PNA) structure and greatly negative Eastern Pacific Oscillation (-EPO) that is trending positive.
Now, for those having trouble following, generally we want to see a -NAO/-AO Atlantic alongside a -EPO and +PNA in the Pacific for a strongly wintry pattern. While we can get snow and cold with other variations of these teleconnections, that overall look is where you want to be for the best chances of big snow events.
The Atlantic teleconnections generally provide the blocking (NAO) and cold loading in Canada (AO) while the PNA is critical to storminess in the US and EPO delivers the cold from Canada and Arctic. For more on teleconnections, visit our 2021 winter forecast here.
Currently, we are seeing the models show a favorable Atlantic pattern for blocking and cold loading. This is high confidence as it is already occurring. As you can see using the top chart, both the AO and NAO are strongly negative. This is important because history suggests that this type of event in December usually leads to additional blocking and cold loading later in winter.
So what's happening? The Atlantic is primed for blocking and cold loading, but the Pacific is lagging. The guidance overall says that the Pacific will allow for cold delivery and storminess, but it will take longer to occur.
There is not an immediate switch flip to cold and snow.
Many view the pattern evolution in the following way and I concur:
Step I--Establishing the Atlantic
This is what we're seeing now and it will continue to evolve over the next few days as the block over Greenland retrogrades west (which is very important) and matures in strength before it begins to transition toward a weaker state.
This is not necessarily a bad thing for winter lovers, as our big snow events tend to happen as the block relaxes. This stage will likely be complete between Dec 7-10. Expect a warm period.
Step II--Gradient Potential
This is where that ridge in the southern US matters. It will likely shift east and become a SE ridge. This makes cold delivery and wintry storm tracks less likely, though it's not a deal breaker especially for interior CT and Southern New England.
The Pacific won't be ready for prime time yet, but with good enough timing we could see a first threat emerge. This is called gradient potential because the position of storm systems relative to the block to our north and ridge to our south can create mixed bag systems that deliver light to moderate winter weather events. Wintry weather is unlikely, but possible. You are not likely to see wintry events show up on operational guidance early with these, so expect a lot of model chaos.
This stage will likely last from December 11-15.
Step III--Establishing the Pacific
This step is the most uncertain. Among the ensemble guidance the EPS develops a textbook New England winter weather pattern around the middle of the month with both the Atlantic and Pacific working in tandem to deliver cold and eventual winter weather threats. The GEFS (GFS Ensembles) are far more uncertain that a favorable Pacific pattern sets up, though it does move toward something that is more serviceable for the middle of the month onward.
I tend to believe that we are going to get a good Pacific pattern around mid-month. However, we should factor in the potential for this to take longer than anticipated today. Once all of the pieces come together we will likely start to see significant winter weather threats show up. It is also important to note that snowfall in CT becomes more likely climatologically after the middle of the month, so the timing of a good pattern works in tandem with climo.
As a result, I would feel pretty good if I were a winter lover, but would temper expectations until at least we get through Step II. The Pacific will be key.
This stage should complete between December 16-20.
Below: The 12z EPS. Like I stated earlier, it takes time, but this becomes a classic winter pattern for the northeast US around the middle of the month and especially after.
For now however, we warm up this week but then start to see a gradient setup that could bring poorly modeled light to moderate winter threats in the following week. However, it is the period after December 15th where higher end winter weather potential should exist.
Does that mean a White Christmas? We'll see...but if the pattern comes close to what we see on the ensemble guidance, especially the EPS, we have a better than normal chance.
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