Although many of us have already seen a significant snowfall on November 7th, we are getting closer to December — the start of meteorological winter — so many of you must be wondering when to expect more snow. I can’t tell you that specifically, but a detailed model analysis may point to a relatively favorable scenario for snow lovers come the first half of December.
Last November and December, there was no high latitude blocking or ridging, which bottled up all of the cold in Alaska and the Poles, and there was no cold air source in Central and Southern Canada. However, there are signs of a strong developing cold air source in Canada, which is one of many steps that need to be taken in order to generate a snowy pattern. How does this cold air source result and will there be any mechanisms to bring that cold air down to the eastern half of the United States?
A below normal temperature pattern has been settled into much of the Northeast United States since the start of November, but aside from the early-month Nor’easter things have remained relatively calm and dry. This time of year is always interesting, meteorologically, as the pattern undergoes tremendous changes as winter starts to take hold. Lower heights, stronger systems, and colder air start getting involved — and forecasters start looking ahead to the winter. Last year at this time, forecasters were staring at data which pointed at an unusually warm start to winter that would continue through at least New Years. Some of which included low snow and ice cover and a lack of any high latitude blocking. The composite image shown to the right illustrates this quite well; there was nothing favorable last year for cold and snow.
A Strong Aleutian Ridge: This feature may be the most underrated signal for pattern changes in the Northern 1/3 of the US. A strong ridge over the Aleutians and into Alaska can successfully dislodge cold air from Northern Canada and the poles into Central Canada and even the Northern 1/3 of the United States. With snow cover already above normal in that area this time of year, this development becomes very important. More cold air and below normal height anomalies for a few weeks over that area will set the stage for the presence of cold air to our north to start the meteorological winter. Additionally, forecast guidance has this feature developing into an anomaly — and a dominant one at that. +2.0 to +3.0 standard anomaly of heights throughout much of the Aleutians into Western Alaska is a common theme on mid range model guidance and ensembles. Even more interesting, down the road, is the potential impact this anomalous feature could have on the pattern. We don’t have to look far to find the last time one of these Aleutian Ridges popped up — and we can also find a familiar theme on the developments it sets off on the model guidance today.
Conversely from last year’s example, if we take a look at November, 2010, we also saw an Aleutian Ridge set up. At this exact day in 2010, there was a strong Aleutian Ridge, and we all know how that December and the rest of that winter turned out. Cold air was dumped into Southern Canada, providing a cold air source for the month of December. Additionally, we also see that there were high heights in Greenland — indicative of a -NAO. For more information about a -NAO and other indices, and what they mean to our overall pattern, click here: NAO And Other Climate Patterns Explained
A -NAO would be a major factor in providing the eastern United States with cold air and snowfall chances. Although we will have a favorable source of cold air in Canada, it is still up to the orientation of the NAO to determine whether December is cold and snowy, or a mixed bag of weather and mixed precipitation events.
Taking a look at the pattern going forward, we see that the 00z GFS ensembles valid 7 days from now show a continually quite strong Aleutian Ridge, leading to lots of cold in Canada. Considering this is an ensemble mean 7 days out, this is quite the strong signal for lots of ridging in the Aleutians, so it’s pretty safe to lock that in. Additionally, we see ridging in the North Atlantic in the form of a developing -NAO. However, on that image verbatim, the ridging is still a bit too far east for sustained cold and snow chances for the east coast. But this is a great start. This image is important because some models are showing a storm system developing in this timeframe. The NAO here, though, is transitioning from positive to negative and is still east-based — and storms in those patterns are generally more wet than white for the NYC area. An established -NAO relaxing is more favorable for snow than a +NAO turning negative. We will still have to watch the trends, though, because there is certainly plenty of cold air available, and if either or both the Aleutian Ridge shifts to the east or the NAO ridge shifts to the west, this could become a colder, snowier scenario. I still think that is quite unlikely at this time.
The big question becomes what will happen after the day 7 period as we approach early December. What is pretty much set in stone is that after that storm departs, the floodgates will open for that cold air source in Canada to dump into the United States. Although the NAO isn’t yet favorable for prolonged weeks of cold and snow, it will still help to keep the cold in the eastern half of the country for several days, at least. Are the signs favorable for this great cold air source to translate into snowstorms?
Now that we have the cold air source in play, it’s time to look at factors that will drive the pattern. One oscillation I like to use is called the MJO (Madden Julian Oscillation). The MJO essentially represents tropical convection in the western equatorial Pacific — its exact location and strength can have a strong influence on the downstream ridges and troughs. If you combine the MJO’s tendencies with what the models are showing in the medium range, you can often extrapolate the pattern and predict snowstorm chances well beyond 10 days in advance. I will attempt to do that now.
The European Model forecast for the MJO is shown below. It is a bit hard to read, but the purple and red lines show what had previously happened with the MJO in October and thus far in November, respectively, the green line is the projected model forecast, and the yellow lines are all of the model ensemble forecasts. Each dot on the green line represents one day of lead time, and there are about 10-12 dots on the map, so the MJO forecast is valid for the end of the month and the beginning of December. As you can see here, the MJO is currently in that small circle, meaning it is at quite a low amplitude and is not having much of an effect on the weather pattern. However, starting a few days from now, there is a good consensus between the Euro and its ensembles that the MJO will leave the “circle of death” and enter phase 1 and stay there for several days or longer. It then may enter phase 2 late in the forecast period or after it.
So, what does this mean for our pattern? Below is an MJO composite for phase 1 patterns in December, which gives a general idea of what one can expect — all else being equal — during an MJO phase 1 in December. This shows lots of low heights in the Aleutians, which yields an amplified +PNA pattern with a ridge in the west, a trough in the east, as well as a -NAO, as the heights are high near Greenland. Now, this seems a bit odd, as we just had a huge Aleutian Ridge, yet this composite calls for the opposite. So essentially, the MJO is “telling” the Aleutian Ridge that it needs to move. Most likely, the Aleutian Ridge is going to retrograde back to the west — but well after it has already provided a great source of cold air. I do not expect our pattern to look exactly like this image, but when you combine what an MJO phase 1 generally favors to the pattern we are currently seeing, it paints a nice picture. This is because the MJO orienting itself at this time allows for the pattern to develop in a nice step-down process. Now that the Aleutian Ridge has done its job, the MJO forcing will allow that ridge to shift to the west, allowing for a strong ridge west of the Aleutians to help provide a strong trough in the Aleutians, and a resulting +PNA ridge on the west coast. Considering there is already a cold air-mass in place from the Aleutian Ridge, this is a great scenario. Getting that Aleutian Ridge to develop in the first place was a major step to help get the ball rolling.
Using the same logic as before, it would also seem apparent that the MJO is “telling” the high heights to the east of Greenland to make an attempt to move towards Greenland, as well. This would favor a west-based -NAO, which is a great pattern for snowstorms. Now, having this develop is certainly not a lock, as this does not guarantee that the negative NAO will be west-based, but it certainly increases the chances. A phase 1 MJO will not create a -NAO out of thin air, but it is an important driver in doing so.
There are several possibilities that could arise from this. An MJO’s effect on the pattern is not instantaneous, so there would not be a +PNA spike immediately starting December, but there very well could be one a few days into December. Combining a +PNA spike with a westward retrograding -NAO could help to favor a strong storm system diving down from Canada straight into the Central US and amplifying into an East Coast snowstorm sometime in the December 5th timeframe, or perhaps a few days after that.
Another possibility is for a relaxation of the NAO during the transition period from phase 1 to phase 2 of the MJO. The phase 2 composite also shows a +PNA — perhaps even more so than phase 1, but it does not have the high heights in Greenland. Intuitively, this would seem to mean that a transition from phase 1 to phase 2 of the MJO would yield a relaxation of the -NAO, which can also lead to east coast snowstorms. The exact timing of the transition between phase 1 and phase 2 is tricky, so it’s hard to say whether that timing will coincide with the threat already mentioned for December 5th (which would probably make that threat even bigger), or if that would coincide with something closer to mid-month, meaning there would be two snowstorm threats before December 15th.
You can also have a situation where the ridging in the Atlantic and Pacific are so amplified, that they form a “bridge” so to speak, covering almost the entire polar regions. This would lock in a cold pattern for probably the first three weeks of December, before the MJO would head into the warmer phases 3 and 4. A scenario that the European weekly models were showing for week 4 (after the December 5th snow threat) was a combination of that with the phase 2 MJO composite — strong Pacific ridging bridging into the Arctic, bringing lots of cold air to the east coast, but not much in the way of Greenland blocking. That would result in a cold pattern but one where heights try to become a tad higher in the southeast, allowing for a “gradient” pattern where storms would fire along that temperature gradient and go over the top of the higher heights in the southeast. Those gradient patterns are generally snowier for New England than they are for NYC, but if that gradient sags to the south a bit, our area would also see snow from that pattern.
This is of course, all speculation at this point, but things could certainly get interesting come the first two weeks of December. Keep following our site as we will keep you up to date as to whether these ideas are coming to fruition.