It's been a long wait for colder weather this autumn across northern Vermont but winter is coming in fast now that the calendar has turned to November and our friends at Sugarbush are celebrating opening day as of this posting on November 24th. As I get older, I feel like the shorter days almost requires one to get excited about the arrival of ski season and the various winter weather possibilities. Yes, the caution flag was raised early this autumn with La Nina lurking and a seemingly endless stretch of anomalous warmth stretching into October but the weather is always evolving and I like to hold back any bold predictions until late November allowing weather conditions to continue to evolve.
The last time cold weather set off any fireworks across the lower 48 was February. Since then, the weather has oscillated between slightly above and the much above range not only across interior New England but for much of the United States and Canada. Locally speaking, what has made this several-month stretch of warm weather unique is the glaring lack of anomalous cold, even for a few days. You have to go all the way back to Memorial Day weekend to find a day in northern Vermont that was at least 10 below (the already recently warmed) 30-year average. To add some perspective to this, we have recorded 28 days since Memorial Day with temperatures at least 10 above average and we've yet to break the streak even with the recent colder weather we've seen the past few weeks. It explains the late foliage year we experienced both in the Great Lakes Region and New England since multi day stretches of anomalous cold are very much a normal part of New England should see from the standpoint of variability. The extended period of warmth has created some problematic feedbacks that we can discuss in a later paragraph but this has certainly been the biggest headline when discussing the weather this past several months. With that, we are ready to take a plunge in to the "what can we expect this winter" domain.
ENSO (El Nino/La Nina)
Let us start with the ENSO situation and get a handle on that. Last year we had a significant La Nina that appeared to be building as we progressed through November of 2020. That particular ENSO peaked in very early December and very slowly subsided as we progressed through the duration of winter, finishing March at less than half the strength of that early December peak. Once again, we are seeing La Nina conditions in the equatorial Pacific but they are weaker than last November by about half a degree. Though this still puts ENSO into what I would consider a more "La Nina" mode, if equatorial waters were to warm, relative to average, by about half of a degree C, we would be be looking at a nearly neutral ENSO. Though I tend to prefer a weaker El Nino as opposed to La Nina, a weakened ENSO of any kind does serve our winter weather needs a little better. This relationship is hardly perfect but based on about 60-70 years of hard data, a strong ENSO of any kind does load the dice for a dud winter. We've had 15 strong ENSO winter's in the last 60 years which comes out to about 1 in 4 and 8 dud winters in that same time frame which comes out to a little less than 1 in 8. About half of those dud winters occurred with a significant ENSO present though which suggests that a strong ENSO about doubles the chances for a 2015-2016-like abomination. This winter marks the 10 year anniversary of the 2011-2012 horror show, a torch-fest largely brought on by some horrific prevailing jet stream conditions in the Pacific. We had a modest La Nina that winter that measured out at about -0.85, not too dissimilar to conditions right now. Interestingly, the 2011-2012 winter came on the heals of a stronger La Nina and one of the snowier winters in New England illustrating the imperfections in this relationship. The best way I can summarize the ENSO situation for this winter is that I am not too worried about it right now and some other combination of factors will allow our weather to sink or swim. Here is a chart I put together showing a recent history of ENSO during the winter months and a few adjectives describing Vermont weather during that winter.
All Things Pacific
Moving on to the all-important Pacific, and I can underscore all important because it's had much to say about the outcome of recent winters. We've seen about 20 months of negative PDO conditions with the index generally amplifying in that direction over that time frame. They've done a number on some of my data sources regarding the Pacific Decadal Oscillation but the index, according to this graph, appears to be hovering around -2 which is pretty significant. We haven't seen anything like this since late 1999 into 2000. Though I consider the state of the PDO very significant, I prefer taking the deeper dive into sea surface temperatures in the mid-latitude Pacific in search for better answers. Why do I prefer this ? Because the actual PDO index seems driven heavily by the state of SST's over a small region near the west coast of North America while the weather pattern can be driven largely by the state of SST's over a broader expanse of the Pacific.
I affectionately refer to the "red blob" as a persistent but vast area of warm SST's in the mid-latitude Pacific. It's been there for a while now, over two years to be exact. It largely ruined the winter of 2019-2020 and was an adverse force last year but was largely neutralized by prevailing (-) Arctic Oscillation conditions. It's still there as of this November but over the past 6 weeks, the area of warmth has shrunk and has shifted south and west. Meanwhile, an area of cooler water has strengthened and now covers a large area of the northeast and east-central Pacific Ocean. I suspect much of this has been weather pattern driven (but I can't say that definitively) but it's been interesting to watch this happen while at the same time, the +EPO has been mitigated and conditions in the Pacific look marginally favorable for cold weather in eastern North America over the next few weeks. Overall, in spite of how negative the actual PDO index is, I consider the conditions in the Pacific to be better overall than they've been the last two years. This doesn't clear all the roadblocks however since we will certainly need help from other fundamentals.
SST Anomalies last November
SST Anomalies more currently
Snow/Ice Buildup
The buildup of snow/ice across both the North American continent and especially the Northern Hemisphere as a whole, in autumn, is certainly something I and many others consistently watch when trying to make a guess on weather for an upcoming winter. Over the last several years, we've seen rapid buildup's of snow across the NH and the trend has been so pronounced and so consistent that it is reasonable to speculate that climate change might be playing a role. More specifically relating how changing thermals in a less icy arctic in autumn might directly cause a more rapid buildup of snow over these areas. A reasonable assertion to make but this is where I punt to the more academic crowd for more definitive answers. In the case of this autumn, 2021, snow coverage in October registered well under 20 millions of sq km for the first time in 10 years. The exact number 18.14, is more in line with the 55-year average but additional data in November is showing that we are now below average we are now on pace to have the slowest buildup of snow since 2008-2009. While the above is certainly noteworthy, so are sea ice conditions in the Arctic throughout the summer and into fall. They made a bit of a comeback this year with substantially less ice melt than in 2019 or 2020 and a fairly rapid freeze-up in October/November. Recall that the Chukchi Sea, the area of water northwest of Alaska, was wide open well into December during the fall of 2019 and has completely frozen as of the middle of November 2021. This contrast in conditions would lend credence to an open water/rapid snow cover relationship and certainly an interesting potential consequence of our warming world.
Snowcover the last several October's
For this winter, the sizable drop in snow cover over the Northern Hemisphere, by 3 million sq km and almost 7 millions sq km in November can't be ignored. In spite of better sea ice conditions the slower buildup of snow is still a warmer feedback this winter that we will have to overcome. Though we've covered this ground before, its always worth a reiteration. On a scale as large as a "hemisphere" a larger snow cover area provides a more fertile ground for cold air pooling and more intense areas of arctic chill.
Atmospheric Tells
Our last section covers the "Atmospheric Tells", a broad category that analyzes whether recent trends in weather both locally and across the continent reveal any behavioral tendencies that might either continue or might portend future weather conditions in any way. I always try to be very cautious in sifting through this data, trying to focus on consequential variables while ignoring those that might be immaterial. This is a very subjective exercise and leans heavily on the years of experience doing this and can rely heavily on persistence-based forecasting. Though a "persistence" forecast, essentially calling for the continuation of weather conditions currently being experienced, isn't exactly as bold as bombing it down Paradise, it deserves a place in the equation. In the case of this year, it's an unfortunate place since the most glaring trend in recent weather has to do with the persistent warmth and particularly the lack of intense anomalous cold discussed in one of the opening paragraphs. This long duration of this recent trend has been self reinforcing because of the negative feedback loop that the region has been sucked into. The warmth has been expansive, especially this fall and covers all the Great Lakes, Northeast and much of eastern Canada. The result has been a Great Lakes aggregate, parts of the western Atlantic Ocean and much of the Hudson Bay that are seeing water temperatures of 5 to as much as 10 degrees F above average. Some of these anomalies have been mitigated in the last few weeks by a changed weather pattern but more work will need to be done. Much of Canada is expected to be quite chilly over the next few weeks going into early December and I am hopeful that we can start to freeze the Hudson Bay close to the usual time. Right now, that very large, nearly 500,000 square mile body of water is wide open making it difficult for intense cold to make the trip into eastern Canada without modifying substantially.
Outlook/Summary
In a final summary, we can start with the more encouraging news relating to the changes discussed in the eastern mid-latitiude Pacific Ocean and an altered weather pattern finally mitigating some of the negative SST feedbacks in the Hudson Bay, Great Lakes and western Atlantic Ocean. La Nina remains a force albeit a little weaker than a year ago. Mostly however, the data is suggesting a mild winter. Even with the recent changes in the Pacific, ocean water temperature configurations there remain adverse and we were only able to overcome that last year with an usually negative and persistent negative Arctic Oscillation. Those conditions are very unlikely to repeat. The worst roadblock on the temperature side has to do with much of what was discussed in the previous paragraph. Recent trends have been horrible for cold and we now have an ugly set of feedbacks to overcome.
It's more easy to retain some optimism on the snowfall side of things. La Nina winters, even the weaker versions, favor a storm track aimed more at us than south of us and though I would expect to see a wide variety of precipitation types, interior New England would be a favored location to "bullseye" a big storm. Warm water in the western Atlantic is not especially positive for the temperature outlook but it can provide a lot of fuel for big storms and I am optimistic we see a few. Barring something anomalous from the teleconnection indices we monitor, we should expect both good and bad periods of weather overall with decent stretches of weather rudely interrupted by an inopportune thaw. Nothing unusual for Vermont. Welcome back everybody and Happy Thanksgiving !
