Incoming stormy pattern to deliver a rain event first and a chance for more interesting outcomes thereafter

 I was hearing a lot of noise a few weeks ago that eastern North America was lined up for a torch in December. To be perfectly fair, climate models were indicating a warm month and there are those that put a lot of credence in the European model weekly forecasts and those were spitting out some very mild temperatures as well. The exceptionally warm November we are experiencing certainly doesn't seem encouraging either and a more persistence based forecast would argue that things that are happening will continue to happen. I am actually a fan of giving some weight to persistence based predictions but I don't put much stake in the climate models or the European weekly forecasts so it does not surprise me that we appear headed toward a rather interesting and very stormy few weeks and a period that is NOT expected to stay exceptionally mild. One thing for sure, it does not bear a resemblance to a typical La Nina winter weather pattern. There's split flow, phasing storms and cool weather in the south. Although La Nina doesn't make those occurrences impossible, they are infrequent. 

Unfortunately Vermont does not appear to be in the best position to hit this La Nina knuckle ball but it will be exciting nonetheless and we should be in a much better place 2 weeks from now throughout the state. November will end mild but only because we end up on the mild side of a powerful and fully phased winter storm - a southern streamer that will take in a polar pacific impulse and explode over the northern Appalachian Mountains Monday. The explosive intensification results in the bowling ball - looking upper air environment and in this case it means the mild air and rain comes our way with the cold and snow impacting the Ohio Valley and Great Lakes. There is no escaping this and it might as well happen now with little to no snow on the ground. Rain will arrive late on Monday and continue through the evening before tapering to the more intermittent stuff by Tuesday. Your smart phone might be telling you rain on Tuesday but most of the day will actually be rain-free with temperatures hovering around 50. All this happens while the snow piles up over portions of the eastern Great Lakes.  There isn't really any arctic air with this storm but it will get colder by Wednesday and we should see mixed precipitation become snow. Again, just intermittent stuff but by Thursday morning, the high country is likely to be white. 

The pattern won't stop delivering after the Monday/Tuesday event. There are strong indications of a 2nd storm next weekend (December 5-6) and although we would certainly like to see more cold air support, there are indications of a bit more help from a storm track that is likely to set up farther east. Storm impact remains 8-9 days out which means inherent uncertainty and it would be unwise to expect a powdery bonanza without the help of much arctic air. This one is worth watching however so stay tuned. 

There are a lot of neutralizing teleconnection indices that are allowing the pattern to become the stormy madness. The Pacific is a little strengthier than I would like but not terrible. AO/NAO will remain relatively neutral while the PNA turns varying degrees of favorable depending on what model you believe. Though arctic cold is not expected to be a part of the equation during the first 6 or so days of December, we should see some enter the weather picture as we approach the time frame of December 10th. If we keep a bit of this storminess alive, and there are indications that we will, we could reach the middle of the month in decent shape.

La Nina spreading some big wings as winter 2020-2021 approaches !!

Though it may not feel like it, we’ve managed to reach late November and the commencement of another ski season. When the SCWB signed off late last year, the worsening pandemic felt like a big black hole enveloping us with no way out. As of November, the black hole still envelops us, but to borrow (and slightly alter) one of Carl Sagan’s most famous quotes, “Us Vermonters have a certain talent for adaptability” and we will apply that adaptability to piece together a ski season while dealing with and overcoming the many obstacles.  As I see it, the weather is presenting a few obstacles as well, but its nothing we aren't used to. For those of you that will never get used to the wordy discussion, please skip ahead to the bottom and receive the less than wordy outlook !

The three months leading us into November were exceptionally normal in terms of temperature, landing .  within a half a degree of normal. There was some concern early in the summer of a worsening drought but a succession of very wet months (August/September/October) has alleviated many of those concerns. The current US drought monitor still puts us in a D1 area - Drought Monitor, but the overall water situation has improved since some very dry months in the spring and early summer. Within the vicinity of the North American continent the two most glaring aspects of weather was the persistent heat and resulting forest fires in California and the remarkable hurricane season which culminated in two historic landfalling storms (Eta, Iota) very late in the tropical season. Many will refer to the 2020 tropical season as record breaking because we blew through the alphabet and well into the greek letters. This is a little deceiving since the technology is available to better indentify weather systems that meet the various thresholds to earn an official tropical name. To see a category 5 storm strike Central America in November is nothing short of remarkable however and truly cements 2020 as a historic one for tropical activity. 

What about this La Nina 

The tropical cyclone activity in 2020, particularly the strength and unusual late presence of that activity deserves a place in the discussion about climate change (and it will certainly get one). That said, a strong and active hurricane season is pretty typical given the presence of La Nina and we’ve had a notable one since late in the summer. La Nina’s are best illustrated by a line of anomalously cold water in the equatorial regions of the Pacific and aside from the aforementioned impact on the hurricane season, can, when strong enough, have a material impact on winter weather across mid-latitude North America. So what are we looking at this year ? La Nina was gradually strengthening, reaching a value, based on a temperature anomaly of  -1.7 C in late October though it has recently subsided a touch since then. Anything under -1 is significant and were it to stay there through the winter will most assuredly have something to say about our prevailing weather pattern.  So what is that weather pattern typically ? A more concentrated storm track in the Pacific, generally north of California (they could certainly use more rain), cold weather confined to the northern tier of the US, warm, dry weather across the southeast U.S. and active weather in the northeast. Active, because usually the storm track takes dead aim at New England and can bring varying results. 

 

  (Above are some of the bigger La Nina's going back 70 or so years, some weaker but notable La Nina events not included)

I am not a fan of strong ENSO events of any kind (La Nina or El Nino) as they bring with them many risks and are infamous for inconsistency. The 70 years of data does present a rosier picture however with several weaker La Nina's yielding huge snow years and a few stronger La Nina's delivering the same. Every inch of latitude-north is important and locations south of 40 N often perform quite poorly. The results really vary across Vermont as illustrated by both attached figures and results are highly dependent on what side of Vermont does the prevailing storm track establish itself. Is this a forecaster speaking from both sides of his mouth ? Absolutely ! but the information is revealing and it makes sense. A storm track aimed at Vermont as opposed to well south of Vermont is actually a positive thing, it just doesn’t always produce positive results. In colder La Nina years, and there are several like 1970-71, 1995-96 (weak La Nina), 2000-2001 (weak La Nina), 2007-2008 and 2010-11, the results are fantastic. This is year 17 of the blog and I rank 2007-08 and 2010-11 as #1 and #2 respectively for MRG ski years and both had significant La Nina’s. Unfortunately 1988-89, 2005-2006 (weak) and 2011-2012 (weak) also featured substantial La Nina’s and were awful snow years. Either way, La Nina’s can produce up and down results not only from one season to another but within that season. It is rare to go more than a few weeks without some classic Vermont-style rain/ice catastrophe. 

 

 (This illustration is pretty worthless to the naked eye except it shows the snowstake at Mt Mansfield which is never worthless and also shows the wide variation in results during La Nina winters)

PDO/Pacific SST

So how can one determine whether we get the good or bad version of La Nina this upcoming winter. It naturally leads us into the Pacific Decadal Oscillation (PDO) part of the discussion which includes a broader overview of sea surface temperature patterns in the Pacific Ocean. The PDO phase is determined by sea surface temperature configurations in the mid latitude Pacific. When cold SST anomalies hug the western North American coastline it illustrates a negative phase with the reverse indicating a positive phase.  A positive PDO phase does correlate well to a more favorable weather pattern for east coast cold and snow but they are rare during significant La Nina winters. Officially however, we remain on the positive side of neutral going into the upcoming winter. I want to convince myself that this is a uniquely positive indicator but the data going back 70 years is inconclusive at best. Several of the snowiest winters in Vermont on this decorated list occurred with both significant  La Nina’s and very negative PDO phases. What is a forecaster to do aside from ramble on and on ! 

Autumn 2019

 

 

Autumn 2020

 

 

 

With each year of blogging and seasonal forecasting, I continue to be a fan of PDO assessments. The winters of 2014-2015 in particular was dominated by very large, sometimes record breaking positive PDO phase underscoring the predictive value of this variable in a seasonal forecast. I’ve also realized however that the nature of the PDO is also of great importance and this speaks specifically to a deeper look at sea surface temperatures in the mid-latitude Pacific Ocean. Last year, the persistent Pacific Ocean warmth stretching from the Aleutian Islands through the Gulf of Alaska left us with an invariably strong Pacific Jet and had us fending off mild air a lot more often than we would prefer. Unfortunately, much of this SST configuration remains and the blob of warmth appears especially formidable south of the Gulf of Alaska. The only difference this year would be that the aforementioned warmth does not extend all the way north into the Gulf of Alaska. Though the map seems to indicate a more negative PDO index, it remains barely positive for now on the basis of some of that warm ocean water extending all the way to the North American west coast. The continued presence of the mid-latitude Pacific warm water is the most concerning variable right now and it underscores the importance of taking that deeper dive into the PDO/Pacific SST assessment. The weather pattern seems intent on dumping a lot of colder air into that region of the Pacific over the next 14 days and it will be interesting to see if that can weaken the area of anomalous SST warmth.  

Snow Cover Expansion

We recorded another strong snow build across the northern hemisphere in November. The 21.22 millions of square kilometers observed in 2020 was the 9^th consecutive October where the land area of snow cover exceeded 20 millions of sq km. Prior to this 9-year run of strong October snow expansion, the average extent of snow cover was about 18 millions of sq km with a standard deviation of roughly 2.5. Relative to that average, all but two of the recent 9 years have been more than a standard deviation above average. To borrow the phrase from former NBA star Rasheed Wallace, “Ball Don’t Lie !“ and neither do these statistics. Either the prior  almost 50 years of data are wrong or more likely, the expansion of snow and ice in October is a byproduct of other trends and changes occurring very quickly at arctic latitudes. The obvious player here is the alarming loss of sea ice as a result of climate change which results in more open water across the Arctic Ocean during autumn and an altered snow climate across northern latitudes. I’ve had to consult with others to verify this hypothesis and I am relatively convinced this is the cause. Without getting too immersed in the weeds, this conclusion changes the way one would use this data and would indicate that the old “normal” doesn’t apply. 21.22 millions of square km is still a very strong number, especially since it is largely driven by the 10.26 millions of square km recorded over the North American continent. That said, this year’s data falls smack in the middle of what was recorded over the last 9 years and is behind what was recorded last year.

Meanwhile arctic sea ice which has, what we think, is a causal and inverse relationship with snow cover expansion been near or at record lows for much of October into early November. We have seen ice expand into the Hudson Bay in the past week or two which conforms to climatological expectations, but as a whole, sea ice continues its longer term decline and that decline is more than alarming when looking through the geological time scale lens. 

 

Atmospheric Poker Tells 

Has the atmosphere given us any poker-style “tells” this year? Some things are obviously very different this year verses last year like the emerging formidable La Nina and the many strong hurricanes. Some things are the same such as the blob of sea surface temperature warmth in the mid latitude Pacific. But how is the weather actually behaving ? We had those succession of “normal” but relatively wet months this fall and what appears to be a relatively mild November. In Vermont, the mild weather has been more garden variety but across the U.S. as a whole, it will likely go down as one of the warmest November’s on record (guessing top 5). I am particularly interested in the behavior of the Arctic Oscillation (AO) since we would all like to avoid a recurrence of 2019-2020. When aggregated over the entirety of last winter, the AO was the 2^nd highest on record and though the season wasn’t completely free of good moments, the persistence of the +AO was the elephant that thrashed the room and proved too difficult to overcome. The AO did surge early this past November, rekindling some of those bad memories but has since faded and is forecast to be close to neutral through early December. You can’t say too much from what we’ve seen so far except that we have yet to see good evidence that the weather pattern is showing a propensity to get blocked. So far it has not !! This combined with the mild turn this November and some more localized SST warmth off the eastern seaboard is a little concerning.  

 Our Winter Outlook 

The outlook is thus as follows. A mild winter can be expected on the whole, but consistent with the traditions of La Nina, there will be interludes of both very mild weather and cold and snow. What will hurt this winter, in my expectation, is the likely inability for a cold, snowier pattern to lock itself in for more than a span of 3 weeks; in fact, we may be lucky to get 3 weeks. That said, I think the winter has some things going for it. La Nina combined with a modestly positive EPO will keep the storm track pointed in our direction and even if extended periods of favorable weather aren’t expected, several broken up decent periods are likely. Additionally, we are in a much better place for snow given this set up than southern New England, the Catskills, Pennsylvania and points south. Latitude will be good and even 10 miles of Rt 100 might make a difference in certain events. I would expect the mild weather will keep snowfall below average this year but we will have our days. Again, I would be surprised to see 2015-2016 revisited. We could also surprise to the upside as several La Nina winters have done. I am shying away from a snowy La Nina forecast based on the SST configurations in the Pacific and recent behavioral pattern tendencies but it wouldn't be unprecedented or that surprising. In the end I am guessing MRG procures around 200 inches of snow.

Hope everyone has and will stay safe through the thanksgiving holiday. A special shout out to those essential folks working through the pandemic helping all of us get through this. It can't be easy and it hasn't gone unnoticed. See you all on the mountain !