Well to start looking ahead, I think I should look back at 2018-19 and compare what I said in the forecast with reality.
Atlantic sea surface temperature anomalies
The Atlantic sea surface temperature (SST) anomalies continue to show this strange profile of cold-warm-cold-warm in the North Atlantic with the cold anomalies forming the shape of a horseshoe which has been the case since Spring generally. This is not favourable for a negative North Atlantic Oscillation (NAO) going strictly by the SSTs alone as they would have to be warmer around Greenland to do so. In case you do not know, the NAO is an index showing the difference in mean sea level pressure between the Icelandic Low and Azores High, two elements which make up the normal zonal pattern for western Europe. A positive NAO involves both of these elements being strong and normally translate to mild, wet conditions in Winter time whilst a negative NAO involves both of these elements either being weak or sometimes, non-existent as the pattern can completely reverse, which usually translate to cold conditions in Winter. This is not always the case with either side of the NAO index as there have been instances of cold conditions with positive NAO just as much as mild conditions with negative NAO. Every Winter since and including 2013-14 has had a positive NAO which has been one of the reasons why Winter hasn't been particularly cold recently and sometimes, record mild or wet as such happened in 2013-14 and 2015-16. Summer has been the opposite for most years since 2007 with every Summer since bar 2013 and now 2018 having a negative NAO. In fact, Summer 2018 was record breakingly positive. Will Winter 2018-19 see a similar flip to negative NAO? Not very likely given the Atlantic sea surface temperature anomalies though not impossible. I did an analogue on Winters following very positive NAO Summers and in fact, the signal was strong for an Atlantic driven Winter. Therefore from this point of view, there is an increased chance of a mild, wet Winter.
This point completely verified. The Atlantic SSTs favoured a positive NAO despite models strongly going for a -NAO. However, I note that I said it increases the chance of a mild, wet
winter. Last winter was quite unique in that regard with it being anomalously mild but dry. Mild winters tend to be unsettled and a mild, dry winter is relatively uncommon especially to the extent of mild that last winter delivered (which was the mildest on record for multiple long-term Irish stations including Valentia Observatory and Phoenix Park). I think it was also unusual given how we only recently had an anticyclonic and mild winter which was 2016-17. The run of +NAO winters since 2013-14 continued despite periods of -NAO at the end of October and mid-November 2018.
A completely different story this year with the opposite North Atlantic SST profile to that of 2018. It has been an extremely blocked season in the high latitudes with records set for -NAO for May to July 2019 including the longest run of consecutive -NAO days on record for any season. Once locked into a certain kind of blocked pattern, it can be a tough puppy to budge. We've seen all sorts of blocked patterns this decade and it has led to some extreme seasons whether it's the very wet summer of 2012, freezing cold Nov/Dec 2010 spells, very cold March of 2013 or summer drought of 2018 for example. I am going slightly off topic here to North Atlantic SSTs but these blocked patterns are theorised to be the result of a meridional jet stream forced either by low solar activity or a smaller temperature gradient between the pole and the tropics. Will we see this -NAO continue into the coming winter as a result? What goes on right now does not mean anything historically as there have been very -NAO summers before followed by +NAO winters, 2011 sticks out like a sore thumb in that department and 2011-12 was indeed one of Ireland's mildest winters on record. However, there is an increased chance of North Atlantic SSTs favouring -NAO compared to 2018.
One thing that could be conducive to a negative NAO regardless of the sea surface temperatures is a sudden stratospheric warming (SSW) event. A sudden stratospheric warming event is where observed temperatures within the stratosphere rise by a few tens of degrees within a short period of time thus why it's so-called "sudden", and the zonal winds in the stratosphere reverse to east compared to how they normally travel in a westerly flow. The impacts of these sudden stratospheric warming events include the risk of intense northern blocking over the Arctic and an increased chance of easterly winds for Ireland. There is no guarantee with them as it depends on the positioning of the high pressure and also the type of SSW. These impacts are felt in the troposphere around 9 days minimum after the initial SSW event. Sudden stratospheric warming events are normally favoured around the transition from easterly to westerly QBO as such happened in the Winters of 2012-13 and 1984-85. This does not necessarily mean we'll see one this Winter as it is based on history and SSW events do not follow patterns. I was confident on one occurring in 2017-18 because we hadn't seen a mid-Winter major SSW since 2013 then. This forecast verified albeit the SSW occurred in February rather than in January of what I originally thought. The CFS model has kept toying with the idea of a SSW sometime in December this Winter and the GFS ensembles recently have shown some instances of minor warming events through December similar in style to December 2017. With this, it's more of a wait and see than what you can necessarily predict. If one does end up occurring, you could say there is the risk of a potent cold spell at some point during the season. Pay close attention to the stratosphere watch thread if you're interested in monitoring the stratosphere.
The Quasi-Biennial Oscillation (QBO) is an index that reflects the variation of the zonal winds in the stratosphere above the equator. These winds travel in a belt around the planet and change direction approximately every 14 months. It is said to be among the most regular phenomena on earth but as 2016-17 showed, this is not always the case, when the QBO was meant to go into an easterly phase. There was an upward displacement of the westerly phase in early 2016 which cutoff the normal downward propagation of the easterly phase and 2016-17 was a second successful westerly QBO Winter. We are transitioning to the westerly phase of the QBO after being in the easterly QBO phase since mid-2017, which should increase the chances of a milder and wetter Winter but there have been cold Winters with westerly QBO (for example, 1978-79) before as much as mild Winters with easterly QBO (for example, 2007-08). There is a time lag before the westerly QBO makes full impact on the weather conditions and one thing that can break this impact before arrival is the occurrence of a SSW event but again no guarantee as described above.
I repeatedly (as did others in the weather community online) mentioned the possibility of a SSW event in December 2018 given how frequent it was signaled by the long range CFS. I mention how we were going into the westerly phase of the QBO and one of the things that could have broken the impact would be a SSW event. However, the SSW of Dec 2018/Jan 2019 was a nSSW so it failed to downward propagate and was the first of its kind since February 2008 following an unusual run of polar vortex split events; Jan 2009, Feb 2010, Jan 2013 and Feb 2018. I didn't put much weight on the QBO as I personally think there is too much historical variance to take it as a good teleconnection for winter but 2018-19 was another mild westerly QBO season that showed its sign well and truly by February with a record strong polar vortex and a very mild period.
The easterly QBO has already started to appear at the top end of the atmosphere and is expected to gradually propagate downwards through the autumn and winter. This is currently not likely to be a full on easterly QBO winter but rather a transitional one. I will be looking at past examples of such for updates that I post and see if a trend appears.
Eurasian snow cover advancement through October
Eurasian snow cover extent through October 2018 was slower than recent years but still relatively above average according to the snow advancement index by Cohen. Comparable years to October 2018 in terms of Eurasian snow cover going by the index include 2010-11, 2004-05, 2000-01, 1999-00, 1996-97, 1982-83, 1978-79 and 1973-74. My analogue of these years showed there is above average heights over and just to the south of Greenland with below average heights in the central North Atlantic into southern and central Europe – even into the east of Europe. This leaves us pulling in north to northeasterly winds. This would be a cold Winter if it were to verify with snow potential.
This part of the forecast did not verify at all. Eastern and central Europe had a severe snowy spell in early January 2019 but otherwise, it wasn't a snowy winter.
El Niño-Southern Oscillation
The El Niño-Southern Oscillation (ENSO) is an irregular index with variation in winds and sea surface temperatures over the equatorial Pacific Ocean. The warming phase is known as El Niño whilst the cooling phase is known as La Niña. Southern Oscillation is the accompanying atmospheric component coupled with the sea temperature change. These phases of ENSO have various impacts on weather patterns around the world but the impacts from it on Europe are unknown due to event to event variation. However, it has been said that El Niño increases the chances of cold later in the Winter whilst La Niña does the opposite of increasing the chances of cold earlier in the Winter. Examples of El Niño Winters doing this include 2014-15 (where February was the coldest of the Winter with a frosty spell during the first half) and 1977-78 (where February produced a severe cold spell with one of the worst blizzards on record to hit the West Country). An example of a La Niña Winter doing this includes 2010-11 (where November/December produced some of the coldest weather on record to occur so early in the Winter including the coldest December on record for most). However there are many odd ones out. For instance, you only have to go back to last year for La Niña and February 2018 was the coldest of the Winter. There has been talks of an El Niño Modoki for 2018-19 rather than a traditional El Niño event. Most of the time, warming occurs in ENSO regions 1+2 but sometimes, you can have another type of El Niño called a Modoki. With this type of El Niño, the warming is focused on the central part of the Equatorial Pacific i.e. ENSO region 3.4. The CFSv2 continues to forecast an El Niño for Winter 2018-19 with the model now showing the possibility of moderate El Niño conditions but this is kind of an outside chance at the moment as it will have been very late for such an ENSO event to occur and the model forecasts a slight drop off in early 2019 too into weak threshold. The overall reanalysis of El Niño Modoki Winters in the past show above average heights over Greenland with below average heights to the southeast of us drawing in an easterly or northeasterly flow and is a clear cold signal. Mind you, this is no guarantee as there were some El Niño Modoki Winters that were mild and or wet such as 1994-95 (the wettest on record for a good part of Ireland before 2013-14) but the majority were cold. The most recent example of an El Niño Modoki occurred in Winter 2009-10. Will we see a similar Winter for 2018-19? Very unlikely, that was the coldest Winter since 1978-79 after all but not impossible! As this will be an El Niño Winter, it is more likely to start off mild with an increased risk of cold later.
In the end, there was no El Niño Modoki at all for 2018-19 despite models hinting at one with the warmth tending to be over to the east Pacific near South America. The El Niño event that did occur was more traditional but also very weak in nature (almost similarly weak to that of the weak La Niña in 2016-17). I haven't posted in the ENSO thread for a long time because it's just been stable generally with El Niño in a weak state although it has since disappeared since early summer and now the east Pacific is in a much cooler state. Models going for ENSO neutral for 2019-20 right now. We haven't had an ENSO neutral winter since 2013-14; 2014-15 was weak El Niño, 2015-16 was very strong El Niño, 2016-17 was weak La Niña, 2017-18 was weak La Niña and 2018-19 was weak El Niño. Rather than starting a new ENSO season thread, I'll post in the 2018-19 one.
At last, I'll discuss my final analogues. The years that cropped up most in my analogues for this Winter are 1941-42, 1963-64, 1979-80, 1994-95, 1996-97, 2010-11, 1901-02, 1940-41, 1968-69, 1987-88, 1995-96, 2001-02, 2008-09 and 1990-91. As to be expected, this batch of years is mixed in terms of their Winter conditions. 1901-02 had an unremarkable December and January whilst February was quite cold. 1940-41 and 1941-42 were cold Winters with some potent cold spells particularly in January of both years. 1963-64 was exceptionally dry and quiet. 1968-69 had a White Christmas and a severely cold February with a mild January despite the NAO and AO being negative then; it was also an extremely cold Winter for parts of Eurasia. 1979-80 was generally on the mild side with a rather wet December though January was a bit cooler. 1987-88 was a very mild and wet Winter and the start of the cluster of three notoriously mild Winters. 1990-91 was changeable with a fairly cool but unsettled December, an anticyclonic January after a stormy start and a cold February with a very cold spell near the start of the month. In terms of the methodology given, 1990-91 provides the most similarities to 2018-19. 1995-96 had an extremely cold spell at the end of December whilst most of January was very mild and dull (dullest January on record for several places) and February was cold, wet and snowy but sunny; a bit of an unusual combination. 1996-97 was a front loaded Winter with a cold spell during the second half of December into the first part of January including some ice days but the rest of January tended to be milder and February was a wet, mild, stormy month. 2001-02 had the sunniest December on record with mainly dry and cool conditions due to frost later on whilst January and February tended to be stormy and mild. 2008-09 had a cold spell at the end of December and start of January with severe frost but spells of sunshine rather than snow along with a very snowy spell at the beginning of February but became milder in the second half. 2010-11 contained two severe and unprecedented cold spells at the start of December (continuing on from end of November) and again from mid-December to St. Stephen's Day. When you put all these years together, you get well above average heights right over the top of Greenland with a trough of below average heights just to the west of Iberia. In this scenario, the mean wind direction is easterly and with cold air coming in from the east across the relatively warm Irish Sea, there would be instances of lake-effect snow. The intensity would depend on the potency of the cold air and the exact state of the sea surface temperatures.
What is the final verdict from all of what I described above? What do I think is likely? Well, the signs from my analogues and long range models are very compelling aren't they for a cold Winter. However, I just don't think it will be as straightforward as some would interpret from these. I don't think this will be a classic cold Winter. Rather, I think a changeable Winter is more likely similar in vein to 2017-18 especially earlier in the season with more of a lean towards a risk of prolonged cold later in the season.
December - A close to average month in terms of temperature if a little bit on the milder side. Above average precipitation is more favoured than below average despite my analogue showing an anticyclonic month on the cards, due to a more zonal flow earlier on and there is a possibility of this continuing through the month.
January - A close to average month again with no large deviation though slightly leaning towards the colder side due to frost or blocking. Precipitation likely to be on the drier side.
February - Holds the greatest prospects for cold currently with the models clearly showing a backloaded Winter for 2018-19. I think February 2019 will be a cold month but not remarkable in the historical record with a deviation somewhere between 0.5 to 1.0c below average. Precipitation likely to be relatively above average due to volatility in the weather conditions as the Atlantic battles against blocking to the north.
Analogues failed completely and did not pick up on any pattern during the winter. One of my best analogue years was 1994-95 which was a mild and weak Niño winter but also very wet unlike 2018-19. I think this shows just how unique 2018-19 was and that every season is different in its own right. They don't follow rules or what has happened on in the past.
I underestimated on December and January's mild nature (although January would have done very well as a UK forecast compared to Ireland due to them having a colder month). February was a disaster all around and I don't think anybody would have been able to call 21c at the end in the UK or just over 17c in Ireland.
The winter rollercoaster of 2019-20 is in operation and has many seats available but warning to those new here, it's going to be a long bumpy ride.
Due to being busy with other things and considering I have two college courses to do this year, I may not be able to post as many updates as previous few years.