A knife-edge winter 2019/2020
Updated: Dec 3, 2019
Winter is here... so here I will go again and try to find some direction to how the winter will progress amongst all the chaos. I will not be looking at analogues for this forecast as I do not believe they are useful anymore in a time which arctic is seeing unprecedented change.
First looking at the drivers which impact weather patterns in UK and afar.
The stratospheric vortex is a vital element in winter forecasting for the UK and Europe but limited use the models have beyond 2 weeks. The vortex naturally strengthens and expands over pole through late Autumn as polar night returns. Typically it reaches its apex strength around late December, it is therefore no surprise Christmas often is mild with no snow boo! The stratospheric polar vortex (SPV) is currently weaker than average but not significantly so. Today (02/12/19) the zonal mean zonal wind at 10hPa from GFS analysis is 15.2 m/s it should be around 30 m/s, it is forecast to remain weaker and weaken further, in a time which it should be at its most rapidly intensifying phase. A growing envelope in GEFS seem keen for a reversal in zonal wind to E'ly and therefore a Sudden Stratospheric Warming (SSW) approaching mid month. I however do not think this is likely as often with GFS we are chasing rainbows, at earliest I see this happening in late December add a few weeks and cold surface impacts?, would be quite a bit into January. With SSW approximately 2 in 3 result in extended cold weather in Northern Europe with nuance the UK is a little island. February 2018 was last recent example which spectacularly benefited W-Europe. If we do see a major perturbation which I'm inclined to believe we will, it is likely we will not see possible positive effects of this until midwinter, January or even into February.
Sea Surface anomalies in the North Atlantic as of now favour a slightly south of normal storm track indeed this has been observed through Autumn. With a shocking lack of gales in Scotland the jet stream has been further south than normal on an unusual axis, depressions which usually ride jet stream have been instead passing through central swathe of England. Sheffield has experienced its wettest Autumn on record, flooding has never been far from news. The current SST anomolies are not particularly promising for a relent in wet weather in these places as we go through December. However, very warm waters around Greenland's southern coasts may help to support ridging in area, indeed which we have seen of late in a colder spell of weather with Greenland blocking. This more hospitable region for blocking in conjunction with a jet stream which has been already more south solar? May allow for significant snowfalls in December and beyond but this is much more likely for northern UK.
Snow cover & ice extent
Snow cover is very high across Northern Hemisphere this October had 5th largest snow cover extent on record. it is believed a strong and extensive snowpack across Eurasia allows for a strong Siberian High, fostering disruption to vortex with a linked connection for a negative Arctic Oscillation (AO). This promoting a southerly tracking jet stream and very cold air bottled up over the high latitudes to spill into mid latitudes a greater risk of cold weather in Europe and US. The Siberian High can become more influential for UK later in winter as robust N Atlantic westerlies die down, I do see the second half of winter featuring coldest weather *spoiler*. While onto sea ice the Atlantic side only really relevant for UK and Europe has seen significant very much above average progress in the last month, something we have not seen in last 10 years. This will mean northerlies are likely to be a little colder than normal this winter and a higher chance of snow falling and accumulating to lower levels and further S.
There is a problem equating a "cold winter" to solar minimum as a direct relationship but it is not truly understood some brilliant winter spells have been seen near to solar maximum such as February 1991 which London did not see equivalent winter weather till 2009. Although dissimilarly this was a solar minimum winter, the observational note in last 10 years has shown a cluster of blocked and colder winters at or following a solar minimum 2008-2013. Some literature suggests there is two year lag, with an uptick in sunspots in 2013 followed a cluster of much milder winters, but again careful pinning it on just one variable. Overall I'd say this winter solar minimum, increases the chances of blocking and therefore a colder spell of wintry weather but where it positions itself is key. A "bendy" jet has been present this autumn not typical flat W-E may already be having an influence with Autumn 2019 being the first cooler than average average to 61-90 average since 2012.
The first month of December is likely to be generally chilly and wet while the jet stream may remain strong it is in my opinion likely to be deflected south. Current modelling paints a likely cold snap/spell around mid month with cold northerly winds, and rather unsettled with it and therefore the risk of snow particularly in the north, significant snowfall(s) a distinct possibility there. I do not think significant cold is likely to feature in this month - but it is not necessary for snow to fall even in SE England, it would just not lay on ground for long. Looking towards pacific forcing the next ENSO cycle expected later this month looks to bring the MJO back into equation, with colder phase 7-8-1, possible this may allow for blocking to develop. So a drier final third I feel but potentially cold or very cold, rather than chilly which could prove good for festive period, with severe frost potential any significant snow I feel this is limited more likely into 2020. The month generally as a whole looks quite cold and seasonable.
This is difficult month I see there being two possibilities the first rests on major perturbation to vortex and of course downwelling, at earliest I can see this truly manifesting on ground level is middle of January. With possible much colder and drier weather, and enhanced risk of disruptive snow just about anywhere. Before this point it could be wet start to January and around average if milder temperature wise, somewhat what I feel like a fair swathe of December will be like. The jet stream remaining predisposed to south, I can see a wonderful season for Scottish mountains. If we fail to see a major disruption to vortex I firmly believe January could be the most stormy month of the winter, with a turbocharged +NAO pattern, this is what I believe seasonal models have gone with. No stratospheric impact for winter 19/20 which may prove wrong in my opinion & if is true wintry weather could be very limited to northern hills. I am placing my bets with first scenario.
It probably will be the driest month of the winter I will not give much detail here but as above in regards to January, this month may be the coldest of winter. With easterlies perhaps bringing exceptional cold from Siberia, in this case not like 2018 possibly much earlier in month, with the QBO transitioning to E by late winter. This aids in the polar vortex not being menacing and on top with a possible major perturbation to vortex and climatology support, northern latitude blocking may prove to be extensive and resilient a very strong -NAO signature this month might have. In contrast to my last winter forecast which I thought would bring an early start to Spring I do not believe this is particularly likely in 2020.
So that is my winter forecast this may be seen as a daring forecast but it is a forecast which I see as being plausible and rooted in science. However, much emphasis on it leaning towards a colder winter rests heavily on a SSW happening and imprinting favourably. If this does not occur then the winter may be rather uninspiring and wet, if opposite is true which I think will be so, expect the winter to become increasingly drier as we progress but also colder. A winter we may not have seen in a number of years, feet firmly on ground but I am looking forward to what may come. I do not for the life of me foresee it being a snowless 2013/2014 winter low solar also to add, expecting for this winter colder than average departures to north and east and more around average for south and west. Thanks for reading. :-)