Well, there goes 2012 and it sure has gone quick, especially when you look back at the variety of quite significant and extreme weather events that were ‘handed out’ to us this past year. Another year is about to start and seeing we are entering into the ‘heart’ of winter, there are some interesting developments which could make for a particularly interesting January.
The winter so far has generally been quite mixed and overall ‘average’. Despite a rather cold first half to December with some fairly frequent frosts and even some snow in places, the second half of the month has clearly become far more unsettled. This has helped to lift temperatures back to nearer average values and not only that cement this year as one of the wettest on records with clearly further flooding to end December and end 2012.
The month ahead, January, I believe could deliver quite a stark variation in conditions with a potentially abrupt change in the weather approximately mid-month, this is primarily due to the expected development of a Sudden Stratospheric Warming event and also other variables including the likes of the QBO (Quasi-Biennial Oscillation).
Taking a look at the first half of the month and the below is what I believe the broader synoptic patterns will look like as an overall average/mean;
Overall there is quite good model consistency between the GFS and ECMWF ensembles for high pressure to build from the south and south-west of the UK during the opening week of January and become a dominant feature just to the south of the country. At times there is a clear signal for the high pressure to build up into the UK and thus produce predominantly settled conditions with a marked ‘ridge’ in the jet stream across more west and north-western areas of Europe. As a result, whilst perhaps not lasting the whole 15 days, the first half of January looks set to be dominated by higher pressure bringing more in the way of settled conditions and with, thankfully, a marked reduction in the risk of further heavy and disruptive rainfall. It should be noted that at the moment the high pressure isn’t likely to be one that brings cold and frosty conditions, at least not a first, but instead a lot of cloud is forecast within the short term with patchy rain and drizzle possible.
Now, onto the interesting bit and this relates to the stratosphere. I have discussed the stratosphere in a blog a few months ago which highlights it has two primary states, that being a well established ‘cold’ vortex and the other being a more disrupted and ‘warmer’ vortex. This blog and information can be read here
Latest forecast models continue to show a warming event taking place within the stratosphere into early January which is forecast to either split the vortex, displace it or perhaps even do both. Some charts of interest are below which highlights this current thinking;
Without going into the details too much, both of the above forecast images show a significant warming event is set to take place through the opening week of January as higher pressure becomes particularly dominant within the stratosphere and with a marked increase in temperatures. You’ll notice on both images that the vortex, or what’s left of it, is well displaced from its usual position across the pole and is also split with one main region over North-east Canada and the other over or near Scandinavia. These charts signal a SSW event.
The other factor of a SSW is the reversal of what is known as the zonal winds within the stratosphere. This is also evident by the negative blue colours on the above image from approximately 60°N and of which are extending from the top of the stratosphere (1hPa) down to near the bottom of the stratosphere and the top of the troposphere (100hPa). This is, yet again, another clear signal for a SWW event. Finally the below image clearly shows the predicted temperatures across certain areas are forecast to rapidly rise above average into early January in association with this warming event;
Now, what I should add here, which I have also mentioned in the blog from a few months back, is that a Sudden Stratospheric Warming event doesn’t automatically mean cold weather for the UK. This is an important variable and just because one is forecast doesn’t mean the UK will be plunged into deep cold for weeks. However, what it does mean is that the affects on the troposphere is likely to be significant and previous SSW events and analysis clearly show that when a SSW event occurs it promotes a highly disorganised polar vortex and allows/aids in the development of blocking (higher pressure) at higher latitudes. If high pressure can be achieved at higher latitudes then this brings the risk of colder weather affecting the UK and also many other parts of the northern hemisphere. As an example of high pressure becomes dominant to the north-west of the UK over and around Greenland, then this ‘blocks’ the Atlantic and often brings cold/very cold north or north-easterly winds for several days.
I will shy away from creating a graphic of how I think the synoptic patterns will evolve beyond mid-January simply because it is anyone’s guess for now. However, the science being the SSW is sound and I do believe that given the above information and also other information, that as January progresses a far more blocked pattern will likely develop and as a result will bring an increased risk of colder conditions affecting the UK towards and beyond mid-January. Clearly the details as to how cold, how long any cold will last and its origins are unknown and will be for a number of weeks. But I urge caution at writing off this winter, there is a long way to go and I do believe eventually the ‘cold’ will arrive.
A Happy New Year to all followers and readers.