The risk of colder weather next week (18th January onwards) is associated with something known as a “Sudden Stratospheric Warming” (SSW), you may or may not have heard of this before. This phenomenon can increase the risk of a sustained period of cold weather in the UK.
Laymans: A SSW is when the temperature of the stratosphere increases by 60C within three days. This occurs at roughly 30,000ft. This can have a effect on the lower levels of the atmosphere, where weather takes place, this layer known as the “troposphere”. The unusually high temperatures above (in the stratosphere) “downwell” towards the troposphere layer altering the circulation from the normal “west to east” flow to an increasingly easterly flow. This is effectively a disturbance in the “polar vortex”, which is the mechanism which produces prevailing westerly winds in the winter. As the polar vortex is disturbed, the westerly winds become weaker and this often leads to easterly flows. In winter, an easterly flow is associated with colder than average conditions. The strength of the westerly (zonal) flow in the polar vortex is defined as the zonal flow. The following chart shows this defined dip in the zonal winds expected in the coming weeks.
Now, only 2 out of 3 SSWs (Sudden Stratospheric Warmings) lead to colder than average conditions in the UK. So, it’s no guarantee that this one will, however signs at the moment are that a response in the surface weather patterns is looking increasingly possible later in the month. One potential reason for this is because the lower troposphere is already in an anonymously weak zonal state. This makes it easier for the above easterly flow to propagate into the troposphere. The following chart from 2018 shows the zonal flow through the stratosphere through time. The zonal flow was already relatively weak to begin with with pulses of easterly flows. The propagation is quite clearly evident.
The current SSW in more detail:
The current SSW (sudden stratospheric warming) is a result of a sustained positive AAM (atmospheric angular momentum) period. The high AAM budget is eventually released in a period of sustained EAMT – a result of an increase in mountain torque at the surface. Such increase in AAM is likely a result of several tropical oscillations being in favoured phases.
This mountain torque propagates to the stratosphere producing a disturbance in the stratosphere which results in a major warming. The warming has occurred at the Siberian vortex and as a result the main vortex lobe has shifted to the European side, this has resulted in a temporary increase in zonality (westerly flow) over the North Atlantic region.
However, there will likely be a sudden reflex tropospheric response due to a mobile downwelling unit, which will increase the risk of significant Mid-Atlantic high pressure amplification around the 18th to 25th of January. This mobile downwelling unit is likely to result in a sharp increase in high pressure. Given the continued tendency for a westerly flow, there may be a combination of low pressure & strong amplification. This could produce a strong northerly wind as the deep low pressure is forced eastwards. Many computer model output is now suggesting this as a possible outcome, as per below. The amplification is increasingly likely to happen especially given the ongoing amplification in this area.
The resultant weather pattern could produce a spell of significantly colder than average conditions in the UK. Snowfall and much colder than average temperatures are likely to feature if this pattern materializes. This particularly true between the 18th and 30th of January. It’s worth mentioning that the usual caveats apply, there is uncertainty with regards to this amplification in the troposphere leading to cold UK weather due to a tendency for high pressure in the Azores which can shift the cold air north east away from the UK.