After what has been a very mild first half to November, especially in the south, more seasonal weather is on the way in the week ahead, as winds turn northerly and increasingly cold arctic air spreads south across all parts Widespread frost will develop overnight where skies clear, while snow is likely to fall and settle over higher ground in the north and will fall to lower levels at times too from mid-week.
Saturday and Friday both saw 16C reached in southern England, Thursday reached 18C at Heathrow and Wisley (Surrey), while temperatures reached the mid-teens earlier in the week. So it will be quite noticeable across the south the change to much colder conditions sweep south through the week, especially by Thursday, as a more potent plunge of cold arctic air arrives from the north behind an area of low pressure moving southeast across the country mid-week. Temperatures on Thursday could struggle to reach mid-single digits even in the south, with highs of 3-6C, some 10C lower than what we’ve seen over recent days in the south!
The last of the mild air is hanging on by a thread across the far south Sunday morning, but by Monday, highs across the south will be a good 8C lower than the last few days, so it will feel noticeably colder. For many it’s looking dry and sunny but chilly on Monday, however, Tuesday and early Wednesday will see an area of low pressure moving SE bring showery rain, turning to sleet and snow over high ground in the north as it clears. Wednesday and Thursday even colder, with wintry showers near coasts, but mostly dry and sunny but cold inland. Temperatures perhaps only reaching 3-6C. Widespread overnight frost likely throughout the week.
The much colder conditions look to ease next weekend, as winds turn more westerly, with Atlantic systems rolling in, bringing milder Atlantic air and the usual spells of rain and windy conditions. However, high up over the higher latitudes of the northern hemisphere, the models have for a while now been forecasting zonal or westerly winds in the stratosphere, which wrap around the polar vortex, weakening and perhaps reversing briefly. If the winds reverse at 10 hPa 60N, i.e. zonal winds falling below 0 m/s, then this would be technically known as a Sudden Stratospheric Warming. This is due to a significant warming of the stratosphere taking place from the Canadian side of the polar region, which looks to displace the stratospheric polar vortex from its usual home over the arctic towards northern Russia and NE Europe.
ECMWF ensemble mean for zonal mean zonal winds at 10hPa 60N reaching 0 m/s, though last few days the mean is above, so slightly less members going for a reversal
00z GFS going for a reversal at 10hPa 60N, the ensemble mean getting close, but not quite there. Note the SPV displaced toward N Russia / NE Europe.
When the polar stratosphere warms and a Sudden Stratospheric Warming takes place, even if the reversal is brief, it can have impacts on weather patterns. This can occur quite quickly after the SSW has taken place, or the changes could occur several weeks afterwards. The response in the troposphere to the SSW, where our weather happens, depends on the type of SSW, whether the event is driven from the troposphere (bottom up) or from the stratosphere (top down).
This possible SSW, which looks to occur around 25/26th November, looks to be driven by warming from the troposphere, which means that troposphere is coupled with the stratosphere. This infers that as both are coupled, then what’s happening in the stratosphere will fairly quickly impact weather patterns in the troposphere. A rise in geopotential heights over the arctic in response to the stratospheric polar vortex being displaced toward Russia / NE Europe is a possible response to a SSW. This means cold arctic air could be pushed further south into northern Europe at times later this month, after turning briefly less cold, and perhaps through December too. So the UK may see an increased chance of further bouts of colder and perhaps wintry conditions as we end the month and head through December, if the SSW takes place.
Hints of northern blocking returning in early December from yesterday's EC weeklies update
Of course, there are no guarantees of colder weather because of a SSW, displacements of the polar vortex don’t always favour colder conditions for NW Europe, a split stratospheric polar vortex more likely brings colder weather, but even then only 2 out of 3 split events on average lead to cold and wintry weather in NW Europe - e.g. late Feb/early March 2018. Split SPVs from a SSW tend to occur later in winter.
A SSW in November is very unusual and hasn’t happened in November since 1968. December 1968 was cold with a white Christmas for some central parts of England, January 1969 was mild though. 1958 also saw a SSW in November. December 1958 was mild and wet, January 1959 cold with CET of 1.6C. So some mixed outcomes after those early events.
We will update over the coming days on the potential for a SSW and whether there is any signal for it to change weather patterns to bring colder weather. Even if a technical SSW is not achieved, the stratospheric polar vortex will be in a weakened state, so the atmosphere will be prone to develop blocking at high latitudes, which can increase chances of colder weather.
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