The latest ECM 32 day model is similar to the previous runs which keeps low pressure dominant across the UK or to the NW for the foreseeable future. The primary difference is it is signalling a more pronounced area of higher pressure across E Europe which as a result allows for the potential development of milder S or SW'lies at times with temperatures around the middle of the month potentially slightly above average.
However and importantly, the final block which now takes us into the opening week of December maintains a trend for pressure to become lower to the south of the UK, whilst pressure becomes higher to the north, essentially creating a -NAO pattern and a signal for temperatures to become below average into early December. That is the reason why if you read the 15 to 30 day update on the UKMO site, they highlight potentially colder conditions long term and it is just to that signal within the ECM 32 day which has been there now consistently, so it'll be interesting to see what happens.
No suprise to see the longer range ECM signalling a rise in pressure to the North and lowering pressure to the south. This is not related to the stratosphere but the lead up to this around mid month (troposperically) will likely have a significant on the stratosphere during the first week December.
Stewarts reactie:
A strong build of pressure over Eastern Europe and Western Russia has a teleconnective influence on Wave 2 activity and I'd be very suprised if we didn't see strong wave 2 response in the stratosphere during early December (wave 2 being more responsible for vortex splits). That places us around late December for a stratospheric influenced pattern. And wouldn't you know it, we have another tropical wave favouring high latitude blocking mid January - good trop / strat / trop/ strat feedbacks]
For any downwelling cold strat influence we need a strong thermal gradient across the tropical and polar stratosphere. At levels below 50 hPa, we don't have this, so any possible signal at 10hPa and 30hpPa is fighting to get through.
Add to that, the lag time for downwelling is 21 days. That takes us to end of the month, when as if luck would have it, the tropopsheric teleconnectors are signalling -NAO. It would be logical to assume that the lower stratosphere would also favourably respond given light polar westerlies.
If you take last year and this year, you have a classic case study as to why an early season cold vortex event can succeed (La Nina and strong solar activity) and why it finds it very hard to engage to lower levels (ENSO neutral / low solar / second season east QBO).
Quote selectie