It is now happening and is shown correctly here:

http://users.met.fu-berlin.de/~Aktuell/strat-www/wdiag/ts.php?plot=temps&alert=1&lng=eng

It is no longer a forecast, but a nowcast.

Originally Posted by: Maunder Minimum 

By the end of the week we'll start seeing some eye candy 😉

Originally Posted by: Snowedin3 

The rationale behind paying so much attention to the strat. at the moment is quite simple really - with the hemispheric setup as it is, a SSW is our best bet for changing the game in our favour.

It's by no means a guaranteed path to success - there could easily be a big freeze to our east with nothing much here, like in February 2012 I think it was - but I can't see much evidence for positive changes being driven by much else in the coming weeks.

In short, what Nick said earlier (good post by the way) 

So, I'm going to be focusing on this aspect until if and when some interesting storms start coming along, by which time the week 2 model output will hopefully be showing some marked changes to our pattern.

Re: Martin's post above, there was a lot of warming last month but it didn't quite have the strength needed to bring about much change away from the areas beneath where the warming occurred, which were on the other side of the pole. Over there, high latitude blocking has been coming along quite often of late and looks to be doing so again during the coming 10 days or so.

As for an impending SSW, I can see strong signs of it on the latest model runs, even GFS, which has been very reluctant until today (below-left shows very strong warming focused over the SW tip of Greenland, below-right shows the impact on the vortex by day 15):

While GFSP remains a bit less keen, it's been trending stronger as well, while ECM has been showing the strongest warming of all, perhaps a full-blown SSW based on the day 10 00z chart (not shown as it's behind a paywall I think - I'm relying on reports by others).

The significance of having warming to our NW is that even if it doesn't go the whole hog, it will act to reduce the westerlies in the Atlantic sector and support high pressure to our NW. Just a hint of that in the latest GFS run.

As it is, the trop. impacts probably won't be seen until the second half of January.

Until then, a strong westerly flow seems likely starting 7-8 days from now with a high risk of severe wind events and large rainfall amounts. Shame we have to take this route but the wave breaking just didn't pack enough of a punch back in December to overcome what was an unfavourable origin as far as the UK is concerned (east Asia).

Edit: just thought I should make clear that an actual SSW remains nothing more than a possibility at the moment - we may just see some fairly strong warming to the NW, which would tip the balance toward blocking to our NW but there's no telling at this stage whether it would be enough.

Despite all this uncertainty, I'd rather analyse it in detail than spend my posts moaning about how dreadful the current NWP output is in terms of the UK's surface weather pattern.

Originally Posted by: Stormchaser 

Hi all,

Here's today's video update - Westerlies Returning For Start Of 2015;

http://www.gavsweathervids.com/

#battendownthehatches

Originally Posted by: Gavin P 

Just throwing this into the mix:

Matthew Hugo
‏@MattHugo81 00Z ECM essentially destroys the strat vortex at 30hPa by 8th of January. Implications possibly felt mid-Jan onwards

And that follows an earlier tweet from MH saying the EC32 shows zonal flow until week 3 of January.

Now we assume that Strat influence is modelled, so if the ECM is destroying the PV, why is it not being shown in the SLP charts by the same model? Perhaps it is not expected to propagate and translate into the synoptics on the surface after all?

Originally Posted by: nsrobins 

Awful. I said a couple of days ago there were some encouraging signs, they evaporated pretty damn quick to give a 2014esque pattern. The high in the arctic has no support further south so is as useful as a chocolate teapot.

Roll on February 

Originally Posted by: Quantum 

No more crying game plenty of time Q chin up

Originally Posted by: Polar Low 

Just throwing this into the mix:

Matthew Hugo
‏@MattHugo81 00Z ECM essentially destroys the strat vortex at 30hPa by 8th of January. Implications possibly felt mid-Jan onwards

And that follows an earlier tweet from MH saying the EC32 shows zonal flow until week 3 of January.

Now we assume that Strat influence is modelled, so if the ECM is destroying the PV, why is it not being shown in the SLP charts by the same model? Perhaps it is not expected to propagate and translate into the synoptics on the surface after all?

Originally Posted by: nsrobins 

Indeed. The 12Z +240 GFS has a fat Azeuro high and a flow across the UK straight from Bermuda. Pure gunk IMO.

Originally Posted by: nsrobins 

Just throwing this into the mix:

Matthew Hugo
‏@MattHugo81 00Z ECM essentially destroys the strat vortex at 30hPa by 8th of January. Implications possibly felt mid-Jan onwards

And that follows an earlier tweet from MH saying the EC32 shows zonal flow until week 3 of January.

Now we assume that Strat influence is modelled, so if the ECM is destroying the PV, why is it not being shown in the SLP charts by the same model? Perhaps it is not expected to propagate and translate into the synoptics on the surface after all?

Originally Posted by: nsrobins 

I will second what James has said about Nick's (nickl) post earlier which was excellent.

It is worth noting that there are several types of SSW including major and minor events as defined in a 1978 WMO commission http://en.wikipedia.org/wiki/Sudden_stratospheric_warming

For a major event to occur there must be an increase in mean temperature poleward from 60N at 10hPa or lower and an associated circulation reversal to a mean easterly flow again from 60N northwards.

For a minor event there must be an increase in mean temperature of at least 25C within a week at any stratospheric level in winter. The zonal mean wind is weakened but there is little or no reversal in the flow and the polar vortex is not broken down.

Typically a major SSW occurs about once every 2 years while a minor SSW can occur several times each winter. The warming we saw in the stratosphere back in November was nowhere near sufficient to qualify as a SSW.

The last major SSW was in early January 2013

This chart shows the sudden increase in temperature at 10hPa
http://www.cpc.ncep.noaa.gov/products/stratosphere/temperature/archive/10mb9065_2013.gif

This chart shows the zonal mean wind changed to easterly (the green area) at the same time
http://www.cpc.ncep.noaa.gov/products/stratosphere/strat-trop/gif_files/time_pres_UGRD_MEAN_ALL_NH_2013.gif

This is also highlighted by the zonal wind anomaly chart (the blue area)
http://www.cpc.ncep.noaa.gov/products/stratosphere/strat-trop/gif_files/time_pres_UGRD_ANOM_ALL_NH_2013.gif

This event was followed by some cold conditions in the UK (which may have been partly a result of the SSW). From 13 - 25 Jan 2013 the mean CET was -0.6C.

The last minor SSW was at the beginning of February 2014

This chart shows a temperature increase of 25C from about -60C to about -35C in early February at 10hPa
http://www.cpc.ncep.noaa.gov/products/stratosphere/temperature/10mb9065.gif

The zonal wind was weakened but only reversed to easterly briefly and only then at 2hPa and below. At 10hPa there was still a weak westerly flow
http://www.cpc.ncep.noaa.gov/products/stratosphere/strat-trop/gif_files/time_pres_UGRD_MEAN_JFM_NH_2014.gif

This event had no discernible impact on UK temperatures.

Applying the above to the current situation and what the models are showing today there are clear signs that a minor SSW is almost upon us. Given what the output is showing this could potentially become a major SSW within the next 10 days.

Warming often starts high up in the atmosphere. This can clearly be seen on the latest chart for the 1hPa level
http://www.cpc.ncep.noaa.gov/products/stratosphere/temperature/01mb9065.gif

This shows a temperature rise of about 19C in the past few days from around -31C to close to -12C. It only needs to rise by another 6C to meet the definition of a minor SSW (the above definition requiring a 25C rise within a week at any level of the stratosphere).

Looking at the model output the current temperature profile at 1hPa is as follows
http://www.cpc.ncep.noaa.gov/products/stratosphere/strat_a_f/gif_files/gfs_t01_nh_f00.gif

This shows an area of very warm temperatures of greater than +15C but also a core of cold temperatures in places below -50C.

The forecast chart for T72 shows the following
http://www.cpc.ncep.noaa.gov/products/stratosphere/strat_a_f/gif_files/gfs_t01_nh_f72.gif

The area of warm temperatures has shrunk a bit but also there are now no areas where the temperature is below -40C. So overall the mean temperature looks like it will continue to rise in the coming days.

Significant impacts are also being modelled at the 10hPa level as per James' post above. As others have said, changes in the stratosphere probably represent at the moment our best chance of seeing colder conditions develop as we move through January. But even if a SSW does occur this does not necessarily mean we will experience colder conditions in the UK, it merely increases the probability that we might.

Originally Posted by: Global Warming 

Will be interesting to see if there's any colder runs around T192 - T240 in the GFS ens - op shows an Artic HP gradually pushing over the pole towards Greenland only for the PV to go a bit nuts and the HP to scuttle off.

Not sure I'm buying that or the subsequent Westerly domination.

Given the flat and zonal Westerly bit then relies on the model having accurately predicted the behaviour of the Artic HP (unlikely) and subsequent behaviour of the PV (unlikely) I think I'll reserve judgement before calling 'zonal train', January's gone etc. for the time being.

That said if I can avoid too many more mornings like this morning from a riding point of view I'll be happy - skinny tyres and lots of ice = bruising

Originally Posted by: Hippydave 

Having looked at the GEFS there is little sign, over the next  15 days, of any northern blocking.Over this period therefore, any cold development would have to come from a  strongly amplified ridge giving the opportunity of a Scandi HP  forming.There  is currently little sign of that in the ENS.

I would go with the METO outlook that no sustained cold is likely for most of January.There is a hint in today's  extended outlook of something colder and more settled towards the end of January but low confidence.The METO contingency forecast also talks of slow easing of the strong  NAO + ve position during January.

I reckon METO have been pretty good so far in their forecasts so hopefully something cold will emerge later in the month. 

Originally Posted by: roger63 

Having looked at the GEFS there is little sign, over the next  15 days, of any northern blocking.Over this period therefore, any cold development would have to come from a  strongly amplified ridge giving the opportunity of a Scandi HP  forming.There  is currently little sign of that in the ENS.

I would go with the METO outlook that no sustained cold is likely for most of January.There is a hint in today's  extended outlook of something colder and more settled towards the end of January but low confidence.The METO contingency forecast also talks of slow easing of the strong  NAO + ve position during January.

I reckon METO have been pretty good so far in their forecasts so hopefully something cold will emerge later in the month. 

Originally Posted by: roger63 

By the end of the week we'll start seeing some eye candy 😉

Originally Posted by: Snowedin3 

Not sure if this has already been posted as I can't be arsed scrolling up but Ian Fergusson earlier today said that there is chances of cold snaps through January as they expect the NAO to weaken as the month progresses. I can't recall him saying anything like that last Winter, it was all doom & gloom. We have plenty of time to be optimistic of more cold weather.

Originally Posted by: Medlock Vale Weather 

Unfortunately it doesn't look as if the Arctic High is going to ridge south and help us and we will need some help from this alleged strat warming event. As Roger says there is nothing much of interest in any of the ens members. I think we can write off the next 2 weeks at least and probably 3.

Originally Posted by: The Beast from the East 

Not sure if this has already been posted as I can't be arsed scrolling up but Ian Fergusson earlier today said that there is chances of cold snaps through January as they expect the NAO to weaken as the month progresses. I can't recall him saying anything like that last Winter, it was all doom & gloom. We have plenty of time to be optimistic of more cold weather.

Originally Posted by: Medlock Vale Weather