Snowy L

Much improved wave forecasts too today with that massive wave 2 breaking beginning to move down the stratosphere on days 9 and 10.

100mph zonal winds yikes, surely this is a sign that wave breaking is putting some pressure on the vortex making it more concentrated, rather than the vortex getting stronger in general?

Figures by NOAA show November was a strong westerly month, so it would take a lot to switch it to Easterly this winter. Even if it were to go easterly in say February, surely there's a lag phase with this?

Yeah it's looking like we may see a strong heights over Scandinavia/Siberia which is a bit of a surprise considering the poor snow advance index. Would be shocked if we got an SSW in late December/early Jan.

I made this a few weeks ago but anyway here's my take on this winter: Methodology ENSO The El-Nino Southern Oscillation (ENSO) is expected to remain neutral/slightly negative through the rest of Autumn and for most of winter. The effect of the ENSO on Europe is a weak one, though it can still influence our weather. In winter, an El Nino event tends to give a warm first half and a cold second half and the reverse is true with La Nina. However with a neutral ENSO it is suggested that it has very little effect on the weather in Europe. ENSO is yet another factor in determining the strength of the polar vortex. It has been shown that El Nino events lead to a warmer polar stratosphere and therefore a weaker polar vortex. El Nino events often lead to anomalously low pressure over the Aleutian Islands, which is a tropospheric precursor to Wave-1 wave breaking. La Nina is more confusing and conflicting things have been said about it. It is thought that it gives a colder stratosphere but observations have shown that most SSWs happen during La Nina. A neutral ENSO gives far less SSWs per winter, though it's also worth noting that a neutral ENSO means that other factors, particularly the QBO can have much more of an influence than the ENSO. So the ENSO suggests we have a smaller chance of an SSW this yea Solar Cycle and the QBO The Quasi-biennial oscillation (QBO) is an oscillation of the equatorial zonal wind between easterlies and westerlies in the tropical stratosphere. How this influences the polar stratosphere depends on what phase the QBO is in. Currently, the QBO is in a strong westerly phase. Until recently it was thought that an Easterly phase resulted in a warmer polar stratosphere with more vortex disruption, and the opposite to be true for when it's in a westerly phase. However in the last decade or so it's been found that the solar cycle can change this relationship. It's been shown this relationship is reversed under solar maximum years. During Solar Maximum and when the QBO is in a westerly phase, the geopotential heights are higher over the North Pole. There is a very clear tendency for SSWs in the west phase of the QBO to occur during solar maxima. In contrast there are very few SSWs in the easterly phase during solar maxima. Also during a westerly phase, SSWs tend to happen later in winter rather than early. The recent solar maximum has been a weak one, though a recent surge in solar activity has seen the solar flux 10.7cm increase to levels found in the lower end of a solar maximum. This, accompanied with the westerly phase QBO, indicates that there is a fair chance of a SSW occurring towards the end of winter. Arctic Ice, Eurasian Snow Cover and the Snow Advance Index Recently, levels of ice in the Arctic have been linked with colder winters. Lower ice is correlated with a weaker polar vortex and a negative NAO. A lower than average ice minimum is also believed to increase the advance of snow cover in Eurasia during Autumn. This occurs because less ice provides a much larger area of open ocean, greatly increasing moisture availability in the area. This tends to make the Arctic warmer and the surrounding land masses colder. We have seen another below average ice cover minimum this year, though not quite to the extent of last year which was record breaking. Also, recent studies have suggested that lower sea ice leads to increased rossby wave incidence in the Pacific/North America region. We have already seen a few wave breaking events though none have caused damage to the vortex so far, due to the low ice we can expect to see more through the winter. The advance in snow cover over Eurasia during October is an important step in the formation of a major tropospheric precursor to stratospheric warmings. This is measured using the Snow Advance Index (SAI). It is derived not from October mean snow cover extent, but rather as a function of how slowly or quickly the snow cover advances across Eurasia in October. This index is highly correlated with the AO. This is because it starts a chain of events where a high SAI aids the development of the Siberian high, which then enhances wave 1 activity, leading to weak vortex events and then to a negative AO. When snow cover advances rapidly across Eurasia in October, this is an indication that the upcoming winter will be more severe for the Eastern US, Europe and East Asia. Unfortunately the index was a poor one this year. As can be seen in the graph below, the rate of snow gain was slow thanks to the very high snow amounts to begin the month and the mild end to the month. Sea Temperature Anomalies While not a great indicator of future weather, a certain type of pattern in the North Atlantic sea temperature profile has emerged that is correlated with a negative NAO. The North Atlantic Tripole is a pattern of sea temperatures that sees a warm anomaly at around 30 degrees latitude, a warm anomaly in the area surrounding the south of Greenland and a colder anomaly in between. I have added a picture (not mine) explaining the pattern as well. The sea temperature profile this Autumn closely matches the typical pattern so this is another factor that could give us increased tendency for the NAO to go negative this winter. Winter Forecast So now I will attempt to mix these factors and make a forecast out of them. My initial thoughts were a cold start to the winter and a cold end, with most of winter in being mild in between and I still expect we will see something along those lines. Currently an anomalous high pressure in the North Pacific is directly affecting the tropospheric pattern and indirectly affecting the stratospheric one. It has enhanced wave-2 breaking which is currently making a few minor dents in the lower stratospheric vortex, allowing some brief meridional patterns to emerge. Therefore I expect the rest of November to follow this pattern and I expect the next few weeks will see us have a chance at a brief cold spell before the vortex fully forms at all levels of the stratosphere. I expect the rest of November to be changeable with cool zonal being the theme. We will see periods of mild weather and colder weather as brief meridional patterns give us short Northerly/Northwesterly blasts. Towards the end of the month and into December I think we will see a better attempt at a cold spell with a solid high pressure building to our Northwest and giving some areas some snowfalls. For the rest of December high pressure will remain close to the UK giving some settled weather but nothing particularly cold or mild. Into Christmas we will see high pressure move to our south, giving us some very mild weather for a time before zonal weather picks up in time for January. I expect most of January to be dominated by unsettled weather. A zonal pattern will become strong and I think we will see some stormy periods of weather. Temperature wise January will be mostly above average but again I do think we will see some periods of brief cooler weather mixed in too. End of the month and into February the pattern will change into much more of a mild zonal with the possibility of the dreaded Bartlett high setting up. This should continue into February too. However, I do expect a big change in the pattern sometime in February. Several factors support the chance of a sudden stratospheric warming happening towards the end of winter. The combination of the solar cycle being in a relative maximum phase plus the QBO, along with the fact that we have seen a low ice year, makes me think we have a fair chance of an SSW, not a great chance but certainly not a slim chance. I think mid-month in February we will see the vortex weakened/split and some form of Northern Blocking will occur. Obviously this is no guarantee of cold but about 70% of the time we will see a colder pattern come from this. The tripole in the Atlantic that often leads to negative NAO events makes me think that if we do get Northern blocking, the Atlantic will be a favoured place for high pressure to build, so the UK should be under this blocking and will see some colder weather. So while I do think we're in for a mixed winter, the key word that would give widespread snowfall is blocking, and unfortunately I don't think we will see that for most of winter. Our best chance is in February I feel and the end of February and March are when our best chances of snow are. Overall this winter will be close to average, I'm favouring slightly above, and precipitation will be above average largely because of the unsettled pattern in January.

It was always going to be that though. I'm starting to trust the Met O a lot more with mid-range forecasts. They will sometimes side with the GFS and will sometimes not side with it, unlike some on here to think it's the gospel. Met O going for a a 3-4 day cold spell, GFS going for a 0 day cold spell, I think it's becoming very clear that GFS should be rubbished for this cold spell.

Eastward shift in this run, with a low pressure system in the Atlantic pushing into the high pressure. Going to be a crazy few days of model watching.

The reason a cold spell is on is because of a shift in the vortex...

Spot on

Nice end to the run with the easterly pretty much locked in

Low pressure building over Spain, that High's going to get sucked Northeast and all it takes is a shortwave to cut off our high from the Atlantic high.

ECM still going for the link up between the two highs. Shortwave in the Atlantic getting very close to Spain. Can't believe the differences at just 96 hours away. It's setting up to be a crazy couple of days of model watching. While UKMO ended up bad, it's still very close to joining the highs. It's just GFS that just completely goes against a link up between the two highs.

I didn't lose count, in fact it only happened once in December. ECM got the January cold spell after the GFS repeatedly telling us that it would never happen, only to back down 2, yes 2 days before the cold spell started. GFS will go for mild close to 100% of the time, so it's bound to get it right at a few times. ECM is much more adaptable and while it is prone to overplaying cold scenarios it will smell the coffee a lot sooner than the GFS.

When is the switch to milder weather going to happen according to the Met Office? The way it's been said here makes it sound worrying but if it's around week 2 of December then it would make a lot more sense. With the vortex expected to split, zonal should surely be out of the question for the next 2-3 weeks, especially as the position of the split is a favorable one for our location.

Both Hirlam and Bracknell FAX charts supporting ECMWF, where they both link up our high and the developing one over Greenland, also JMA obviosuly did more than support the ECM yesterday with that 12z run. Not sure where the idea of no support for ECM has come from? UKMO is almost there, once again GFS is the one firmly on its own.

Just look at that vortex reform so quickly in early FI. oh GFS... you don't help yourself do you?

GFS eventually seeing the split close to t+180, I'm guessing we'll get a block on this but it's pointless speculating. It's still got the t+96-t+144 period wrong imo.

Warm sector seems smaller in this run, GFS actually going for some snow in the precipitation.

And the GFS has been guilty of reverting to zonal in 99% of the outer reaches of its output yet we still hear everyday about how zonal will take over any day now...

It was strat related and very similar to this year. If I remember correctly what Chiono said it was a wave breaking event over Greenland that caused the disruption to the lower and mid stratospheric vortices even though they continued to cool down, exactly the same thing is going on now.

JMA knows what's going down

The signs were there for this weeks cold spell, the stratospheric charts showed low pressure into Europe and a ridge getting as far as Iceland but not penetrating into the polart vortex, which inevitably leads to mid-lattitude blocking rather than high-latitude blocking. This time the vortex is splitting with each lobe moving away from the North Pole. This should enable Northern blocking for a time Edit: Example of this:

Northern Hemisphere pattern is the encouraging part, no point talking about specifics 200 hours away.

I'd say it's pretty representitive of what's going to happen over the high lattitudes. The things that will win it or lose it for us in the mid-lattitudes are shortwaves.

Arctic high completely in control of the North pole, the only logical solution after today's stratospheric charts. ECM very much leading the way.