SUDDEN STRATOSPHERIC WARMING (Watch 2012-2013)

Since the SSW has peaked out, its time to look at model performance of GFS and ECMWF in the last 31 days (ending on 14.1, so its fresh). Might be interesting for some, and boring for others. The area of the verification is 20N-80N, basically where all the main action was.

First we have the 10mb temperature bias. We can see that GFS had a warm bias which increased with the forecast time, while ECMWF had a cold bias, also increasing with the forecast time. This means that GFS has slightly too warm forecasts with time, and ECM slightly too cold.



The 10mb GPH bias. Both GFS and ECM had a negative bias for heights, increasing with forecast time. Basically meaning, they were promoting a stronger vortex with the increased forecast hour, especially the ECM.


The "die off" for 10mb GPH. Skill score reducing with time (GFS still very good at 240h). Basically GFS outperformed the ECM in the stratosphere, especially on the actual SSW event, as I will show below.



10mb 20N-80N, 10-day zonal wind forecast. At the time of the SSW event, GFS outperformed ECMWF (higher the score, better the forecast). This is the accuracy of the forecast issued 10 days before the forecasted date.



50mb Temperature forecast. GFS outperformed the ECM in 10day forecasts, in the SSW period.



10mb Temperature. GFS outperformed the ECM in the 10 day forecasts, except for the last 2 days.



10mb GPH, 10day forecast skill. GFS nicely outperformed the ECM in 10 day forecasts of the SSW.



GFS basically had better performance with the 8-10 day forecasts of the SSW. But notice the very high skill score of the 10 day forecasts, prior to the SSW, where there is much less complex dynamics. And that is what I was telling about the power of the model stratosphere forecasts in the 240h-300h period, compared to tropospheric reliability.

Thanks Ed, appreciated, was going to try and run through a few charts myself as its been rather quite here, but as ever that answers my questions! As an additional note the EC32 maintains a signal for higher than average pressure over Greenland and the North Pole in general really throughout the forecast period but it also signals a particularly -ve pressure anom across and to the W of the UK signalling a particularly cyclonic and 'active' mid and eastern Atlantic and into the UK. Clearly whether this occurs or not I don't know, but it is attempting to model lows on a more southerly track, but overall the EC32 day doesn't paint a particularly cold outlook towards the end of January with temps forecast to rise to near average. Clearly it'll be interesting to see whether it is way off or not.

I can't but help think that if we do get the scenario you describe then we are really looking at a prolonged period of cold weather for the UK and potentially very cold if that N or NE'ly flow can be achieved. It's interesting to note that some of the recent GFS ENS are particularly cold later in the month. All to play for throughout the second half of the winter and I certainly believe there is little chance of the vortex restrengthening or returning to a more organised state, enough to affect the troposphere.

One question I have for you, which reply when you get chance is as follows;

What variables evident did you acknowledge or know to look for weeks in advance to give you a solid grounding to suggest that a SSW would occur in January?. I remember this being mentioned way back in early to mid-December at a time when the ECMWF charts were clearly not even touching late December, let alone early January and the GFS model may have only been reaching late December as well?.

What is fascinating here is that whilst we all acknowledge the conditions in the stratosphere are more 'stable' than that in the troposphere I would be interested to hear your thoughts, detailed if possible, as to what was being acknowledged at the start of the winter to gauge the event we have just experienced. I am certainly playing 'catch up' here in terms of what signs and signals to look for on a longer term basis.

Now plenty there for the lot of you to get your teeth into !

Stratospherically we are still seeing a NH pattern dominated by the recent SSW. Contrary to some opinions, this has downwelled from the mid stratosphere and affected the troposphere. It is always hit and miss how and where the SSW can have knock on effects in the troposphere and it appears that we have been lucky to see the split vortex favourably positioned to prevent the jet stream powering through during the two week mid January period.

However, that is about to change as further warming of the stratospheric Canadian vortex which allows it's tropospheric counterpart to break free. West to east momentum initially is set to carry this across the Atlantic to its Siberian counterpart as can be seen in this 30 hPa chart.




As this occurs there will be a corresponding increase in Atlantic mobility allowing the westerlies to reach the UK rather than be held at bay. The stratospheric vortex even though weak will be positioned on the Atlantic sector meaning a far less meridional jet stream whereas the Pacific sector will have a far more disturbed flow due to the upper ridge holding strong. This pattern will be as of a direct consequence to recent stratospheric events - but the wheel of fortune will be dropping off the meridional cold flow elsewhere in the NH.

So the main question that everyone is asking is will we likely to see a pattern re- emerge that can bring back the cold and blocking to our shores. And the answer is not immediately but definitely not no!

With the weakened vortex conditions likely to persist for a period of time then it is only a matter of time before realignment occurs in line with the MJO and GWO.

The stratospheric forecasts during the next 10 days right up to 10 hPa are showing and will show signs of increased fluctuations in the exact position of the vortex.

There are signs of another split occurring around day 10 and this will need to be watched carefully.





If this does occur then we could quite easily see the door to returning cold opened back up again. I think we will need to keep an eye on the strat forecasts very closely in this period.