The models over the last day or so have continued to trend and show the possibility of some colder weather as we move towards the end of November and perhaps into early December. There remains a great deal of uncertainty at the moment and I think it’s important to note that these cold weather updates are not forecasts.
This article is going to be more technical than the last few and I’m going to talk about some of the things the models are showing that I believe are key if we’re going to see some colder weather, I’ll also be taking a look at current Stratosphere conditions and how this may play into the weather as we move into the first part of winter.
Firstly, lets take a look at a Northern Hemisphere chart from the latest GFS 18z model. I’ve highlighted three key areas and I’m going to go into these below.
Area 1: This area has High Pressure building into it, this is otherwise known as the Kamchatka High and when we see this it generally teleconnects to high pressure over Southern Greenland or Scandinavia, not always but there is good correlation based on historic patterns to suggest this to be the case. This is a tick towards eventually seeing a pattern develop that’ll bring cold weather to the UK.
Area 2: Here we see High Pressure across the core of the Arctic, this not only helps split and weaken the Polar Vortex but it’ll also play an important role on increasing cross polar airflow from the Pacific all the way to our side of the world, this again helps to further weaken the Polar Vortex reducing the mean Westerly winds.
Area 3: Moving a little closer to home now, we see an area of high pressure across Russia and gradually extending towards Scandinavia. Whats happening in Area 1 and 2 will help this high pressure in area 3 retrogress North-Westwards towards Scandinavia and hopefully later, over towards Greenland.
Looking at things from a Northern Hemisphere view above helps us to spot trends on a much broader scale, the exact details for our tiny little island will come later when the forecast period is much closer, but for now, we’re seeing a good trend towards the potential for colder weather. Next we’re going to take a quick look at the GFS 18z Ensembles.
When it comes to forecasting further out in the forecast period, ensembles can be a big help in determine what the most likely outcome is. I’ve drawn a yellow line on the above chart to highlight the -5c isotherm line, as a rule of thumb for snowfall we need temperatures to around or colder than -5c. The yellow box shows the period in the forecast we’re currently interested in, being so far out in the forecast period things change with each run as models try to get a grip on whats likely to happen.
Taking a look, whilst there are a couple of ensemble members drop below this line, generally at the moment there’s a little support for any significant cold. The trouble is, these ensembles are run at a lower resolution than the operational runs and because we’re looking towards the East for our weather, they tend to handle these setups quite poorly. During 2010 the ensemble members didn’t start to pick up the cold signal until quite close to the forecast period when they suddenly jumped in one run so whilst they can be helpful, they can prove to be quite useless.
The ECM model tends to be more accurate when forecasting the type of setup the models are hinting at, the ECM Ensembles for London cool off a fair amount and indicate that colder weather could be on the way, important to note that some ensemble members keep things mild too.
So what does this mean? Well, at the moment there’s definitely a trend towards colder synoptics. The cross Polar heights begin to develop just outside the reliable time frame but once this begins to develop I think the models will start to become a little clearer over whats likely for the end of the month. Given how things stand at the moment, I’d say there’s currently a 40/60 chance of colder weather over milder weather.
Stratosphere Temperature Update
Moving onto something more important in terms of longer range prospects and the winter. Currently, the Polar Stratosphere is near record breaking low values. Considering we’d like the Stratosphere to be at, or even better above average this definitely isn’t a good sign.
The good news is, current forecasts suggest the Polar Vortex is going to be fairly elongated keeping the mean zonal winds quite spread out across the Arctic which really helps prevent the Polar Vortex from ramping up too significantly, for now at least.
Further good news is that Wave Breaking 2 is forecasted by the ECMWF to occur later in the period and this could help generate some warming as it propagates upwards into the Stratosphere bringing temperatures up from their current very low position.
Whilst cold weather isn’t impossible for us here in the UK during cold stratospheric winters, the chances of any prolonged cold weather are significantly reduced. A cold Polar Stratosphere increases the temperature difference between the Arctic and the Tropics and this only helps to power up the polar vortex which really inhibits high pressure systems setting up for any long period of time.
So, whilst conditions are currently very poor for the Stratosphere, if these Wave Breaking forecasts come to fruition then we could be in a better position a couple of weeks down the line. If not, then my next monthly update at the end of the month could be talking about the possibilities of a milder than average December.