Some early thoughts on the UK Winter 2019-2020 forecast

Date of “official” winter 2019-20 forecast release: 20th November 2019.

In this blog I will touch on a few of the “factors” that are used in winter forecasting, just a touch-base to establish some basis before the release of the main winter forecast.

Disclaimer: winter forecasting is not designed to be “definitively accurate”, more so a guide as to how heavy loaded the dice are in a given direction. Be it loaded for cold, average, or mild. Ultimately winter forecasts are limited to our understanding of complicated seasonal weather and ocean processes, which is fairly limited.

The importance of the stratosphere:

In recent years, well since Winter 2013/14 anyway, which was the first mild winter of many in recent years, cold spells have been few and far between. They have also relied rather heavily on the influence of the stratosphere. I’m sure you’ve heard of “Sudden Stratospheric Warmings”, these events originate near the surface in “waves” from the Troposphere (the layer of air nearer to the surface), and extend upwards, producing large swings in temperature in the Stratosphere. Temperatures in the Stratosphere, around 6 miles + high, can shoot up by around 30C or more for a Sudden Stratospheric Warming.

The last proper Stratospheric warming occurred back in February 2018, and was the most likely cause for the beast from the east. What was quite stark about this event was that it also resulted in a split of the Polar Vortex. When this occurs, the ribbon of air that circulates around the Northern Hemisphere from West to East is cut in half, and in stead starts rotating in the opposite direction. Due to the troposphere and stratosphere being interconnected, a “split” in the stratosphere can often produce a split in the troposphere to, resulting in a reversal of the West – East flow closer to the surface.

On a large scale that is what happened during February 2018. The graphic below shows the polar vortex during February 2018, vs a “normal”, organized polar vortex a month earlier.

Reversal during February 2018:

“Normal” polar vortex during January 2018:

See the difference? Although this is not necessarily a “factor” of the winter forecast, it shows just why winters are difficult to predict, sudden stratospheric warmings are almost impossible to predict a few months out, although computer models get a good grip on these events and forecast them fairly well within a month. So, that’s one thing that can significantly change our winter prospects within a fairly short time scale. Winter 2017/18 was due to be another “benign” winter until the Sudden Stratospheric Warming changed that all.

Saying that they are unpredictable, there are a few things we can look for to help determine just how likely a Sudden Stratospheric Warming (SSW) is. And also, perhaps even more importantly, how likely the SSW is to bring the UK cold weather. During Winter 2018/19 a fairly significant SSW occurred, however failed to produce anything really cold in the UK.

1) Background factors. Often a negative QBO, which is another climatic index, is a good thing. When the QBO is in it’s negative phase, it means that the easterlies produced by the SSW are fought with less friction, and as a result are more pure when they reach the troposphere. Contrastingly, a positive (westerly) QBO is thought to do the opposite, and make it “more difficult” for those easterlies to propagate.

2) State of the troposphere section of the atmosphere to “begin with”. If the atmosphere is already in a state of “weak zonality” (the term used to describe an already weak normal west – east atmospheric flow), then the SSW or polar vortex split will be fought with less opposition. An example of this is 2018, during winter 2017/18 there were various bouts of weak zonality, so that when the major SSW came along, it had an easier time reducing them even more, to the point of a reversal. Compared with 2019, with more active zonality in the troposphere, which resulted in more “friction”, and a less extensive reversal.

SSW thoughts for Winter 2019-2020: given the fact that SSW’s have been fairly frequent in the past couple of years, I would suggest that the chance of a SSW this winter is around 50%, which is fairly normal.

El Nino / La Nina, importance for the Winter 2019/20 forecast:

ENSO (El Nino Southern Oscillation) plays a big part in winter forecasts across the pond, in America, and not so much in the UK. Super El Ninos and strong ENSO events (strong El Ninos, strong La Ninas), are thought to reduce the likelihood of a cold winter in this country.

ENSO Forecast for Winter 2019/20: please bear in mind that ENSO forecasts, like others, are prone to change. Often El Nino and La Nina events can develop fairly quickly. Having said that, the CFS is currently suggesting that sea surface temperatures around the equatorial Pacific will remain fairly close to normal, if anything a little cooler than normal, but not enough to warrant a proper La Nina.

What might this mean for Winter? As explained, this factor is potentially one that could develop to “favor” a colder than average winter when the final forecast is produced, but is prone to change. Interesting nonetheless.

Next, Solar Activity.

Undeniably the sun has now reached what is known as “solar minimum” this is characterized by many consecutive days of spotlessness on the sun. In the scientific community there has been arguments that low solar activity (solar minimum) can be corresponded with an increase in earthquakes and volcanoes, and also unusual swings in the jet stream. So, another interesting pointer for this winter.

Global Warming:

Also undeniable is that the planet is warming. A combination of human influences by burning of fossil fuels, and perhaps also the urban heat island affect and other types of warming, are affecting the temperature and weather patterns on planet earth. The severity of this warming and just how much of recent warming can be attributed to human induced warming can not be deduced. However, it is having an affect, and the climate is warming (as per Met Office image below).

Impacts of global warming on the Winter 2019-2020 outlook: ultimately with a warming climate, the likelihood of very cold winters may be reduced.

Summary: as has been mentioned in this blog, this was only designed to be a teaser, a touch base. There will be a couple of updates in September, October and early November that also glance on some factors that have yet to develop to the point that we can make predictions based on them, for example snow cover through the Autumn season in Siberia, and the development of ENSO towards Winter, as well as some others. However, if I were to make a “risky” conclusion already, I would probably say the following: It is too early to make any bold claims, however the dice seem to be loaded in various directions thus far. The QBO has been positive for a while, however in contrast ENSO and solar activity seem to be favoring the possibility of colder shots this winter. Therefore, there is a slight chance, nothing more slight, of a similar season to 2011/12 or 2005/06, in that a mixture of weather systems that would be characteristic of both a “cold” and “mild” winter season look possible.