A cold start to winter?
1st November 2016 - Today the Met Office has released its long-range outlook for November 2016 to January 2017, highlighting the risk of a cold start to the winter for the UK.
This does not necessarily imply that the UK will experience cold and snow - in fact the most likely outcome is for conditions to be relatively normal on average over the next 3 months. So how should the outlook be interpreted, and what enables us to look at the weather so far ahead?
The Met Office’s long-range predictions rely on the fact that the weather patterns we experience in the UK can be ‘steered’ to a degree by various climatic factors around the world. Although day-to-day changes in UK weather become impossible to predict beyond about 10 days ahead, the existence of these global effects allows us to get an idea of the themes that the weather might favour over the coming months.
The effect of this on the UK is expressed in the outlook as changes in the likelihood of getting specific temperatures or levels of precipitation. Nothing is ruled out, but any given value may become more or less likely than normally expected. In the temperature schematic below, the highest likelihood is where the curve is widest. For a normal year the most likely outcome is the long-term average, as would be expected.
Schematic illustration of the November to January outlook probabilities
With the current outlook, however, the curve shifts towards lower temperatures. This increases the likelihood of the coldest values. Overall, the chance of the UK experiencing temperatures in the ‘cold’ category is 30% - one-and-a-half times the usual risk. Contingency planners and others with sensitivity to cold weather might need to consider this elevated level of risk.
While the peak likelihood in this outlook also moves downwards compared to normal, it is still relatively close to average conditions. The fact that the probability for cold is 30% means there is a 70% chance of other temperatures - ranging from moderately below average to mild. A cold season is therefore still not as likely as a more normal one. Cold winters in the UK tend to happen only every few years; most are not noted for persistently cold weather.
It should also be noted that at this stage the outlook is only for the early part of the winter. An outlook for what might happen later in the winter will only be available closer to the time, and could, of course be quite different. The Met Office maintains a regular schedule of updates with releases towards the end of each month.
What factors have a bearing on the signals seen in the outlook?
During this year there has been a wholesale change in conditions in the tropical Pacific Ocean. The start of the year saw much warmer than normal temperatures, related to one of the strongest El Niño events on record. El Niño is the warm phase of the El Niño-Southern Oscillation (ENSO), in which temperatures vary between warmer- and cooler-than-usual states from year-to-year. The El Niño dissipated by mid-summer and now temperatures are hovering just above the threshold for La Niña – El Niño’s cold counterpart.
Over the next few months, ENSO looks likely to either continue at about the same level or move into moderate La Niña conditions. La Niña has a tendency to reduce the strength of the westerly winds which normally bring mild air to the UK in early winter, so is expected to increase the chance of colder-than-average weather. The effect of a moderately cool tropical Pacific Ocean on the UK could potentially be boosted by higher-than-average rainfall expected to persist in the eastern Indian Ocean.
A further potential influence is the stratosphere – the layer of the atmosphere between altitudes of 10 and 50 km - which lies above the troposphere where our weather occurs. The tropical stratosphere is home to the Quasi-Biennial Oscillation (QBO) – a pattern of winds that alternates between an easterly and westerly direction, with the whole cycle taking approximately 27 months. The QBO is very regular and predictable, although an unprecedented disruption in the cycle was observed in the early part of the year. Despite this, the QBO appears to have resumed its normal progress after this ‘reset’, so there is high confidence it will be in its westerly phase this winter. As long ago as the 1970s, Met Office scientists recognised that a westerly QBO also tends to increase westerly winds at middle and high latitudes (both in the stratosphere and at the surface).
The stratospheric polar vortex - the wind system that whirls around the North Pole in winter – has not strengthened as much as usual this autumn. This weakness is forecast to continue over the coming weeks. The tendency for a disrupted stratospheric vortex increases the chances of weakening of the westerly winds across the Atlantic and colder-than-normal conditions in early winter, in spite of the westerly QBO.
Modern computer modelling techniques are used to simulate the overall influence of global factors on emerging weather trends. Current output from the Met Office prediction system for the November to January period points to an increased likelihood of high pressure to the northwest of the UK, in the vicinity of Iceland and Greenland. This ‘blocking’ signal opposes the progress of depressions which usually track towards the UK in winter and bring mild and wet weather. It therefore implies more frequent north or north-easterly winds and an increase in the likelihood of colder- and drier-than-average conditions, consistent with the discussion of the effects of current global climate above.
Prediction systems from other leading forecast centres also show a blocking tendency, giving confidence that this pattern, and the cold weather it could bring, are indeed more likely to be themes in the weather over the next 3 months. Nevertheless, the increased risk of a cold start to winter needs to be put in proper perspective – the most likely outcome is still that conditions in the UK will be relatively normal in this 3 month period. It is only for contingency planning, where the impacts of cold weather could be high, that a heightened risk of a relatively low likelihood event may warrant some mitigating actions.