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Thursday 21 September 2017

My weather alert -- beware sting in the tail of our windy winter storms

AS the winter of 2011/12 approached, vivid memories returned of snow and ice. People stocked up on snow shovels, salt, and even winter tyres.

Nature, though, had other ideas. The last four months of 2011 saw windy, wet and mostly mild weather, and the early days of 2012 have brought little change. Why have the winds been so strong over the past few months, and in particular over the past few weeks?

Ireland lies between 52 and 55 degrees north, in the "mid-latitudes". Around these latitudes there is a band of westerly winds circling the earth, not just at the surface but also at high altitude. The strength and direction of these winds at the surface is controlled by the high and low pressure centres, familiar from the weather charts. When high pressure builds close to Ireland it acts as a barrier, and we get light winds and settled weather. When, more usually, high pressure is down over the Azores, with a low pressure centre close to Iceland, the winds rush between these pressure systems, bringing us into the stream of strong westerlies.

Temperatures also play a part. In the winter there is a greater contrast between the cold air in the Icelandic Low and the warm air in the Azores High. This temperature contrast drives the rapid development of weather systems.

These weather systems have their own life cycle, typically a few days, in which warm and cold air interact to form eddies, or whirlpools, in the Atlantic westerlies. These whirlpools grow and deepen into low pressure centres, adding extra rotational energy and bringing stronger winds, together with cloud and copious amounts of rain in the "weather fronts".

When forecasters analyse weather charts daily, they quickly come to recognise situations where there is a lot of "energy" available to weather systems -- such as large temperature contrasts between adjacent airmasses. This is when forecasters expect very vigorous storms to develop -- although predicting the exact storm tracks and timing remains difficult.

For most weather systems, there is a well-understood progression as the system moves from west to east. First the build-up in cloud, then rain, followed by an increase in temperature in the "warm sector" between warm and cold fronts. Often the strongest winds are within this warm sector, just ahead of the cold front. Once the cold front passes (often with a short burst of heavy rain), there is a drop in temperature and the winds ease.

For the most vigorous storms, however, the weather fronts get "wrapped around" the low pressure centre into a spiral. This wrap-around cloud can help generate another region of strong winds behind the cold front. It is here that a "sting jet" is sometimes experienced. This is a complex weather feature. Air descends from on-high, where the winds are stronger in any case, and brings those stronger winds to surface level. The name arises because these extra-strong winds occur when, in a more normal storm, the winds might be expected to ease -- the "sting in the tail" of the storm.

Finally, does the recent windy period prove or disprove global warming? Neither -- we have had such windy winters before (such as in the early and late 1990s) and we will have them again. Evidence for global warming must be sought in the longer-term records of weather.

Are there more storms to come? The weather looks set to remain mild and fairly windy over the next week, although the more vigorous storms are likely to take a more northerly track than recently and spare us the worst. Nevertheless the forecast team will keep a close eye on developments. In weather, even more than in politics, a week is a long time.

Gerald Fleming is head of forecasting at Met Eireann

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