This was an extreme period of weather that saw a marked change from dry conditions to very wet conditions in a very short period of time.
Following the exceptionally wet weather of late spring 2012 the Met Office provided a full report into the possible reasons for the switch from dry to wet conditions. Our report states that the advice provided in the long-range outlook for April to June 2012 issued in March 2012 ‘was not helpful’ to our government customers.
However, looking at the skill of these outlooks over many individual forecasts clearly shows that they provide useful advice to their specialist users on over 65% of occasions. In addition these outlooks are never used in isolation but form one part of a range of forecasts from the Met Office including regular monthly outlooks and highly accurate 1 to 5 day forecasts and warnings.
Facing up to the challenge of long-range forecasting
The science of long-range forecasting is at the cutting edge of meteorology and the Met Office is leading the way in this research area. We are continuing to work hard to develop the science of long-range forecasting. We are confident that long-range outlooks will improve progressively and that the successes we have achieved in other parts of the world already will, in the future be mirrored in the UK.
The Met Office constantly reviews the accuracy of our forecasts across all time scales and is recognised by the World Meteorological Organization as one of the top two national weather forecasting services in the world. We also routinely verify our short-range forecasts on our website.
The ‘big switch’ of April 2012
During March 2012 the La Nina event that had persisted from 2009 was finally waning in the Pacific (as predicted by the seasonal forecast system), although many parts of the global oceans and tropical weather patterns still retained characteristics associated with La Nina. In the northern hemisphere the jet stream was very disturbed, resulting in a wave pattern of high and low pressure regions. The UK was positioned under a strong high pressure region resulting in very dry and warm conditions. In April, the wave pattern underwent a significant shift to bring the UK under the influence of strong low pressure, with prevailing south-westerly flow and heavy rainfall.
As detailed on ‘Today’, one of the potential causes of this shift in the northern hemisphere circulation may have been associated with a shift in tropical weather patterns. In particular, this may have been caused by a strong Madden Julian Oscillation (MJO) which occurred in March. This is a large-scale tropical phenomenon which leads to disturbed weather patterns over a timescales of typically 30-60 days. This changes originating over the Indian Ocean may have influenced our northern hemisphere weather regimes. Understanding the initiation of an MJO event is, however, largely unpredictable, and remains one of the great unsolved challenges of tropical meteorology.
Due to the fact that the initiation of an MJO is largely unpredictable – combined with knowledge that often subtle, and sometimes small, shifts in hemispheric circulation patterns can make all the difference between fine, dry weather and unsettled, wet weather over the UK – it is very unlikely that its impacts could have been anticipated in any forecasts for the coming months issued in early and mid-March.
A complicated world
Finally, although one reason for the switch in the fortunes of our weather in 2012 may have been the MJO, there are other parts of the climate system which we increasingly recognise as having an influence on our weather patterns. These include the North Atlantic Ocean temperatures, solar variability, the circulation of the upper atmosphere – the stratosphere – and of increasing interest, the changing state of the Arctic.
Better understanding and representing the drivers of predictability in the global climate system that influence our weather patterns is as ever a priority for Met Office research in order to deliver improved advice and services on all timescales.