The MET Office, the UK’s National Weather Service, relies on more than 10 million weather observations from sites around the world, a sophisticated atmospheric model and a £30 million IBM supercomputer to generate 3,000 tailored forecasts every day. Thanks to this advanced forecasting system, climate scientists were able to predict the size and path of Monday’s St. Jude’s Day storm four days before it formed.

The IBM machine, upgraded in 2012 for a total of 1.2 petaflops of processing power, brings a new level of accuracy to weather modeling. ATelegraph article describes how the machine was able to predict the storm based on two areas of turbulent weather over Canada and the United States which came together over the western Atlantic to form one large low-pressure system.

Such depressions are common and generally harmless, but in this situation a fast-moving jet stream carried the weather mass across the Atlantic where it encountered a band of unusually warm air over Britain. The warm air ‘energized’ the depression, and transformed it into the storm that swept across the UK on Monday.

The Met Office supercomputer was able to forecast this storm and many other weather events using data from millions of sources across the globe. These crucial sites include weather stations, satellites, aeroplanes, boats, buoys and argo floats, which report on water temperature – a important weather variable.

On Sunday, October 27, the Met Office warned “a major Atlantic storm is set to move across the UK over the next 24 hours, bringing some heavy rain and very strong winds to parts of England and Wales.” The office issued Severe Weather Warnings advising of the potential disruption from both the strong winds and the rainfall.

While this wasn’t a catastrophic weather event, the advance notice gave people the opportunity to take necessary precautions and make arrangements to delay travel.

Met Office spokesperson Dan Williams describes how the forecasting process played out:

The accuracy of [forecasting] these days is enough to pull out factors that could lead to the storm that we saw last night. There were factors that played a role in that, there were two weather systems over the Americas which coalesced in the western Atlantic forming a fairly innocuous area of low pressure.

But then because we have a particularly strong jet stream at the moment it rattled across the Atlantic fairly quickly and it was at that point just off the South West of the UK that we could see there was an area of particularly warm air that was going to be there just as that low pressure reached the UK.

Those two things combined meant that this low pressure rapidly deepened just off the coast of the UK and energised it, making it much more vigorous and giving it the power it then took across the UK.

The Met Office is slated to receive a new £100 million supercomputer in 2015, a replacement for the current £30 million machine. Scientists and government officials alike say the new equipment is necessary to increase forecast accuracy, especially for reliable seasonal predictions.