The Met Office has transferred to a new off-site supercomputer - which could make 14-day forecasts as accurate as seven-day equivalents.
Operating using Microsoft's cloud-based Azure system, the new computer could also help predict extreme weather further in advance and advance climate research.
The weather agency used its first supercomputer 60 years ago, but until now it has almost always been run on site.
For more than a month, Azure has been running off-site simultaneously with the Met Office's previous supercomputer, which has now been switched off.
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Chief executive officer Penny Endersby said on the agency's website that "a big change like this is like changing the engines when you're flying over the Atlantic" - noting that the new computer had already been running in parallel with its old system for more than a month.
"You can never have a stop, which is why we did the long parallel run with the old supercomputer," she added.
"So, success at this phase is that nobody can tell the difference."
Charles Ewen, the weather agency's chief information officer, said the Met Office uses numerical weather prediction "to predict the future state of the atmosphere" which "takes the laws of physics that are fairly well understood and applies them at scale".
He added: "To do that is very, very computationally expensive. It's simulating the future state of the atmosphere.
"Operationally, that's 200 to 300 terabytes of information a day."
Using Azure, he said the Met Office will be able to start fresh projects without having to build new infrastructure.
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Segolene Berthou, head of a research team working on the Met Office's environmental prediction capabilities, added that the supercomputer will also allow for a model to be run several times with slightly different parameters.
"The coupled system we're preparing is running faster and more smoothly on the new supercomputer," she said.
"This is very good news because it means we can now be even more confident in our climate projections and have longer slices of time running this model."
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