Most major weather services (US NWS, Britain’s Met Office, etc) have their own supercomputers, and their own weather models. But there are some models which are used globally. A new paper has been published, comparing outputs from one such program on different machines around the world. Apparently, the same code, running on different machines, can produce different outputs due to accumulation of differing round-off errors
▻http://science.slashdot.org/story/13/07/28/137209/same-programs--different-computers--different-weather-forecasts