The cause of the problem, which occurred at around 3 a.m., local time (6 p.m. GMT), in systems of the Japan Meteorological Business Support Center, is unknown and the system hadn't been brought back online as of 1 p.m. local time.
As a result of the problem, the Meteorological Agency and many news organizations haven't been unable to publish current weather data and weather forecasts. The most up-to-date forecast on the agency's home page is that from 11 p.m., local time, Sunday night. On a typical Monday it would have been updated at least twice before lunchtime.