It sounded weird that they'd have reached the 1500 monthly tweet limit so quickly, but apparently there's also a limit of 50 tweets per 24-hour period. In one prefecture that they use as an example, there are 45 cities, towns and villages, and each one needs it's own specific warning. So if a big storm comes through, they'd use up all their daily tweets on the first warning, and there'd be no room for updates. So that's what the issue was.