And a lot of hard work, sounds like: https://droughtmonitor.unl.edu/About/AbouttheData/DroughtCla...

> [Authors] bring together the physical climate, weather and hydrology data and reconcile that with local expert feedback, impact reports and conditions observations. The author is also responsible for weighing different indicators based on what’s most appropriate for a particular place and time of year. In the West, for example, winter snowpack has a stronger bearing on water supplies than in the East