The API was occasionally returning empty forecasts for some locations between 11:27PM ET and 11:57PM ET. This was caused by our forecasting system generating a corrupt data file that the API was unable to consume. While we have checks in place to avoid such files from being written in the first place, it is difficult to identify all possible forms of corruption, and it was one of these subtle cases that snuck past our checks.
For the moment, we have added better monitoring of these cases and improved our on-call documentation so that such issues can be more quickly resolved in the future. We will also be adding automated checks for this kind of error so that it can automatically be fixed by our forecasting system in the future.