Olmo
For any forecast to be possible server would have to "queue" weather change in advance and that I think is not such trivial upgrade.
Queuing weather changes could be actually pretty straightforward and trivial - circular buffer class keeping let's say 4 weather patterns in store, current and 3 upcoming. Then as weather change occurs, change the head pointer to point at next in queue, let rng create the upcoming fourth weather pattern and be all set. Using circular buffer due speed and storage effectiveness.
If the wetaher needs to be in database as well, the patterns could be lazily written to DB from server memory as the system resource usage allows or as a part of larger DB server batch update.