Is your feature request related to a problem? Please describe.
Right now, the saving of forecast data is very tricky. Currently, @Malte96 finds an approach that might be useful for general use cases.
The basic concept is to use a data structure like this:

{
"id": "WeatherService:001",
"type": "WeatherService",
"forecast": 
{
"type": "StructuredValue",
"value": {
"00:00": 20.2,
"00:30": 20.3,
"01:00": 20.4,
"01:30": 20.5,
"02:00": 20.5,
"02:30": 20.6,
"03:00": 20.6,
"03:30": 20.6,
"04:00": 20.6
}
}
}

Describe the solution you'd like
Add a tutorial to cover these aspect.

• How to create a data model for such use case?
• How to save the forecast data in a time series database? How must the Subscriptions look like?
• How to utilize the historical forecast via QL?