JSDoc Polymorphism + type casting

Webstorm is throwing up a warning (correctly) when we are trying to assign/return polymorphic types.  Here is an example:

In a compiled language (like C#), the above code wouldn't compile without a type cast, and in this case, we can safely cast.  How can I achieve the same in JSDoc land?  (P.S., if derivedSet returns a BaseModel, everything is happy, but derivedSet should be returning DerivedModel).

Thanks!