How best to store multi-dimensional timeseries data in a (R)DBMS environment?
I think you're over-complicating things.  For this example



Modeling this as a relational table is simple:

create table Forecast
( 
  reference_time datetime,
  latitude float,
  longitude float,
  precipitation float,
  temperature float,
  constraint primary key (reference_time,latitude,longitude)
)


It may not be extremely efficient for storage space and processing, depending on data structures and algorithms used by 
the RDBMS that implements it.  But it's logically very straightforward to model this as a relation.