You don't seriously think that weather underground has weather forecasting (radar, etc) facilities? This is expensive stuff.
Perhaps OS, but even then my experience has been that using the third party apps gives a degraded result as they sometimes miss updates from the weather bureaus (for example with ex-cyclone Oswald, Bom gave extra updates which were often missed by the 3rd party apps).
Please show me something that says they use extra data than from the BOM?