for Gonzo!
For a slightly more serious answer, as it appears that no-one else is going to - in my experience, ATC offers advice about where weather returns are strongest and the crew combine this with what the wx radar shows and between them agree which way the aircraft will go.
My own personal thoughts based on working with largely unprocessed radar pictures in an terminal/approach environment. Controllers of my vintage and experience certainly knew of the limitations of the information that the radar picture offered - it was primarily showing rain and obviously could not show turbulence or lightning. Additionally, depending on the type/wavelength of the radar being used, the picture was affected by the size of the water droplets in the atmosphere. For a short period, I had two radar systems available (one a replacement being commissioned) with very different characteristics which I imagine could provide a controller with additional useful information given a bit of experience in heavy weather. I would note that the place I was working whilst it 'ticked over' was not routinely a high traffic density environment which meant that controllers had the time to offer advice and to use increased separation which allowed aircraft to deviate without much risk of losing separation.