Shrewsbury Weir is a playspot that comes into condition only when the Severn is very high. At lower levels it can form a dangerous stopper and should be avoided - note that the level suggested here is only an indicator that it should be running. If it looks nasty, it probably is - don't get on! The wave flushes when the river rises further.
The weir is just downstream of the town centre itself. There is ample parking along Sydney Avenue - just find somewhere the water won't be start lapping at your car whilst you're strutting your stuff.
Rainchasers is best experienced as an app.