The A465 between Glynneath and Hirwaun has now reopened, South Wales Police say.
One person was taken to hospital earlier in the evening after 12 vehicles were involved in a collision.
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Staying mostly fine in the southeast and far northwest with some brighter spells. Cloudy in between with outbreaks of rain.
Levels over the next three days are moderate to high.
An 18-year-old man has been arrested on suspicion of murder following the incident in the Ely area of Cardiff.