Taking Llantwit Major as being equivalent to St Athan (one adjoins the other) last night was one of those cases where the city of Cardiff gets far more rain, lying as it does, a little to the north of the Exmoor rain shadow, which sometimes benefits the coast around Llantwit / St Athan.
24h totals to 06h this morning were 23mm at St Athan (hit badly enough again, no Exmoor benefit then) but as much as 36mm at Cardiff (Bute Park). 26mm fell there overnight.
Julian Mayes