Health expert Marcus Longley warned that changes to the NHS in Wales, with fewer major A&E departments, could make the pressure on ambulances transferring patients worse, as response distances and times are increased.
Professor Longley said, though, that current problems lie within the system more broadly, and "the health service needs to get ahead of the problem, rather than just forever scrambling to catch up with it."
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The heaviest bursts are expected in the south and east, with some western areas either remaining dry or becoming dry by dawn.
Many places will start dry, but rain in the southeast will spread to most for a time overnight.
The family of Isaac Nash who went missing after being swept out to sea last summer have presented a cheque for £10,000 to the RNLI.