Health chiefs say they 'continue to be concerned' at the number of cases of measles in the Swansea and Neath Port Talbot areas.
Nearly 200 people have been diagnosed since the outbreak started in November, with 20 new cases in the last week alone.
More than 30 schools and nurseries have been affected so far.
Public Health Wales says it is working closely with Abertawe Bro Morgannwg University Health Board to combat the outbreak.
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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.