Officers using specialist equipment are examining land around the Cariad Farm site in Marshfield, Newport.
The search - prompted by suspicion a body has been buried there - is expected to last for several days.
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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.