Flood alerts have been issued for the Don and Dearne rivers in Barnsley.
The Environment Agency issued them on Wednesday afternoon.
It says the Dearne and its tributaries from Denby Dale to Mexborough are at risk of flood, as are the Don and its tributaries from Penistone to Sheffield.
The agency's forecasters predict heavy and persistent rainfall through the afternoon and overnight. Heaviest rainfall will be over the hills.
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A spokesman said: "Washlands at Old Moor, Bolton Ings, Cudworth, Houghton, and Wombwell are expected to flood. If river levels rise further, Washlands at Adwick, and Dearne Mouth are expected to flood."
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Heavy and persistent rainfall is expected over the Pennines from midnight, affecting the Don and its tributaries.
