The air-raid-warning-style flood siren at Walsden Water in west Yorkshire was sounded this afternoon.
The Environment Agency said flooding was expected in the area and people living in 245 properties nearby should take immediate protective action.
The area has a loud siren to warn people of imminent flooding, due to the nature of the landscape.
Environment Agency environment manager Jacqui Tootill said: “Flood water is dangerous, particularly in these areas where water cascading down from the steep surrounding hills means flooding can happen very quickly and with little warning. We would ask people not to put themselves at danger by driving or walking through fast moving flood water.
“We would urge people to remain vigilant to local weather conditions and make use of our three day flood forecast on our website as well as the Met Office Severe Weather Warnings.”