The Environment Agency has issued a flood warning for Walsden Water at Walsden, near Todmorden.
Calderdale Council tweeted: "The flood siren in Walsden is sounding. Please act now & follow your flood plan. Turn off gas, water & electricity & move important items to safety. Take extra care & don't travel unless necessary. Teams are out monitoring river levels. For advice visit http://eyeoncalderdale.com"
The Environment Agency said in a statement: "River levels across the Upper River Calder Catchment have risen as a result of persistent rainfall and are now approaching levels where we expect properties to flood.
"Consequently, flooding of property, roads and farmland is expected today 19/01/2021 soon after 21:00.
"The siren at Rochdale Road has now been sounded.
"Further persistent rainfall is forecast over the next 36 hours.
"We expect river levels to remain high through Wednesday and into Thursday.
"Areas most at risk are properties in Walsden, especially around the station. The railway line will also flood. We are closely monitoring the situation.
"Our incident response staff are liaising with emergency services and checking defences.
"Please remain safe and put your flood plan into action and activate any property flood protection products you may have, such as flood barriers and air brick covers.
"This message will be updated in 8 hours, or as the situation changes."
Twitter user Dazpigsy captured the noise of the siren, tweeting: "So our town again is at risk of flooding. #todmorden and #walsden #floods."
Calderdale Council CEO Robin Tuddenham tweeted: Siren activated in #Walsden by @EnvAgencyYNE where river levels at highest. @Calderdale @WestYorksPolice staff on site supporting local residents. All flood plans to be activated. #StormChristoph"