Calderdale Council is asking residents and businesses at risk to activate their flood plans - turning off gas, water and electricity, and moving important items to safety.
People are being asked to avoid using low lying footpaths near local watercourses and plan driving routes to avoid low lying roads near rivers, which may be flooded.
The flood siren in Walsden sounded earlier this morning to urge residents to take extra care.
At Walsden, flooding is expected from Bottomley Lock to Clough Mill, including Rochdale Road, Walsden Station and Walsden School.
Levels at Walsden were forecast to continue to rise until about 8am this morning, with a second peak occurring late this afternoon.
The area of concern includes the railway line and properties next to the railway station.
In Halifax, flooding is expected at Dean Clough Industrial Park and Dean Clough Mills.
Levels at Halifax were forecast to continue to rise until about 8am this morning.
The area of concern includes Dean Clough Industrial Park and Dean Clough Mill.
Hebden Beck in Hebden Bridge was also under a flood warning, with properties and roads by the beck most at risk.
For the River Calder, flooding is expected between Brearley and Luddenden Foot, including Station Road in Brearley, Ellen Royd Bridge, and Station Road and Mill Stream Drive in Luddenden Foot.
Levels at Brearley were forecast to peak at mid-morning today.
The area of concern includes the Tenterfields Business Estate.
The Environment Agency and Calderdale's flood wardens are monitoring the situation closely and will be updating their websites and social media channels throughout the day.
Meanwhile, Yorkshire Water has closed Victoria Road and Palestine Road in Hebden Bridge because of a burst water main.