The Envionment Agency said water levels at the river, from Brighouse to Dewsbury had risen and warned that they could peak this evening and early tomorrow morning (Tuesday).
Water levels are currently high at the Lower River Calder and its tributaries, including Brighouse, Castleford, Chickenley Beck, Spring Mill Beck in Ossett and Waindike Beck, Normanton.
The agency said it was closely monitoring the situation from its Leeds incident room.
The flood alert said: “We are now starting to see more of a river response to the persistent rainfall in the Lower River Calder and its tributaries.
“The River Calder Lower Catchments are already saturated meaning river levels are being very responsive to this rain and will remain high.
“There are likely to be two peaks in the river response, one peak around teatime and a second peak, which could potentially be higher, in the early hours of Tuesday morning (February 9).
“It is possible that Flood Warnings could be issued for some locations, depending where any heavier rain falls.”