The Long Point Region Conservation Authority expects flooding in low-lying and poorly drained areas.
Spokesperson Matt Churly says a Flood Watch is now in effect for all watercourses and the Lake Erie Shoreline.
He says between 60 and 70 millimetres of rain and precipitation fell over the weekend and another 10 is expected today (Monday).
The Ministry of Natural Resources and Forestry is currently forecasting lake levels to surge to 1.75 metres at Port Colborne and more than 1.5 metres at Long Point with wave heights of up to 1.85 metres.
People are urged to stay away from the shorelines as well as ditches, streams, river and frozen water bodies.